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					chapter 6>>   visual perception

visual perception                                                         six stages of visual perception
                                                                          >>key knowledge>>                                              >>chapter overview>>
Possibl the best illustration of the way body and mind interact to
                                                                           Application of psychological perspectives to explain           Six stages         Sensation
enable us to Perceive the world around us is in the Process of vision.
                                                                           visual perception:                                             of visual             > Reception
our eyes receive the stimulus energy of light, reflected from an object    •	 characteristics	of	the	visual	perceptual	system	and	        perception            > Transduction
in the outside world, light-sensitive cells in the eye send messages to       the	visual	processes	involved	in	detecting	and	                                   > Transmission
the brain and then we interPret and understand what we see.                   interpreting	visual	stimuli
                                                                                                                                                                > Selection
                                                                           •	 the	effect	of	psychological	factors	on	perceptual	set                             > Organisation
                                                                           •	 	distortions	of	visual	perceptions	by	illusions.                                  > Interpretation
                                                                           	(VCAA	Study	Design	2009)                                      The processes      Reception and light energy
                                                                                                                                          of visual          Reception and absolute threshold
                                                                                                                                          perception         The role of the eye in visual perception

                                                                                                                                          The principles     Selection and organisation of visual
                                                                                                                                          of visual          stimuli
                                                                                                                                          perception            > Perceptual constancies
                                                                                                                                                                    > Size constancy
                                                                                                                                                                    > Shape constancy
                                                                                                                                                             Gestalt principles of perceptual
                                                                                                                                                                > The phi phenomenon
                                                                                                                                                                    > Figure-ground organisation
                                                                                                                                                                    > Camouflage
                                                                                                                                                                    > Closure
                                                                                                                                                                    > Similarity
                                                                                                                                                                    > Proximity
                                                                                                                                                             Depth perception
                                                                                                                                                                > Binocular depth cues
                                                                                                                                                                    > Retinal disparity
                                                                                                                                                                    > Convergence
                                                                                                                                                                > Monocular depth cues
                                                                          fig 6.1>>[caption]                                                                        > Accommodation
                                                                                                                                                                    > Pictorial depth cues
                                                                          The actual image on our retina is                                                         > Linear perspective
                                                                          > upside-down                                                                             > Interposition
                                                                          > back-to-front                                                                           > Texture gradient
                                                                                                                                                                    > Relative size
                                                                          > blurred
                                                                                                                                                                    > Height in the visual field
                                                                          > crisscrossed by a network of veins                                               The effect of psychological factors on
                                                                          > patched by holes.                                                                perceptual set
                                                                              Yet when this image has been sent to the brain, it is
                                                                                                                                          Distortions of     The Ponzo Illusion
                                                                          processed so that we see a crystal clear picture!               perception         The Müller-Lyer Illusion
                                                                              Sometimes our brain can be tricked and we see what                                > The carpentered world hypothesis
                                                                          we expect to see. This is what happens when we see visual                             > The perceptual compromise
                                                                          illusions.                                                                              theory
                                                                                                                                                             The Ames Room Illusion

                                                                          A study of the way in which we perceive the world around
                                                                          us through the sense of vision is a wonderful example of the      One of the key objectives in this chapter is to examine
                                                                          way the brain and body work together, depend on each other     how the physical reality of what we see (sensation) is
                                                                          and help each other as we go about our everyday activities.    influenced by the psychological processes that enable

chapter 6>>           visual perception

us to interpret it (perception). Sensation refers to the          From the time we receive an image to when we can
process where the structures of the eye receive information    identify what we see, six stages have been identified,
about the environment and transmit that information            some of which occur at about the same time. The first
to the brain. Perception is where the brain organises and      three stages make up sensation: the physiological part of
interprets this information (visual image). Thus, the visual   perception. The second three stages make up perception:
perception process is broadly divided into ‘sensation’ and     the psychological part of the process. The sequence is as
‘perception’. The process itself—from receiving an image       follows:
to ultimately interpreting what we see—is complex and
has been studied extensively by a number of psychological
perspectives as discussed in Chapter 5.                        1 Reception: Light enters the eye through the cornea,
   The processes involved in sensation and perception            a tough transparent tissue covering the front of the
are also thought to be adaptive. From an evolutionary            eye. It then passes through the pupil—the hole in
perspective, the ability to see, hear, touch and smell has       the middle of the coloured part of the eye (the iris).
                                                                                                                                                                                                                                       fig 6.3>> the brain makes
developed over thousands of years and through millions           The lens then focuses the light on the retina, which                                                                                                                  sense of what we see.
of changes—leaving our senses perfectly suited to our            contains the photoreceptors—light-sensitive cells called
environment and to help us survive and reproduce                 rods and cones.
(Tooby & Cosmides 1992, cited in Westen et al. 2009).          2 Transduction: Light energy is converted by the rods
Just like frogs, which have an inbuilt ‘bug-detecting’           and cones into nerve impulses. This allows the visual         4 Selection: We can’t possibly pay attention to all the                where the object is in space (in relation to our visual
function in their visual system designed to activate when        information to travel along the fibres of the optic nerve         millions of stimuli that enter the eye at the same time,           field and us). Patients who have damage or tumours in
a tasty insect is in view, humans have specialised areas         to the brain.                                                     so we pick out the ones that are important to us and pay           parts of the temporal lobe responsible for identifying a
in the brain that allow the perception of faces and facial     3 Transmission: Within the retina, the rods and cones               attention to those. At this stage of the process, the image        visual stimulus may be unable to recognise an object or a
expression. This can be seen in infants, which have an           send the neural signals/impulses via the bipolar cells            is pulled apart by specialised cells called feature detectors      familiar face (prosopagnosia). Patients who have damage
innate or inborn tendency to show greater interest in            to the ganglion cells. The ganglion cells have receptive          that respond to lines, size, length and orientation. The           to the parietal lobe will be able to recognise an object but
objects that look like a human face (Adophs, Damasio,            fields that are activated by the pattern of the image             various feature detectors correspond to the receptive              may constantly bump into furniture or misjudge picking
Tranel & Damasio 1996).                                          received by the rods and cones. Next, these neural                fields in the ganglion cells in the retina and are found in        up their knife and fork.
   The eye is a fantastic organ—it is very complex in            signals travel along the ganglion cell’s axons that form          the striate cortex of the primary visual cortex (brain).
construction, but we only need to know about a few of its        the optic nerve to the occipital lobes, at the very back of   5 Organisation: When visual information reaches the
structures.                                                      the brain.                                                        brain (visual cortex), it is reorganised so that we can
                                                                                                                                   make sense of it. We do this by using certain visual
                                                                                                                                   perceptual principles:
                                                                                                                                   > perceptual constancies
                                                                                                                                   > Gestalt principles
                                                                                                                                   > depth and distance cues
                                                                                                                                   These are explained in the next section of this chapter.
                                                                                                                                   Once the image is reassembled using these principles,
                                                                                                                               it travels along two pathways simultaneously: to the
                                                                                                                               temporal lobe, to identify the object, and to the parietal
                                                                                                                               lobe, to judge where the object is in space (in relation to
                                                                                                                               our visual field and us).
                                                                                                                               6 Interpretation: This is the process whereby the visual
                                                                                                                                   stimulus/object is given meaning. The temporal lobes
                                                                                                                                   identify what the object is by comparing incoming
                                                                                                                                   information with information already stored in memory.
                                                                                                                                   Past experience, motives, values and context help us
                                                                                                                                                                                                   fig 6.4>> do you know why a person cannot see clearly
                                                                                                                                   understand what we are looking at as well as perceptual
                                                                                                                                                                                                   underwater? the cornea of the eye is designed to refract or
                                                                                                                                   set (this will be discussed later in the chapter). At the       bend light rays travelling through the air—not through water. so
                                                                                                                                   same time as information is sent to the temporal lobes, it      when you put swimming goggles or a diving mask, the layer of air
fig 6.2>> movement of light entering eye.                                                                                          also travels to the parietal lobes, which assist in judging     between the water and cornea allows you to see more clearly.

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chapter 6>>           visual perception

6.1 review                                                                                                                                                     reception and absolute threshold

Copy this table into your workbook or use a computer. Complete the table                                                                                       The second important element in the eye detecting the light stimulus is that
using the information provided for each stage of the visual perception                                                                                         the light energy that falls within the visible light spectrum must be intense
process. (Note that the image prior to entering the eye during the reception                                                                                   enough for the human eye to see. In other words, it must reach absolute
stage is the correct way up but by the time it reaches the retina, it has been                                                                                 threshold. The absolute threshold is the minimum amount of physical/light
turned upside down and back to front.)                                                                                                                         energy needed for an observer to perceive a stimulus in ideal conditions.
                                                                                                                                                                   One method psychologists use to measure absolute threshold is by
 Reception             Transduction                     Transmission                Selection                       Organisation           Interpretation      presenting a light stimulus at different intensities to see at what point the
                                                                                                                                                               person can see it. If that person detects it during the experiment about 50
                                                                                                                                                               per cent of the time at a particular intensity (the point at which they actually
                                                                                                                                                               perceive it), then absolute threshold has been reached.
                                                                                                                                                                   Imagine this. You are walking at night on a deserted country road with a
                                                                                                                                                               friend (an enthusiastic psychology student) and there are no lights in the area.
                                                                                                                                                               It is a clear night and you have arranged for another friend (also a psychology
                                                                                                                                                               student) who lives 60 kilometres away to begin walking towards you. You have
                                                                                                                                                               also asked the friend walking towards you to hold a lit candle. Research has
the processes of visual perception                                                                                                                             found that when your friend gets to approximately 50 kilometres from you,
                                                                                                                                                               you will only then be able to detect the light from the candle. Hence, absolute    fig 6.6>> your eyes can detect a candle
reception and light energy                                                                                                                                                                                                                        light up to 50 km away on a clear dark
                                                                                                                                                               threshold has been reached (Brown, Galanter, Hess & Mandler 1962).
The process of light entering the eye is an important part of our ability to                                                                                       Absolute threshold for other senses are:
receive and interpret a visual stimulus/image. However, before the eye can                                                                                         > Hearing: ticking of a watch six metres away.
receive the visual stimulus/image, there are a couple of elements that must be                                                                                     > Smell: one drop of perfume in a large house.
in place.                                                                                                                                                          > Taste: one teaspoon of sugar dissolved in 10 litres of water.
   First, the light energy must be within the visible part of the electromagnetic                                                                                  > Touch: the wing of a fly falling on the cheek from a height of one
spectrum. Wavelengths of between 360 and 370 nanometres form the visible                                                                                               centimetre.
spectrum (1Nm = 1 billionth of a metre). The energy that enables us to see is                                                                                      > Vision: the flame of a candle 50 kilometres way on a dark, clear night.
what we call light energy, the visible part of the electromagnetic spectrum.                                                                                       These are based on sound scientific research but may not be the same
                                                                                                                                                               for everyone and can vary depending on a range of environmental factors
                                                                                                                                                               (noise, amount of light) and psychological factors (fatigue, motivation, stress,
     Wavelength in      400                                        500                                      600                                  700
                                                                                                                                                               expectations). For example, if a person has had someone break into their house,
                                                                                                                                                               they will be more highly attuned to sounds at night, and this may decrease their
                                                        The Visible Spectrum                                                                                   usual absolute threshold for sound.

                                                          Ultra-                                        FM Television Short          Broadcast
                     Gamma                                               Infrared
                                    X-rays                violet                           Radar                      wave             bands
                                                                           rays                                                                  AC circuits
                                                                                                                                                               the role of the eye in visual perception
                                                                                                                           Radio waves
  Wavelength in                                                                                                                                                As already mentioned, light enters the eye through the cornea, a tough
  meters                     10 – 12 10 – 10             10 – 8     10 – 6        10 – 4     10 – 2 1                        102         104     106
                                                                                                                                                               transparent tissue covering the front of the eye. It then passes through the
                                                                         Wavelength in nanometers                                                              pupil, the hole in the middle of the iris. The lens then focuses the light onto
                                                                                                                                                                                                                                                  fig 6.7>> your taste buds can detect one
                                                                                                                                                               the retina, which contains photoreceptors (light-sensitive cells).
                                                                                                                                                                                                                                                  teaspoon of sugar in 10 litres of water.
                         400                                       500                                      600                                  700              The retina is nerve tissue that covers more than 50 per cent of the inner
                                                                                                                                                               surface at the back of the eye. The retina contains two types of photoreceptors:
                                                                                                                                                               rods and cones.




                                                                                                                                                                  Rods: There are 125 000 000 in each eye.
                                                                                                                                                               > They are responsible for vision in low light (that is, they are very sensitive
                                                                               Perceived colour                                                                   to light).
                                                                                                                                                               > They are responsible for peripheral vision (out of the corner of the eye).
fig 6.5>> electromagnetic spectrum, including the visible spectrum.                                                                                               They are concentrated at the edges of the retina.

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chapter 6>>           visual perception

                                                                                                                             the principles of visual perception
                                                                                                                             selection and organisation of visual stimuli
                                                                                                                             At any given moment, a vast amount of visual stimuli enters the eye —far
                                                                                                                             more than we can pay attention to. The brain selects and organises visual
                                                                                                                             information according to a number of visual perceptual principles.

                                                                                                                             Perceptual constancies:
                                                                                                                             Perceptual constancies enable us to maintain a stable perception of a stimulus
                                                                                                                             although the image on the retina may change (e.g. in size or shape). The more
                                                                                                                             familiar we are with the observed object, the more likely it is that we will
                                                                                                                             maintain perceptual constancy of it. Perceptual constancies are usually learned
                                                                                                                             early in childhood.
                                                                                                                               size constancy
                                                                                                                               This term refers to the fact that we maintain a constant perception of an
                                                                                                                               object’s size even though the size of the image on the retina alters as the
                                                                                                                               object moves nearer to or further from us. For example, when we look at
                                                                                                                               a friend from a distance of five metres, he produces a retinal image twice
                                                                                                                               the size of the one he produces when he is ten metres away. Because we are
                                                                                                                               familiar with his height we don’t change our perception of it, but we use the
                                                                                                                               size of the image as a cue to provide information about how far away he is.
                                                                                                                               shape constancy
                                                                                                                               An object is perceived to maintain its known shape despite the changing
                                                                                                                               perspective from which it is observed. This is a learned skill—a toddler
fig 6.8>> structure of the retina                                                                                              may have difficulty perceiving a familiar toy if it is viewed from an unusual
                                                                                                                               angle. Objects that are familiar to us can be accurately interpreted when
> They have low visual acuity (they can’t register detail).                                                                    viewed from any direction. For example, as a door opens, its retinal image
> They can register only in black and white.
                                                                 6.2 review                                                    changes shape from a rectangle to a trapezium, but we easily maintain our
  Cones: There are 6 500 000 in each eye.                        1 What is the range of the visible light spectrum in          perception of a rectangular door.
> They are concentrated in the middle of the retina.               nanometers?
> They are responsible for vision of detail.                     2 Draw a visible light spectrum in colour and show where    gestalt principles of perceptual organisation
> They are responsible for colour vision (and black-and-           ultraviolet and infrared rays are situated.
  white vision in daylight).                                                                                                 Gestalt is a German word that means ‘good form’. Gestalt psychology
                                                                 3 Explain what happens in transduction.
> They require high levels of light to enable them to respond.                                                               developed early in the 20th century and is based on the principle that ‘the
                                                                 4 In a table, write down the characteristics for rods and   whole is greater than the sum of its parts’. In terms of sight, it deals with the
did you know?                  Psychologist have
                                                                   cones (number, whether they are for daytime vision
                                                                   and so on).
                                                                                                                             tendency for our visual system to perceive what we see as a meaningful whole.
                                                                                                                                How can this be? This answer is described in the Gestalt principles of visual
learned a great deal about our vision through
                                                                 5 You have soccer practice after school during winter       perception.
experimentation on animals. they discovered how                                                                                                                                                                  fig 6.9>> size constancy—the object
                                                                   and your coach wants you to play till it’s almost dark.                                                                                       appears larger to the eye as it moves
receptive fields in ganglion cells respond to an image
                                                                   Which photoreceptor is better suited to low light         The phi phenomenon                                                                  closer, but we know it is the same size, so
after it has been transduced (converted to neural
                                                                   conditions?                                               Although less common than they used to be, you can still find some local            we use the information to establish the
impulses) by inserting a tiny electrode into the                                                                                                                                                                 distance away.
                                                                 6 Grandma is busy knitting you a pair of woollen socks.     stores—often ‘Fish and Chip’ shops—with a line of light bulbs round the
brain or retina of an animal. by holding the animal’s
                                                                   She is sitting in her lounge with her spectacles and a    outside of the window. At night these bulbs light up one at a time, and as you
head still and flashing light to different sections of
                                                                   bright light. She needs to focus on what she’s doing.     watch it looks as if one light is running around the window. It’s not doing that,
the visual field, they were able to identify and map
                                                                   Which photoreceptors are best suited to activities that   of course, but the property of motion is present in the whole, rather than in
the receptive fields of ganglion cells of the retina
                                                                   require clear vision in bright light?                     any of the individual parts (the light globes).
(westen et al. 2009).

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chapter 6>>             visual perception

   This can also be seen outside Crown Casino in
Melbourne, where lights look like bubbles flowing out of
                                                                   did you know? animals such as                                           We tend to perceive rows of ‘X’ and ‘O’ in group A and
                                                                                                                                        columns of ‘X’ and ‘O’ in group B.
                                                                   leopards need to hide in trees so they can surprise
the top of a champagne bottle.
                                                                   their prey. thinking about the principle of figure-
   This effect is known as the phi phenomenon. Have                                                                                     Proximity
                                                                   ground, complete this sentence:
you seen it anywhere else? Of course you have! It is the                                                                                When the individual parts of a stimulus pattern are
principle that makes cartoons, movies and television                  ‘the leopard’s black spots on its yellow fur mean                 close to each other, we tend to group them together as a
work. One still picture after another gives us the                 that when it is in a tree its               is broken up,            meaningful ‘whole’—a single unit.
impression of movement on the screen.                              it no longer stands out as a                 against the
                                                                                                                                                      A                             B
                                                                                of the leaves and speckles of sunlight,
                                                                                                                                             X    X   X   X   X         X   X   X   X   X   X   X
Figure-ground organisation                                         and as long as it remains still it will be very hard for
                                                                   other animals to see.’                                                    X    X   X   X   X         X   X   X   X   X   X   X
The first principle of perception that develops in infants
is figure-ground organisation. The part of the visual                                                                                        X    X   X   X   X         X   X   X   X   X   X   X
field being attended to and focused on is the figure and its          closure
                                                                                                                                           You can choose how to perceive group of shapes (as
surroundings are the ground. Figure and ground are separated          Closure is when we perceive an object as being whole
                                                                                                                                        rows or columns) but tend to perceive the ones on the
by an imagined contour (line). The contour is ‘owned’ by the          despite its actually being incomplete, we are employing
                                                                                                                                        right as rows, not columns.
figure. In children’s drawings, we often see a black line around      the principle of closure. Many company logos use such
                                                                      incomplete figures in their designs.                              > Many other Gestalt principles of visual perception exist     fig 6.12>> cartoonists use the principle of closure when they
a figure emphasising the separation of figure and ground.                                                                                                                                              draw characters with little detail.
                                                                                                                                           but these examples show how they work.
     camouflage                                                       similarity
     Camouflage is where the Gestalt principle of figure-             Similarity is when the individual parts of a stimulus
                                                                      pattern are similar (e.g. in size, shape or colour), we tend to
     ground is used to ‘blend’ the contour of the figure                                                                                  6.1    investigate
     (which usually stands out) against the ground                    group them together as a meaningful ‘whole’—a single unit.
                                                                                                                                          1 Make your own animation!
     (background), making it more difficult to see. We not
                                                                                                   A                                        > You will need three sheets of A4 paper cut into 12 equal rectangles.
     only see camouflage in nature with leopards; the armed
                                                                                          X    X   X   X    X                                 This will give you a total of 36 pages for your animation.
     forces of every country around the world use it to blend
     into their surroundings to avoid detection.                                          O O O O O                                         > Place the 36 pieces together to create a small booklet and secure
                                                                                          X    X   X   X    X                                 one end with a clip or staple.

                                                                                          O O O O O                                         > Draw a stick figure of your choice on each sheet, with each figure
                                                                                                                                              slightly different. Think of each piece of paper as the different frame
                                                                                                   B                                          of a cartoon or movie.
                                                                                          X O X O X                                         > Once you have drawn a progression of pictures on the 36 pages,
                                                                                                                                              hold the secure end and flick through to watch your animation
                                                                                          X O X O X
                                                                                                                                              come to life. You have just created your own phi phenomenon!
                                                                                          X O X O X
                                                                                          X O X O X                                       2 Use of Gestalt Principle
                                                                                                                                            Look through magazines, newspapers and the Yellow Pages. How many
                                                                                                                                            company logos can you find where one or more Gestalt principle is used
                                                                                                                                            in a clever way in the design? Make a table of your findings using these
                                                                                                                                            headings: ‘Company’, ‘Logo’, ‘Gestalt principle used’.

                                                                                                                                        depth perception
                                                                                                                                        The point made above about Gestalt principles also holds for depth cues: there
                                                                                                                                        are many other depth cues besides those listed below. But these are the depth
                                                                                                                                        cues listed in the VCE Psychology Study Design, and therefore only these can
                                                                                                                                        be examined.
fig 6.10>> figure-ground—a special case. this figure may be
                                                                                                                                           Depth and distance cues are vital to us. This is because we exist in a three-
seen as either a vase against a dark background or two faces       fig 6.11>> can you see the soldier in camouflage? the figure-
in profile, depending on which is chosen as figure and which is    ground is used to blend the contour of the figure against the        dimensional world but have only two-dimensional images on our two retinas
chosen as the ground.                                              background.                                                          from which to judge depth and distance.

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  Depth cues may be binocular (using both eyes) or                    patterns viewed from about 20 centimetres. Each eye            Monocular depth cues
monocular (using one eye alone or both eyes together).                observes a slightly different view of the same scene and       Two broad monocular depth cues enable us to judge depth and distance using
                                                                      the brain fuses the two images together in the same way it     one eye: accommodation and pictorial cues.
Binocular depth cues                                                  would when observing a real (three-dimensional) scene.           accommodation
     retinal disparity                                                   If you can look ‘through’ the picture below, you will         Accommodation involves the lens of the eye (located behind the iris)
     Retinal disparity is a cue that comes about due to the           see a ‘three-dimensional’ star in the middle of it.              changing shape so that it can focus light rays onto the retina. Small muscles
     fact that our eyes are set about 6–7 centimetres apart.          convergence                                                      called ciliary muscles control whether the lens bulges (for closer objects)
     When an object is within about 7 metres of a viewer              Convergence is also a binocular depth cue. As an object          or flattens (for further away objects). At the same time, the tension in the
     (with the strongest depth cue occurring within 4 metres),        comes closer to us, the eyes turn inwards to keep the object     ciliary muscles is received by the brain to confirm the location of the object
     each eye receives a slightly different image on the retina,      centred on the retina. This cue, again, operates for objects     being viewed. The greater the tension, the closer the object.
     resulting from the different angles of view from each eye        within about seven metres. The brain reads the amount of             Try this yourself. Take a pen and close one eye. Move the pen as close to
     to the object being observed. The brain fuses these two          turning from the tension of the muscles and uses this to         you as you can whilst maintaining focus. You should be able to focus on a
     images together in stereoscopic vision. The more different       make judgments of distance. The more the turning, the            close object (between 8 to 10 cm away—depending on your eyesight). Keep
     the two images, the closer the object will be to the viewer.     closer the object is to the viewer.                              focusing on the pen until you feel the tension within your eye. That’s your
         Try this yourself:                                              Try this yourself:                                            ciliary muscles at work keeping your brain informed.
     > Hold a pen at arm’s length and look past it at the             > Hold your pen vertically at arm’s length and slowly            Pictorial depth cues
         other side of the room.                                         bring it closer to your nose, watching it with both           Pictorial depth cues are named ‘pictorial’ because they are used by artists
                                                                                                                                                                                                                           fig 6.14>> linear perspective shows
     > Close one eye and then the other and watch how far                eyes all the time.                                            to create a three-dimensional perception of something that exists on a two-
                                                                                                                                                                                                                           parallel lines converging in the distance.
         the pen ‘jumps’ from side to side.                           > As the pen gets close to your nose, you can feel your          dimensional surface.
     > Now bring the pen closer—about 40 cm from your                    eyes turning and soon you go ‘cross-eyed’.                    > Linear perspective (first described by Leonardo da Vinci) is one of the
         eyes—and repeat the process. What do you notice?                                                                                 most basic skills an artist uses to create apparent depth. Parallel lines are
         The process of retinal disparity is artificially recreated                                                                       made to converge as they extend along the page to an imaginary point
     in ‘magic eye’ pictures from two flat, two-dimensional                                                                               (where in theory they meet) at the horizon.
                                                                                                                                       > Interposition (overlap) is based on the partial blocking or obscuring of
                                                                                                                                          one object by another. The obscured object appears to be further away
                                                                                                                                          than the object obscuring (overlapping) it. This is an effective cue for
                                                                                                                                          determining which objects are closer than others, but it is not as effective
                                                                                                                                          for actually judging distance.
                                                                                                                                       > Texture gradient is used to make surfaces in a picture appear to recede
                                                                                                                                          into the distance. Artists draw less and less detail as a surface is more
                                                                                                                                          and more distant, the same way we see it in real life. This is shown in this
                                                                                                                                          boardwalk at Rhyll, Phillip Island. In the foreground we can see every
                                                                                                                                          detail of the wood and the mesh, but as the boardwalk gets further away
                                                                                                                                          it becomes much less detailed.
                                                                                                                                       > Relative size is a pictorial depth cue based on our tendency to perceive
                                                                                                                                          the object producing the largest retinal image as being the nearest, and
                                                                                                                                          the object producing the smallest retinal image as being the farthest. For
                                                                                                                                          this cue it is necessary to know the real size of the objects so that accurate
                                                                                                                                          comparisons can be made. Think about watching a game of football from
                                                                                                                                          behind your team’s goal—you realise that the players at the opposite goal
                                                                                                                                          are far away; you don’t think they are tiny!
                                                                                                                                       > Height in the visual field is a pictorial depth cue that shows depth
                                                                                                                                          by portraying objects further away as being closer to the horizon. For
                                                                                                                                          example, aeroplanes, clouds and birds will be perceived as further away as
                                                                                                                                          they become lower in the visual field (closer to the horizon). On the other      fig 6.15>> objects in the distance
                                                                                                                                          hand, trees, houses and people will be perceived as further away as they         appear smaller than those closer – the

fig 6.13>> magic eye picture—what can you see?                                                                                                                                                                             principle of relative size.
                                                                                                                                          become higher in the visual field (closer to the horizon).

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   The picture on the next page was used by researchers     > We would apply the depth cue of ‘height in the visual      the effect of psychological factors on perceptual set
to discover that members of some African tribes did not        field’ to judge that the hunter is about to spear the
use the last two cues to estimate depth and distance. The                                                                Expectations that are created
                                                               zebra, not the rhinoceros, which is in the distance.                                                     Previous
Africans were very surprised at what they thought was                                                                    by our previous experiences, the
                                                            Cross-cultural research has allowed psychologist to                                                        eXPerience
a picture of a hunter attempting to spear a very small                                                                   environment in which a stimulus
                                                            understand that even though every human being receives
rhinoceros!                                                                                                              is seen, our motivation and even
                                                            exactly the same visual information (sensation), their                                                     motivation
                                                                                                                         our emotional state can combine to                                                                             selection
> We know that a rhino is much bigger than a zebra, so      individual and cultural experiences (perception) can alter                                                                                   PercePtual
   we apply our knowledge to judge that the zebra is much                                                                predispose us to perceive the world in
                                                            their interpretation of what they see because of what they                                                                                       set
   closer to us—at about the same distance as the hunter.                                                                a certain way. Perceptual set is:              emotion
                                                            have seen in the past.
                                                                                                                         > a predisposition to perceive
                                                                                                                             stimuli in a specific way—that is,
                                                                                                                                                                        conteXt                                                       interPretation
                                                                                                                             to interpret what we see according
                                                                                                                             to certain preconceptions              fig 6.17>> factors effecting our perceptual set.
                                                                                                                         > a predisposition to attend to only certain features or aspects of our field of view (selection).
                                                                                                                             Previous experience can affect visual perception, especially if the experience holds significant
                                                                                                                         personal meaning. The same stimulus can be interpreted differently by different people.
                                                                                                                             Context also sometimes has an immediate effect on our expectations. Context refers to the
                                                                                                                         environment in which a perceived stimulus is observed.

                                                                                                                         did you know? researchers found that when they presented a picture of a
                                                                                                                         farmyard to a group of subjects and then asked them to describe it from memory, they
                                                                                                                         were able to do so quite well. however, when shown the picture below, the subjects were
                                                                                                                         surprised by the picture of a giant squid outside the barn—it is out of context, so they
                                                                                                                         concentrated on this and failed to pay attention to the other items in the picture.

fig 6.16>> which animals is to be speared?

  6.2    investigate
  You be the artist
  1 Draw a picture using the following pictorial depth      3 Why do we receive a slightly different image on each
    cues. (Alternatively, find a picture on the Internet      retina?
    and use that instead.) Label your picture with the      4 Create a poem or a song using monocular and
    pictorial cues, making sure that you explain each         binocular depth cues. Make sure that each term is
    one to illustrate your understanding.                     either defined or explained in some way and that
                                                                                                                         fig 6.18>> Quickly look at this photo. close your eyes and recall what you saw.
     >   linear perspective                                   your poem/song makes sense. Tune suggestions:
     >   overlap/interposition
                                                              >   School anthem
     >   texture gradient
                                                              >   Mary had a little lamb                                 Bugelski & Alampay (1961) performed the very famous rat-man experiment.
     >   relative size
                                                              >   Itsy bitsy spider                                      One group of participants was shown a series of line-drawings of faces and
     >   height in the visual field.
                                                              >   Come up with one of your own!                          then the ambiguous rat-man stimulus—the majority identified it as an old
  2 What is the key difference between monocular and          >   Present your creation to the class!                    man. The other group was shown a series of line-drawings of animals and then
    binocular depth cues?                                                                                                the ambiguous stimulus—the majority identified it as a rat or mouse. This             fig 6.19>> rat-man. can you see the rat
                                                                                                                         shows how the perceptual set created by prior experience influences perception.       and the man?

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review 6.3                                                                                                                   distortions of perception
                                                                 8.   What is retinal disparity and why is it important to
1     Describe how the brain uses certain principals to               depth perception?                                      Visual illusions are mistaken perceptions of visual stimuli. We may misjudge length, curvature,
      organize visual information.                                                                                           position, speed or direction in a visual illusion. It is the brain that is tricked, not the eye!
                                                                 9    Convergence is also a binocular depth cue. Explain
2     What is the Gesalt Principle?                                   how this works.                                           A visual illusion occurs when perception consistently differs from objective reality.
3     Explain the Phi Phenomena using local examples.            10   Why are monocular depth clues important to us?
                                                                                                                             the Ponzo illusion
4     What is figure-ground organization? Why is a contour       11    What techniques have been developed by artists to
      line important to this organization?                            show visual representations of three dimensions on a   In the Ponzo Illusion, for example (Fig 6.16), the upper horizontal line is perceived to be longer
5     Why do we find it so difficult to see animals                   two dimension surface?                                 than the lower horizontal line. Why? We mistakenly perceive this because of the depth cues of linear
      camouflaged in their environment?                                                                                      perspective and height in visual field—both of which make the upper line appear to be further from
                                                                 12 Do these visual clues apply to all humans? If not why
                                                                                                                             us. Because it casts the same size image on the retina, we perceive the ‘more distant’ line to be longer.
6     Explain how ‘closure’ work.                                   not?
7     Give examples of similarity and explain why the            13 What psychological factors influence our visual
      example is seen as a single unit.                             perception?

    6.3   investigate
    1 Trace the drawings below onto pieces of card or paper

                                                                                                                             fig 6.20>> the Ponzo illusion

       <insert a/w
                                                                                                                             the müller-lyer illusion
       0636—six simple
       line-drawings of                                                                                                      Another illusion, the Müller-Lyer Illusion (Fig. 6.17),
       faces and six of                                                                                                      consists of two lines of equal length, each having opposite
       animals—approx                                                                                                        shaped patterns on the ends. One line has regular
       7 cms high>                                                                                                           arrowheads; the other has inverted arrowheads (feather-
                                                                                                                             tails). The line with feather-tails is perceived as being
                                                                                                                             longer than the arrow-headed line.
                                                                                                                                There are two theories to explain the Müller-Lyer Illusion.

                                                                                                                             The carpentered world hypothesis
                                                                                                                             The carpentered world hypothesis (Fig. 6.18): British
                                                                                                                             psychologist Richard Gregory believes that the illusion
                                                                                                                             involves the ‘misapplication of size constancy’. In a three-
                                                                                                                             dimensional world, we maintain a constant perception of the
                                                                                                                             size of a familiar object by considering its distance from us.
                                                                                                                             Gregory argues that we mentally make a three-dimensional
                                                                                                                             form of each figure, using familiar features of buildings to do
    2 Show either the ‘faces’ group of cards or the ‘animals’ group, one at a time, to volunteer participants.
                                                                                                                             so. This causes the observer to perceive the arrow-headed line
    3 Show the ambiguous rat-man stimulus to each volunteer and record the response.                                         as the leading vertical edge of a building’s outer wall (closest
    4 Compare the responses of the two groups. Did the ‘faces’ group identify an old man more than the ‘animals’ group?      to the observer). The line with the feather-tails is perceived as
                                                                                                                             being further away (the inside corner of a room). Because it is
    5 Repeat the experiment with other volunteers, but this time show them a mixture of three ‘faces’ and three
                                                                                                                             perceived as being the more distant of the two, and each line
      ‘animals’ before showing the rat-man stimulus.
                                                                                                                             produces an equal length image on the retina, we interpret          fig 6.21>> the müller-lyer illusion—which
      > This is an important step in the research. Why should this be done?                                                  the line with feather-tails to be longer.                           vertical line is longer?

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chapter 6>>          visual perception

                                                                                                                                   the ames room illusion
                                                                                                                                   The Ames Room shows that we
                                                                                                                                   maintain shape constancy (of
                                                                                                                                   the room) at the expense of size
                                                                                                                                       The room is constructed in
                                                                                                                                   a trapezoidal shape, but made
                                                                                                                                   to appear square when observed
                                                                                                                                   through a peephole using only one
                                                                                                                                   eye, so that the strongest depth cues
                                                                                                                                   (binocular cues) do not work. The
                                                                                                                                   rear wall actually runs from right
                                                                                                                                   to left, away from the viewer. The
                                                                                                                                   far-left corner is twice as far from
                                                                                                                                   the peephole as the far-right corner.
fig 6.22>> which room has the higher walls?                                                                                        To add to the illusion, the ceiling        fig 6.26>> the ames
                                                                                                                                                                              room is not really
                                                                                                                                   is not parallel to the floor. It slopes
                                                                                                                                                                              rectangular, but
                                                                      A               B                     A                B     upwards from right to left, while
   Evidence to support this theory came when the illusion                                                                                                                     because of our lifetime
                                                                                                                                   the floor slopes downwards from            of experience with
was shown to country-dwelling Zulu people who live in
                                                                                                                                   right to left, so that the height of the   rectangular rooms, we
round huts with rounded doors and windows. They did
                                                                                                                                   room at far left is double that at far     perceive it to be bounded
not perceive the lines to be different lengths as they had                                                                                                                    by right-angles.
                                                                                                                                   right. When a person walks from the
never learned to judge distance from corners and angles.
                                                                                                                                   left corner to the right corner, the       in fact the ‘smaller’
Zulu people living in cities in South Africa were fooled by
                                                                                                                                   observer maintains shape constancy,        person is simply twice as
the illusion like everyone else.                                                                                                                                              far from the observer as
                                                                                                                                   but cannot also maintain size
                                                                                                                                                                              the ‘larger’ person.
                                                                                                                                   constancy—so the person appears to
the perceptual compromise theory                                                                                                                                              the shape of the room is
                                                                                                                                   be growing rapidly in size.
                                                                                                                                                                              as shown at right.
   The perceptual compromise theory by Ross Day,
an Australian psychologist, proposes a different theory.
1 Both parallel lines cast identical-sized images on           fig 6.23>> the blue lines           fig 6.24>> our mind averages
                                                               show how our mind applies the       out the black and blue lines      6.1   investigate                                                  review 6.4
   the retina.
                                                               gestatt principles of closure b     and we perceive the length as
2 The arrowhead or feather-tail lines at the ends of the                                                                             Visual illusions                                                   1 Describe the Ponzo Illusion.
                                                               appears larger.                     shown by the green lines.
   figures create ‘open’ figures that cause us to apply the                                                                          1 How do visual illusions occur?                                   2 Draw some of your own Muller-Lyer illusions and
   Gestalt principle of closure.                                                                                                                                                                          try them out on your fellow students.
                                                                                                                                     2 There are two theories to explain the Müller-Lyer
3 This creates a more ‘solid’ figure as shown by the blue                                                                                                                                               3 Do these visual clues apply to all humans? If not
                                                                                                                                       Illusion. Choose one of these theories and briefly
   lines in figure 6.23.                                                                                                                                                                                  why not?
                                                                                                                                       outline the key points.
4 Because of the perceptual compromise made, we
                                                                                                                                                                                                        4 Explain why the Ames Room illustration confuses
   perceive each figure to be the length of the average                                                                              3 How does the Ames Room work? Describe the
   between the internal (black) line and the external (blue)                                                                           construction and how this influences what is seen
   lines (see figure 6.24).                                                                                                            when looking through the peephole.                               5 Why does the person that is viewed through the peep
5 As a result of this, figure A is perceived to be much                                                                                                                                                   hole appear to grow and shrink before your eyes?
                                                                                                                                     4 Using the Internet, search ‘visual illusions’ and
   shorter than figure B, each being perceived to be as                                                                                create a PowerPoint presentation of the illusions                6 Make a model of the Ames room. You can find some
   long as the distance between the green lines shown in                                                                               you have found to share with the class.                            excellent designs on the Internet to assist in your
   see figure 6.24.                                                                                                                                                                                       construction.
                                                                                                                                        Make sure that you define ‘Visual Illusions” at the
   Evidence in support of this theory is that the illusion                                                                                                                                              question or two
                                                                                                                                        beginning of your presentation and create at least
occurs even when the ends of the lines are ‘U’—shaped or       fig 6.25>> if the arrow ends are replaced with ‘u’ shapes or
                                                                                                                                        10 slides.
completed circles.                                             circles, the theory is unchanged.

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chapter summary                                                                                                                    test your understanding
> The physical reality of what we see (sensation) is              > For a visual stimulus to be perceived by a person, it          multi-choice questions                                    5   As I was driving along the freeway a huge truck
     influenced by the psychological processes that enable            must reach absolute threshold. This is the minimum           1   Which of the following statements is incorrect            came hurtling up behind me. It seemed to take ages
     us to interpret it (perception). This occurs through six         amount of light energy needed for an observer to                 concerning the stimulus energy for vision?                for the long body to get past as it overtook me.
     processes, some of which operate at the same time.               perceive a stimulus in ideal conditions 50 per cent of           a    Electromagnetic energy of about 380                  Afterwards it gradually disappeared up the road in
>    These processes are thought to be adaptive—our                   the time.                                                             nanometers is perceived as blue or violet.           front of me. Which of the following principles would
     sensory systems (smell, touch, vision, hearing) have         •   The retina, which covers over 50 per cent of the back            b Electromagnetic energy of about 760                     I have applied to help me realise that the truck
     evolved to ensure our survival as a species and to assist        of the eye, contains two types of photoreceptors. The                 nanometers is perceived as red.                      remained the same object although its image on my
     us in reproduction.                                              125 000 000 rods in each eye are responsible for vision          c    Electromagnetic energy of about 380                  retinas changed?
>    The six stages of visual perception are divided between          in low light and for peripheral vision; they only register            nanometers is not visible to humans.                 a     shape constancy and size constancy
     sensation (the physiological part of perception) and             black and white. The 6 500 000 cones in each eye are             d Electromagnetic energy of about 800                     b size constancy and orientation constancy
     perception (the psychological influences).                       responsible for vision of detail and colour vision (and               nanometers is not visible to humans.                 c     brightness constancy and orientation constancy
>    The first three stages (sensation) are (1) reception, when       black-and-white vision in daylight).                         2   Rods are most responsive to electromagnetic energy        d brightness constancy and size constancy.
     light energy enters the eye and is focused on the retina;    >   The visual perceptual principles include perceptual              of which colour?                                      6   In the figure below, we tend to perceive a building,
     (2) transduction, when the light energy is converted by          constancies, which enable us to maintain a stable                a    violet/blue                                          rather than a series of disjointed lines.
     the retina’s rods and cones into nerve impulses; and (3)         perception of a stimulus in terms of size, shape,                b blue/green
     transmission, when the neural impulses travel via the            orientation; Gestalt principles (the phi phenomenon,             c    green/yellow
     optic nerve to the occipital lobes at the very back of the       figure-ground, closure, proximity and similarity), which         d orange/red
     brain.                                                           relate to the tendency for our visual system to perceive     3   Which of the following lists the groups of visual
>    The second three stages (perception) are (1) selection,          what we see as a meaningful whole; and depth and                 perceptual principles that we use to organise and
     when the image is pulled apart by cells called feature           distance cues, which govern our ability to view the              help interpret stimuli?
     detectors that respond to lines, size, length and                world in 3-D.                                                    a    perceptual constituencies; Gestalt principles;
     orientation; (2) organisation, when visual information       >   Binocular (two eyes) depth cues are retinal disparity                 depth and distance cues
     reaching the brain’s visual cortex is reorganised                and convergence. Monocular (one eye) depth cues                  b perceptual set; Gestalt principles; expectancies
     using various visual perceptual principles; and (3)              are accommodation and pictorial depth cues (linear               c    depth and distance cues; perceptual
     interpretation, when the brain’s temporal lobes identify         perspective, interposition/overlap, texture gradient,                 constancies; Gestalt principles
     the object by comparing incoming information with                relative size, height in the visual field).                      d visual illusions; Gestalt principles; depth and
     information already stored in memory, and the parietal       >   Previous experience and context can influence the                     distance cues.
     lobes assist in judging where the object is in space.            way we perceive a visual stimulus. This is referred to as    4   Which of the following correctly pairs the names          Which two Gestalt principles are mainly responsible
>    The energy that enables us to see (light energy) is              perceptual set.                                                  of two sets of visual perceptual principles with              for this perception?
     the visible part of the electromagnetic spectrum—            >   Visual illusions are mistaken perceptions of visual              examples from those sets?                                 a   closure and similarity
     wavelengths between approximately 360 and 760                    stimuli, including misjudgments of length, curvature,            a    Gestalt principles—linear perspective; depth         b closure and figure-ground
     nanometers (1 Nm = 1 billionth of a metre).                      position, speed or direction.                                         and distance cues—convergence                        c   figure-ground and similarity
                                                                                                                                       b perceptual set—expectancies; Gestalt                    d similarity and figure-ground.
                                                                                                                                            principles—closure                               7   The binocular depth cues are:
                                                                                                                                       c    depth and distance cues—retinal disparity;           a   linear perspective and convergence
                                                                                                                                            Gestalt principles—orientation constancy             b retinal disparity and accommodation
                                                                                                                                       d Gestalt principles—figure-ground; depth and             c   relative size and linear perspective
                                                                                                                                            distance cues—texture gradient.                      d convergence and retinal disparity.

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test your understanding
8  The strongest depth cues are:                             12 As her family was driving across to Perth from            14 The Ames Room Illusion is one in which we                short answer questions
   a     convergence and retinal disparity                      Melbourne, seven-year-old Jodi called out, ‘Look             mistakenly perceive a person to change in size as        16 When we are in broad daylight looking at an object,
   b linear perspective and convergence                         Mum, there’s hundreds of rabbits!’ As they gradually         they cross a room. The illusion is caused by the fact       we like to look straight at it and hold it in the centre
   c     pictorial cues                                         got closer, Jodi said ‘Oh! They’re actually great big        that we:                                                    of our visual field. When it is almost dark, we cannot
   d monocular cues.                                            kangaroos!’ What had caused Jodi’s initial error of          a    maintain size-constancy for the room at the            pick out colours and we see objects better when they
9  One evening I saw a crowd of tiny dots in the sky.           perception?                                                       expense of size-constancy for the person               are on the edge of our field of view. With reference
   These dots became larger and suddenly a large flock          a    previous experience with rabbits, causing a             b maintain shape-constancy for the room at the              to the retina and photoreceptors, explain why this is
   of mutton-birds began to land. I was not surprised                perceptual set that led to selection of specific             expense of shape-constancy for the person              the case. (2 marks)
   that the dots I had seen in the distance were actually            parts of the scene to be attended to                    c    maintain size-constancy for the room at the         17 The processes that are carried out by the
   big sea-birds because I had applied the principle of:        b previous experience with rabbits, causing                       expense of shape-constancy for the person              photoreceptors during visual perception include
   a     shape constancy                                             a perceptual set that led to a particular               d maintain shape-constancy for the room at the              reception, transduction and transmission.
   b orientation constancy                                           interpretation of the scene                                  expense of size-constancy for the person.              Explain what is meant by transduction. (1 mark)
   c     brightness constancy                                   c    the context in which the animals were seen,          15 The Müller-Lyer Illusion is one in which we mistakenly        Explain what is meant by transmission. (1 mark)
   d size constancy.                                                 causing a perceptual set that led to selection of       think that one parallel line is longer than the other    18   With reference to the picture shown below, identify
10 Jack and his team were doing fielding practice,                   specific parts of the scene to be attended to           because of the pattern on the end of each of the              four depth cues that are shown and describe how
   catching a ball that the coach hit high in the air.          d the context in which the animals were seen,                lines. The best description of the illusion is that:          each one enables us to perceive depth in this
   After a few people had dropped catches, the coach                 causing a perceptual set that led to a particular       a    the line with the arrowheads on the ends is              picture. (4 marks)
   insisted that all players hold their hands in front of            interpretation of the scene.                                 perceived to be shorter than the line with the      19   Explain, using an example, the Gestalt principle of
   their face and watch the approaching ball through         13 Psychologist Colin Turnbull took a pygmy from the                 feather-tails on the ends                                similarity. (2 marks)
   their fingers. Jack was surprised that his success           rainforests of South America to the plains of Kenya in       b the line with the arrowheads on the ends is            20   Explain what is meant by the term ‘perceptual set’.
   rate improved using the new technique. What is the           Africa. The pygmy looked at the herds of thousands                perceived to be longer than the line with the            Give an example that shows your understanding of
   reason for his improved performance?                         of wildebeest in the distance and perceived them to               feather-tails on the ends                                this principle. (2 marks)
   a     Using the new method, he could use                     be a large colony of ants a few metres from him. The         c    the two lines are perceived to be the same          21   The Ames Room Illusion is a visual illusion.
         convergence and retinal disparity cues to help         reason for this error in perception is:                           length but at different distances from us                a.    Define ‘visual illusion’ (1 mark)
         him judge depth and distance.                          a    previous experience with ants, causing an               d the two lines are perceived to be the average               b. i. Describe the Ames Room Illusion.
   b Using the new method, he could use                              expectancy that led to selection of specific                 length of the outermost and innermost                             (Be careful to state what is perceived.)
         accommodation and convergence cues to help                  parts of the scene to be attended to through                 measurements.                                                 ii. Why does the illusion occur? (1 + 1 = 2 marks)
         him judge depth and distance.                               perceptual set
   c     Using the new method, he could use pictorial           b previous experience with ants, causing an
         cues to help him judge depth and distance.                  expectancy that led to a particular interpretation
   d Using the new method, he could use monocular                    of the scene through perceptual set
         cues to help him judge depth and distance.             c    the context in which the animals were seen,
11 In drawing a landscape, John used the cue of height               causing a perceptual set that led to selection
   in the visual field to give the impression of depth and           of specific parts of the scene to be attended to
   distance. This meant that:                                        through perceptual set
   a     objects that were further away were drawn              d the context in which the animals were seen,
         higher up in the plane of the picture                       causing a perceptual set that led to a particular
   b objects that were further away were drawn lower                 interpretation of the scene through perceptual
         down in the plane of the picture                            set.
   c     objects that were further away were drawn
         closer to the horizon in the picture
   d objects that were nearer the horizon were drawn
         smaller than similar objects in the foreground.

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