Visual Perception by ROAoRY4


									Visual Perception
         Simon Bartlett

This area of study focuses on
 perception and the general
characteristics of the visual
      sensory system.
      What are the stages?
• Reception – eye senses a stimulus
• Transduction – changes it so brain can
  understand it
• Transmission – sends it to the visual cortex
• Selection – aspects selected of stimulus
• Organisation – grouping of elements to form a
• Interpretation – given meaning with the aid of
  psychological factors
• Electromagnetic energy (light) sensed from the

• RODS (black and white) and CONES (C for
  colour) are photoreceptors that pick up the
  electromagnetic energy

• This energy must be CHANGED for the brain to
  be able to interpret it…
• The electromagnetic energy is converted,
  changed or ‘transduced’ into electrochemical

• This is the energy all neurons use.

• This means that information sensed by the rods
  and cones can be sent as neural impulses to
  the visual cortex.
• Neural impulses once triggered are sent to
  visual cortex of the occipital lobe

• Like any neuron, the “all or none” law applies –
  there is no such thing as a big or small impulse.
  The stimulus must be ENOUGH for the
  threshold to be reached to make the neuron fire.

• They are sent via the optic nerve.
• This is where discrimination and identification of
  the FEATURES of the stimulus takes place.

• Feature detectors are cells that are in the
  retina, optic nerve and visual cortex, that
  respond to patterns, lines, edges and angles

• Selection is also happening at a basic level in
  reception with rods and cones responding to
  different electromagnetic energy wavelengths.
• This can only happen once the brain has
  received the neural impulses.

• Single elements are grouped to form a
  whole, using perceptual principles that
  work like ‘rules’ of organisation.

• These principles are called Gestalt
• The ‘whole’ that has been created is given

• Memory is used to compare incoming
  information with what is already known.

• This creates a perceptual hypothesis.

• Interpretation involves the brain using
  psychological factors in order to make sense
  of what it is considering.
       Absolute Threshold
• The minimum amount of energy needed
  to detect a stimulus under ideal conditions
  50% of the time.

• VISION: A candle flame 50km away on a
  clear, cloudless night with NO light
  pollution (moon, other lights, cities)
   Differential Threshold or JND
• The smallest difference in the intensity of a
  given stimulus that a specific sense can detect
  50 % of the time.

• This is about DIFFERENCE in the intensity of a
  stimulus; there must be comparison.

• If there is enough change, there will be enough
  stimulus for receptor cells to fire and send a
  message to the brain to be interpreted.

• VISION: a dimmer switch does not turn a light off
  but changes the intensity of the light.
Physical Structure – the Eye
        Let there be light!
• Receptor cells respond to 380 nm (violet)
  to 760 nm (red) of the electromagnetic
  radiation spectrum.
• This is the visible light spectrum to which
  humans are able to respond.
• To remember the pattern of light in order,
  think ROYGBIV (red, orange, yellow,
  green, blue, indigo, violet).
 Eye structure and function
• CORNEA – light enters through the cornea and
  is bent onto the lens.

• PUPIL – light passes through the SPACE called
  the pupil.

• IRIS – a band of muscle that contracts/expands
  to manage the amount of light entering the pupil
  and hitting the lens.

• LENS – the lens bends the light more and
  focuses them on the retina – in particular, on the
  FOVEA. The lens can bulge or stretch to help
  the light reach the fovea.
  Eye structure and function…
• RETINA – registers the electromagnetic energy,
  processes the incoming information and
  transduces the energy to a form the brain can

• BLIND SPOT – where the optic nerve attaches
  to the retina – no photoreceptors.

        Rods – 125 Million
        Cones – 6.5 Million
                   Rods VS Cones
               RODS                                CONES
•   Black and white vision            •   Daytime and colour vision
•   Operate well in low level light   •   Excellent visual acuity –
    – night vision                        sharp and clean images
•   Sensitive to brightness,          •   Concentrated in the fovea
    darkness and movement             •   Not useful at night – can’t
•   Mainly located in outer part          discriminate colours
    of retina                         •   Outnumbered by rods 19:1
•   Poor acuity (low sharpness
    and focus)
•   Excellent for peripheral
    vision rather than direct
•   19x the number of cones
      GESTALT Principles
• Theory that the whole is greater than the sum of
  its parts – Gestalt means ‘form’ or ‘shape’.
• We group individual elements of a visual
  stimulus into a complete form.
• This allows us to perceive objects in the most
  simple way possible.

• BOTTOM-UP = building an image from the parts
  into a whole

• TOP-DOWN = working to move from the whole
  image to the parts it is assembled from
    What are the 4 Gestalt

• Figure Ground
• Closure
• Similarity
• Proximity
    • We tend to separate the
      important aspects of the figure
      from the surroundings
      (background). We focus or
      give our attention to the
    • Real or imagined contour lines
      separate the figure and ground
    • Camouflage depends on
      difficulties with establishing
      contour lines
    • Reversible figures change the
      ownership of the contour lines
Love and Death?

Proximity is the grouping of elements that
are close to each other to form an overall
figure or pattern. Also known as

When the elements of a stimulus or pattern have
similar features (size, shape, colour) we tend to
group them together. Similarity is stronger than
proximity if the two are together in a stimulus.
What do we see?

We perceive an object as a whole, despite it
being actually incomplete. We group the
individual elements to make ‘one’ by filling in
the missing contour lines so that it makes sense.
      Depth Perception
• Depth cues which are internal and
  involve the eye and the brain and are
  known as primary cues.

• Depth cues which are external to the
  body and part of the environment or
  pictures are known as secondary cues.
           Binocular Cues
• Binocular cues are primary cues and involve
  the eye and its functioning.
• Binocular cues ALWAYS involve BOTH eyes

• Convergence

• Retinal Disparity
What do I need to know about
• We use this for objects up to 6m away
  from our eyes.
• When we view objects close to us, our
  eyes turn inwards or ‘converge’ so that
  a single image is formed on both retinas.
• Changes in muscle tension are detected
  and interpreted by the brain and used to
  determine the depth and distance of an
• Our brain detects that the closer the
  object, the greater the convergence.
 How does retinal disparity work?
• Because our eyes are 6-7cm apart, the retinas
  receive slightly different images. An object must
  be under 10 metres away for us to use retinal
• The retinal images are combined together and
  compared by the brain.
• Any disparity or difference between the 2
  images provides us with information about the
  depth of the object and its distance from us.
• You can check this by only using one eye, in
  turn, to view the same object on your desk.
    Monocular Depth Cues
  This is a primary monocular depth cue that
  involves the lens of the eye changing shape.
  Information about how much the lens bulges or
  flattens is used by the brain to determine depth
  and distance of an object.
Object close? Lens bulges to direct the image
 clearly onto retina.
Object distant? Lens flattens or elongates to
 direct the image clearly onto retina.
 Secondary Monocular Depth Cues

• Linear
  Perspective     Pictorial Cues are
• Interposition   secondary cues, as
• Texture         they occur in the
                  environment – they
                  are NOT a function
• Relative size   of the eyes!
• Height in the
  visual field
 Secondary Monocular Depth Cues

  Linear Perspective             Interposition
• Parallel lines appear   • The object that is
  to converge towards       closer obscures part
  the horizon and an        of another, more
  imaginary’ vanishing      distant object.
  point, creating a       • It is useful to indicate
  sense of distance.        which objects are
                            closer than others
                            but not actual
 Secondary Monocular Depth Cues

   Texture gradient                 Relative Size

• The amount of detail in a   • If separate objects are
  scene decreases as the        predicted to be the same
  distance increases or         size, then the one that
  moves away from the           appears larger, is
  viewer.                       closer.
• The foreground is highly    • The retinal images of the
  detailed and the              objects are different and
  background is less            we can only use relative
  detailed and less             size if we KNOW the size
  focussed.                     of an object to be able to
 Secondary Monocular Depth Cues

    Height in the             • If air objects appear
                                smaller and closer to the
     Visual field               horizon, they appear to
                                be further away.

• NEVER just call this        • HOWEVER – land based
  ‘height’!                     objects will be LOW in the
• If land objects appear        visual field if close and air
  smaller and closer to the     objects will be HIGH in
  horizon, they appear to       the visual if close.
  be further away.
Henri Cartier-Bresson
Concert crowd - Bristol, UK
    Perceptual Constancies
• These are learned visual perception
  principles that let us perceive or make
  sense of stimuli.
• This means when objects change in
  shape, size or brightness, we still see
  them as being what they are – a ‘constant’
  and familiar thing.
• Our perception stays the same even
  though the retinal images may have
  altered; we are familiar with the objects.
           Size Constancy
• We maintain an object’s perceived size
  even though the size of the retinal image
  alters due to the object’s distance from us.

• If we look at a truck outside our homes, we
  KNOW that the truck isn’t smaller if we
  stand and look at it as it drives down the
  street. We don’t think it ‘shrinks’ as it is
  driving away from us.
     Brightness Constancy
• We maintain the perceived brightness of
  an object, despite actual changes in the
  amount of light being received by the

• Because we look at objects in their
  immediate environment, we perceive that
  everything else has altered by the same
  amount of brightness too and so know that
  the brightness has remained constant.
         Shape Constancy
• We maintain an object’s perceived shape
  even though the image cast on the retina
  changes if the object is observed from a
  different angle.

• If we see a closed door, a half open door
  and a fully open door, we still understand
  that the door has a rectangular shape,
  even if it appears different when closed,
  half open and fully open.
    Orientation Constancy
• This is our tendency to maintain an
  object’s location in ‘space’ as constant –
  and perceive the world as being upright
  and vertical.

• If we hang upside down on monkey bars,
  we don’t believe that the world is no longer
  vertical and that the trees are upside down
  and sideways! Again, we compare and
  use our body’s signals to help us work out
  what’s going on.
 Psychological factors affecting Visual
       Perceptual Set or ‘Expectancy’
• We interpret stimuli by comparing it to
  what we already know and have
  experienced = prior experience.
• We tend to see what we EXPECT to see –
  this includes ignoring some stimuli and
  giving attention to others.
• Factors like emotions, motivation, context
  or setting, cultural background and past
  experience all affect our perceptual set.
Physiological factors affecting Visual
Alcohol… Slows us down, changes
coordination, reduces attention,
concentration and ability to make

Old Age…cataracts, long-sightedness,
age-related macular degeneration, retinal
detachment, diabetic retinopathy,
glaucoma. Our eyes age as we do and
become more susceptible to disease and


  How do Optical Illusions work?
• A visual stimulus misleads our perception
  (or meaning-making) of that stimulus.

• This happens because we APPLY
  perceptual constancies to what we are
  seeing – they are our RULES.

• We make a false judgement because we
  misjudge length, position, speed, direction
  or curvature.
         MULLER LYER Illusion
There are 2 theories to explain this illusion!
The Muller Lyer illusion consists of 2 lines that are
equal in length to each other.
   •One has ‘arrow-heads’ attached.
   •One has fish-tails’ attached.
Theory of Perceptual Compromise

1. The length of the solid horizontal lines of
   each figure are actually equal in length.
2. BUT the length of each WHOLE figure is
   not the same.
3. The lengths between the added tips
   (heads or tails) is very different. We use
   closure to estimate the length and
   compromise somewhere between the
   actual length and the perceived length.
Diagram of Perceptual
The Apparent Distance Theory
• We use our knowledge from depth cues and
  size constancy.
• We liken the line with ‘fish-tails’ as being the
  inside corner of a room in a building.
• We liken the arrow-heads as the closest
  outside corner of a building .
• BOTH lines cast the same size image on the
• As the fish-tails are perceived to be further away
  (inside the building), we apply size constancy
  and interpret that the line must be longer.
Diagram of Apparent Distance
          The Ames Room
• We must only use MONOCULAR depth cues in
  order to perceive the Ames room as an illusion.

• It is a trapezium - a shaped room that is much
  higher and longer on one side than the other,
  however, it LOOKS like a rectangle to the

• The back corner on the left hand side is double
  the distance away from the viewer, compared to
  the back right hand side of the room.
         The Ames Room

• The roof is also higher on the left hand
  side of the room, compared to the right
  hand side of the room.

• All of the decorations, windows and
  furnishings are also not ‘square’ – there
  are no right angles to be seen. Even
  clocks are not circles but odd ovals.
Ames Room construction
         The Ames Room
• A person standing in the back left hand
  corner of the room is going to appear very
• A person standing in the back right hand
  corner of the room looks like a giant.
• If a person crosses the room, they appear
  to shrink or grow in size.
• This creates a conflict for the viewer!!! The
  retinal images of the room’s corners
  (etc) are the same shape and size.
The Ames Room

 If the viewer maintains shape constancy,
 they continue to perceive the room as
 means they see the person shrink and
 grow. Most people do this.

 It is difficult to maintain size constancy in
 Ames room as it means that the brain
 must remain totally confused! The retinal
 information (the same sized retinal image
 of the room’s corners) cannot be corrected
 due to the lack of accurate depth
 information – even if we know what we are
 ‘seeing’ is not possible.
Perception Deception
Is this picture
REALLY moving?

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