HUL 211 Location in object perception and memory - Perception of distance

Document Sample
HUL 211 Location in object perception and memory - Perception of distance Powered By Docstoc
					 Location in perception and memory
Perception of depth/ distance/ space

            Snehlata Jaswal

                 Perception of Space
Human conception of space may be described in terms of three
dimensions or planes: height (vertical plane), width (horizontal
plane), and depth (sagittal plane).

These planes all intersect at right angles, and their single axis of
intersection is defined as being located within perceived three-
dimensional space – that is, in the "eye" of the perceiving individual.

The horizontal, vertical, and sagittal planes divide space into
various sectors: something is perceived as "above" or "below" (the
horizontal plane), as "in front of" or "behind" (the vertical plane), or
as "to the right" or "to the left" (of the sagittal plane).
Space perception refers to perceptions in all these three planes.
Though it traditionally implied only distance or depth perception, it
also includes the perception of our body in space, particularly the
knowledge of it being vertical.

                         HUL 211 OBJECT PERCEPTION AND MEMORY
              Orientation in space
We human beings have a keen perception of vertical relative
to gravity.

Experiments in which subjects were placed on a tilting seat
indicate that we can detect a tilt angle if less than 1 degree
from the true (gravitational) vertical (Benson, 1982).

Much of this sensitivity depends on information from the visual
and skin senses. But kinesthesis (the sense of muscle
contraction) and the vestibular sense (which provides
information regarding the position and movement of the head
in space) also play a role. In a state of total darkness the
orientation of an individual in space is mainly dependent on
sensory data deriving from vestibular stimuli. In fact, the major
function of the kinesthetic and the vestibular senses is to help
us to stay upright.
                     HUL 211 OBJECT PERCEPTION AND MEMORY
            Orientation in space
On casual consideration, it appears that perception of space
is based exclusively on vision.

However, this visual space is supplemented perceptually
clearly identifiable cues from auditory (sense of hearing),
kinesthetic (sense of bodily movement), olfactory (sense of
smell), and gustatory (sense of taste) experience. In addition
to these, a number of other spatial cues, such as vestibular
stimuli (sense of balance) and other modes for sensing body
orientation, must be taken into account.

These various (e.g., visual, olfactory) "spaces" are not found
to be perceptually independent of one another; indeed they
interact to produce unified perceptual experiences.

      Distance and proximal senses
Vision is a distance sense; it is capable of gathering
information from extremely distant points in the environment,
reaching out to the stars themselves. Hearing is also
considered a distance sense, as is smell, though the space
they encompass is considerably more restricted than that of

All the other senses, such as touch and taste, are proximal
senses – i.e., conveying information about the space in direct
contact with the perceiving individual's body. However, strictly
speaking, the latter senses may also function over distances.
The smell of rotten eggs (produced by hydrogen sulfide gas)
several yards away may be associated with a taste
                     HUL 211 OBJECT PERCEPTION AND MEMORY
            Perception of distance
The term depth perception implies two rather different kinds of

   The distance from an observer to an object is called the
   absolute distance.

   Relative distance, the distance between one object and
   another or between different parts of a single object.

                    HUL 211 OBJECT PERCEPTION AND MEMORY
  The problem of depth perception
The problem in depth perception was first enunciated by the
Anglican bishop Berkeley at the beginning of the 18th century.

He queried how the third dimension (depth) could be directly
perceived in a visual way when the retinal image of any object
is two-dimensional.

Berkeley concluded that the ability to have visual experiences
of depth is not inborn (nativistic) but can only result from
logical deduction based on empirical learning through the use
of other senses.

                    HUL 211 OBJECT PERCEPTION AND MEMORY
              Depth cues
Non visual


Monocular Monocular
 Static    Dynamic

                        Non visual cues
Oculomotor cues (Kinesthetic cues from the eye muscles):

   When one looks at an object at a distance, the effort arouses activity in two
   eye-muscle systems called the ciliary muscles and the rectus muscles.

   The ciliary effect is called accommodation (focusing the lens for near or far
   vision), and the rectus effect is called convergence (moving the entire

   Each of these muscle systems contracts as a perceived object approaches.
   The effect of accommodation in this case is to make the jellylike lens more
   convex, while the rectus muscles rotate the eyes to converge on the object as
   it comes nearer.

   One's experience of these muscle contractions provides one with cues to the
   distance of objects.

   Accommodation and convergence provide reliable cues when the perceived
   object is at a distance of less than about 30 feet (nine metres) and when it is
   perceived binocularly (with both eyes at once).

                            HUL 211 OBJECT PERCEPTION AND MEMORY
                        Non visual cues
Gross tactual-kinesthetic cues:

   The body of the individual seems to function as a perceptual frame of
   reference--that is, as a standard against which the distances of objects are

   In his perception of the distances of objects located in nearby space, one
   depends on the tactile (tactual or touch) sense. In tactile experience, however,
   kinesthetic experience (sensations of muscle movements and of movements
   of the sense-organ surfaces) normally is so closely associated that
   investigators lump the two as tactual-kinesthetic cues.

   One's perception of the body may vary from time to time, and so its role as a
   perceptual standard is not always consistent.

   It has been found that the way in which the environment is perceived also
   affects one's perception of the body. Thus, for example, one experiences
   one’s arm as growing longer when one is using it to point at some object off in
   the environment.

                            HUL 211 OBJECT PERCEPTION AND MEMORY
                        Non visual cues
The vestibulo-ocular reflex

   It refers to the automatic combination of the eye movements and head
   movements to keep the visual image stable on the retina. This reflex is
   controlled by neural connections between the semi-circular canals in the ear
   and areas of the brain that control eye movements.

Auditory cues

   Auditory cues for depth perception include sound intensity (loudness), auditory
   pitch, and the time lapse between visual perception and auditory perception
   (for example, one hears a distant cannon after seeing the flash and smoke of
   the explosion).

   Changes in pitch function as depth cues because when a moving object (e.g.,
   an automobile) is emitting sound waves (e.g., from its horn), the pitch of the
   sound seems to rise when the object is approaching the perceiver; to fall when
   it is moving away.

                              HUL 211 OBJECT PERCEPTION AND MEMORY
                         Non visual cues
Information from other senses:
    The proximal senses, such as taste and touch, convey information about space
    in direct contact with the perceiving individual's body. They may also occasionally
    function over distances. The smell of rotten eggs several yards away may be
    ‘tasted’ when the gas dissolves in the perceiver's saliva. Similarly the pressure of
    the air in the skin is a cue in space perception that helps to maintain the
    orientation of the body in space. The sense of smell operates at intermediate
    distances. A faint smell obviously suggests greater distance than a strong one.

   Olfactory cues are extremely important in navigation particularly by lower
   animals. Earthbound species of animals often use olfactory (smell) signals in
   recognizing paths of varying distance; this is encountered both among social
   insects (e.g., ants) and among many mammals (the dog sniffing from tree to
   tree). Many animal species act to defend a specifically delimited territory against
   interlopers. Territorial behaviour depends on a rather precise perception of space
   on the basis of olfactory cues. The social distance maintained by primates (e.g.,
   human beings and apes) is also theorized to be the result of this tendency to
   mark the territory on the basis of olfactory cues. We dislike it when someone gets
   too close to us and violates our personal space. The information regarding such
   proximity is largely based on olfactory cues.

                             HUL 211 OBJECT PERCEPTION AND MEMORY
  Visual cues: Static monocular cues
Static monocular cues are
based on features of
stationary objects and require
only one eye to produce the
perception of depth.

Since they can be
represented in flat two-
dimensional pictures, they
are also called pictorial cues.

Most of them depend on past
experience and learning.

                      HUL 211 OBJECT PERCEPTION AND MEMORY
Visual cues: Static monocular cues
• Interposition: When one object overlaps another, it is
  perceived to be closer of the two.

• Relative size: The larger of two similar objects, presented
  simultaneously or in close succession, appears to be

• Familiar or assumed size: This cue is different from that of
  relative size, because by definition, it depends on
  familiarity or past experience. If we know the real (distal)
  size of an object, and are used to the retinal image it
  subtends on the eye (proximal size), we have a good
  potential indication of its distance.

• Relative height in the field of view: Higher objects appear
  to be more distant.
Visual cues: Static monocular cues
• Linear perspective: Parallel lines appear to converge as
  their distance from the eye increases.

• Texture gradient: If we look at a textured surface, the density
  of the texture elements increases with increasing distance.

• Aerial perspective: We look at objects through particles
  suspended in air. The more distant the object, the more
  particles we look through. As a result faraway objects look
  fuzzier than nearby objects.

• Colour perspective: Objects change colour as they increase
  in distance from the observer. The faraway mountains
  appear to be blue green, whereas the nearer ones appear to
  be bright green.
Visual cues: Static monocular cues
• Object brightness: A brighter object appears to be
  nearer. In experimental studies it is found that the
  brighter an object appears, the closer it seems to be.
  Thus, a white card against a dark background seems to
  recede or to move forward as the level of illumination on
  the card is experimentally varied. Similar effects can be
  induced by changing the colour (hue) of an object--e.g.,
  from bright red to dark red.

• Shadowing: We tend to think of light as always falling
  from above. The interplay of lights and shadows can
  change the appearance of a round or angular surface.

 Visual cues: Dynamic monocular cues
As objects move in the world, they provide dynamic monocular
  cues for depth: motion parallax and motion perspective

Movement parallax:

  Parallax, in general, refers to the change in the visual field
  resulting from a change in the observer’s position. In
  movement parallax, distance cues are obtained from retinal
  changes that depend on the movement of our own head and
  the interposition of objects in space. Thus, when the
  individual moves his head either from side to side, or forward
  and backward, the retinal image of an object also moves. If
  we fixate at a point somewhere in the middle distance, closer
  objects move in the opposite direction, while the farther
  objects move in the same direction. This relative movement
  called motion parallax is a cue for depth.
                      HUL 211 OBJECT PERCEPTION AND MEMORY
Visual cues: Dynamic monocular cues

Motion perspective:

  Not only do objects move with the observer, but they also
  move at different velocities. Observed frequently while
  traveling, this cue was emphasized by Gibson (1950). As we
  move through a scene, objects close to us flow by more
  rapidly than distant objects. The image of a nearby tree
  moves more rapidly, while that of a distant tree moves less
  rapidly. This pattern of streaming of the retinal image is a
  strong cue for depth.

                      HUL 211 OBJECT PERCEPTION AND MEMORY
                    Binocular cues
Humans do not ordinarily perceive a binocular space (a
separate visual world from each eye), rather they see a
Cyclopean space, as if the images from each eye fuse to
produce a visual field akin to that of Cyclops, a one-eyed giant
in Greek mythology.

Though the image of an object in space falls on two different
points in the retina, binocular fusion that takes place in the
brain automatically ensures that we see a single object located
in space.

Nevertheless both eyes do send different images to the brain,
and there are some cues to depth that are dependent on the
use of the two eyes together. These binocular cues give very
precise depth information.
                      HUL 211 OBJECT PERCEPTION AND MEMORY
                   Binocular cues
Double images:

The significance of double images in depth perception was
noted very early by Hering (1861). If a near and a far object
are both in front of you and you fixate at the near object,
you get double images of the far object, as it is seen by the
right eye as lying to the right of the fixated object, and by
the left eye as lying to the left of the fixated object.
Conversely, if you fixate on the far object, thus getting
crossed double images of the nearer object, the right eye
sees it as lying to the left of the fixated object, whereas the
left eye sees it lying to the right. Crossed disparity can be
decreased by converging the eyes, and this may be used a
cue for distance.
                     HUL 211 OBJECT PERCEPTION AND MEMORY
                   Binocular cues
Binocular disparity:

Perhaps the most important perceptual cue of distance and
depth is binocular disparity or stereopsis. Since the eyes
are embedded at different points in the skull, they receive
slightly different (disparate) images of any given object. The
two retinal images of the same object are combined
perceptually in the brain into one three-dimensional

                       HUL 211 OBJECT PERCEPTION AND MEMORY
Binocular cues

                    Binocular cues
Binocular disparity (contd.):

 The optical axes show the alignment of the eyes and the
projected point on the retinas. There is another point in
green which is also projected. Those points have the same
distance L and R to the optical axes on both retinas, they
are corresponding points. Each point on the horopter, an
imaginary line in space, has these characteristics. Points
which are in front or behind the horopter have different
distances, L' and R', to the optical axes on the retina. The
difference between R and L is the so called disparity,
which is positive or negative dependent on the position in
front or behind the horopter. Points on the horopter have a
disparity of 0.

                      HUL 211 OBJECT PERCEPTION AND MEMORY
                     Binocular cues
The degree of disparity between the two retinal images, also called
binocular parallax, depends on the difference between the angles at
which an object is fixed by the right eye and by the left eye. Thus, in
reading the indicator needle on a pressure gauge, one makes slightly
different readings because of parallax if one uses first the left eye
alone and then the right eye.

The greater the parallax difference between the two retinal images,
the closer the object is perceived to be.

Binocular disparity functions primarily in near space because with
objects at considerable distances from the viewer the angular
difference between the two retinal images diminishes.

Nevertheless, using optical devices that magnify the parallax distance
separately for each eye can involve visual disparity in estimating
greater distances. Such devices include artillery range-finding devices
and old-fashioned stereoscopes.
                 Interaction of cues
By learning about systematic relationships that exist among a number
of simultaneously available cues, people can perceive distances more
or less correctly. However, none of the cues can be regarded as
crucial, because depth can still be perceived if any one of them is
eliminated. Nevertheless, the more cues that are available, the more
accurately people can detect depth and distance.

Experiments have also shown that the distance (in depth) between
selected objects in photographs is most accurately estimated when
the objects have been filmed in a richly organized environment--e.g.,
many people standing at different distances from the camera.

On the other hand, it is most difficult to reliably perceive even the
relative depth of two vertical rods when they are presented against a
background in which other cues have been reduced or eliminated.
                       HUL 211 OBJECT PERCEPTION AND MEMORY
      Nativist vs. empiricist debate
According to empiricists, spatial perception develops primarily
as the result of learning; indeed, during the early stages of
development, the individual gradually learns something about
the significance of observable (empirical) spatial cues.

The nativist view is that space perception is based on the
innate (hereditary) structure of sense receptors and the
nervous system; for example, the ability to detect differences
in the size of two-dimensional figures seems to be inborn (that
is, the differences seem to be immediately recognizable
without training or learning).

                    HUL 211 OBJECT PERCEPTION AND MEMORY
    Development of space perception
Visual cliff experiments
Visual cliff designed by Gibson and Walk (1960)

   Even 3 day old infants show an increase in heart rate when placed
   on the deep side
                        HUL 211 OBJECT PERCEPTION AND MEMORY
  Development of space perception
Special spectacles – reversal experiments
Reverse the right-left or up-down dimensions of images as
normally received at the surface of the retina. E.g: Shimojo
and Nakajima (1981) made the subjects wear left right
reversing spectacles for nine days. At first the person
becomes disoriented, but when he wears such a device all the
time, he gradually learns to cope with space correctly while
the special glasses are on. Indeed, he reorients himself to his
environment to the point that he visually begins to perceive
objects as if they were in the correct positions again. The
process changes direction when the optical distorting device
is removed; at first the basic visual dimensions appear
reversed to the subject who has adapted to the spectacles.
Within a short time, however, a new adaptation occurs; the
subject quickly reorients himself to the earlier, well-learned,
                           perceives his environment as normal.
normal visual cues and211 OBJECT PERCEPTION AND MEMORY
    Development of space perception
The role of touch in development

The development of human ability to perceive space normally depends
on the interaction of the sense modalities of sight and touch.

Toward the end of its first year, a child eagerly begins to touch and to
explore objects with his hands. Compared with his visual apparatus,
which begins to function more efficiently at a later time, the child's
sense of touch at this stage of development is delicately and effectively
sensitive. By touching objects that are placed at various heights and
distances from him, the child learns excellent tactual ways of
perceptually evaluating what must be a highly ambiguous world as
transmitted by his relatively immature visual apparatus.

The part played by other modalities (e.g., hearing) does not appear to
be as fundamental in perceptual learning among young children.

                         HUL 211 OBJECT PERCEPTION AND MEMORY
         Thank you


Shared By:
Description: Object Perception and Memory Lecture Series