Vision by Sn2DG0


									               Sensation and Perception II: Vision

        What is your most important sense? Many answer "vision." Our brain does
devote a large proportion of the cerebral cortex (about 1/3) to processing visual
information, and one would not expect natural selection to have allocated so much
brain power to a sense that is not very important. On the other hand, blind people do
learn to get along OK. One could argue that hearing is more important, since hearing
alerts you to dangers from all around you, not just in front of you, as is the case with
vision. I would argue that some of our special senses, such as pain and the internal
senses that monitor things like blood pressure and the oxygenation of the blood are the
most important senses -- without them, we soon die. So, one may be able to argue that
vision is not the most important sense, but it is the one about which we seem to know
the most, judging from the number of pages devoted to it in our textbook.
      Many animals have photoreceptors, sensory receptors that detect light, and
many of them do not much resemble the human eye. For example, earthworms have
photoreceptors all over their bodies. We shall focus on the photoreceptors found in the
human eye, and the eyes of similar animals.

Anatomy of the Eye

        As shown in Gray's Figure 8.1 (above), light enters the eye through the
transparent cornea, which helps to focus the light on the retina, which is found at the
back of the eye, and which is where the receptor cells are found. After passing through
the cornea, light shines through the pupil of the iris (named after the Greek god of the
rainbow). Muscles in the iris control the size of the pupil, generally enlarging it in dim
light and constricting it in bright light. After passing through the pupil, light passes
through the flexible lens, whose focusing power can be altered by being bent by
muscles. This allows us to focus on objects at different distances from us.
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        Each photoreceptor in the retina has a photochemical which when impacted
with light of an appropriate wavelength will be chemically altered and cause an electrical
potential in the receptor cell. The receptor potentials produced by the photochemicals
are gathered, processed, and sent to the brain via the optic nerve. The point where
the optic nerve exits the retina is called the blind spot, because there are no
photoreceptors on that spot. Light that falls in that spot is invisible to you.
Common Problems with Visual Acuity
       Some people have an eyeball that is too long, resulting in the image from distant
objects being focused in front of (rather than on) the retina, leading to the problem
called myopia or nearsightedness. The last time I asked (many years ago), my myopia
was 20:400, which means that I see as well from 20 feet as does a person with normal
20:20 vision from 400 feet away.
       Persons with overly short eyeballs are farsighted -- the image from close objects
is focused behind the retina, so it is blurred on the retina. With age the lens often
hardens, loosing some of its ability to change shape and focus. When this happens
one starts having trouble focusing on close objects, even if one is nearsighted, and
reading glasses or bifocals are in order. This condition is called presbyopia.
        Astigmatism is the most common condition affecting visual acuity. In the
perfect eye, the cornea is equally curved in all directions. In astigmatism, the cornea is
curved more in one direction than in the other (less often, the problem may result from
asymmetry of the lens) As a result, light rays do not all come to a single focal point on
the retina. Instead, some focus on the retina while others focus in front of or behind it.
Photoreceptors and Other Cells in the Retina
          Most humans have two types of photoreceptors, rods and cones. Cones provide
us with the ability to see fine detail and, usually, colors, but they work well only in bright
light. The cones are most dense in the fovea, which is the area of the retina onto which
light is focused when we look right at an object. Try to read something with peripheral
vision. Rods are found throughout the retina, except in the fovea, and are most dense
about 20° away from the fovea. Rods work best in dim light, but provide little acuity
(ability to see detail) and no color vision. If you can get away from the terrible light
pollution around here at night, try looking at a very dim star at night. If you look right at
it, it will disappear, because the light from that star is being focused on your fovea,
which has only cones and is not very sensitive to dim light. It you look just off to the
side of the star, it will reappear, as the light from it falls on your rods. Rod vision is so
sensitive that you could detect the light from a single candle 30 miles away if we had
clean, dry air and no objects blocking the view.
       The rods and cones do feed their information directly to the brain. A fair amount
of processing of information takes place in the retina. Rods and cones connect to
bipolar cells, and the bipolar cells connect to ganglion cells. The axons of the
ganglion cells form the optic nerve, carrying already partially processed information to
the brain. Within the retina, horizontal and amacrine cells form lateral connections
among adjacent bipolar and ganglion cells.
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Rod Vision
       For rods, the photochemical is rhodopsin, which is derived from beta carotene.
If you have problems with night vision, eating more carrots, a rich source of beta
carotene, might help.
       Rods do not respond to long wavelength (red) light. Sailors usually wear red
goggles before they go on night-time watch duty so that their rods will not be
"bleached" by normal light. Without the red goggles, normal levels of white light would
break down the rhodopsin, greatly reducing the ability of the rods to detect dim light It
takes about 45 minutes of darkness for the rods to fully replenish the visual purple for
maximum night vision. Were the sailor's rods not already adapted when he started his
watch he might miss something important! You don't look right into oncoming
headlights at night, do you?
        Nocturnal animals often have only rods, no cones, so they can't see red light.
One way that zoos and animal researchers trick these animals (so that the humans can
see the animals during the animals' active period without having to watch them at night)
is to use a reversed light cycle, with white lights on at night (making the nocturnal
animals sleep) and red lights on during the day. Since the animals can't see the red
light they think it is night and they are active. Since we can see the red light we can see
the animals well. I have observed many nocturnal animals this way, including vampire
bats while they were feeding.
        Dogs may be considered nocturnal, and it was once thought that they have no
color vision, but we now know that they do have limited color vision. Their foveas have
only 10% cones, not 100% like our foveas, and they have only two types of cones
(like people with red-green color blindness). If you have a special interest in dog vision,
I recommend the site at Those
interested in the visual abilities of cats can check out
        Neural convergence accounts, in part, for the reduced acuity but increased
sensitivity of rod vision, as compared to cone vision. The receptive field of a ganglion
cell is that portion of the retina which, when stimulated by light, will cause altered
electrical activity in the ganglion cell. Ganglion cells that receive information from
cones in the fovea may have one-on-one connections -- that is, each ganglion cell
receives information from only one cone (or just a few cones), and this results in such
ganglion cells having very small receptive fields. These small receptive fields allow
for great visual acuity, the ability to discriminate between light sources that are very
close together. Ganglion cells that receive information primarily from rods typically
receive connections from many rods, the information from the many rods being
“funneled” down to a single ganglion cell. Such convergence greatly increases
sensitivity to dim light, but at a cost of having larger receptive fields, and thus
reduced visual acuity.
Color Vision (by Roy G. Biv)
       Electromagnetic energy consists of particles (photons) which pulse like waves as
they travel. The distance between the peak of one pulse and the peak of the next pulse
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is the wavelength of the energy. Our eyes can detect only a very small portion of the
entire spectrum of electromagnetic energies. We can detect wavelengths between
about 400 (violet) and 700 (red) nanometers. A nanometer is one billionth of a meter.
Gray‟s Figure 8.8 (below) nicely illustrates the visual spectrum within the much broader
spectrum of electromagnetic radiation

       Color Mixing. A person with normal color vision can create any color she can
see by mixing red, green, and blue lights, as nicely illustrated in Gray‟s Figure 8.10
(page 283, not available here). Do note that we are mixing lights, not pigments. Mix
red and green lights and you get yellow, but mix red and green pigments and you get
muddy brown. Red pigment looks red because it absorbs all but the red light, reflecting
only red to your eye. Green pigment absorbs red light (and reflects green). A mixture
of red and green pigment absorbs most everything, and thus looks dark. If you look
very closely at the screen of your color television (especially if it is an older model) you
may be able to see the tiny red, green, and blue dots with which it makes all colors. For
example, if you look at a yellow area on the screen you will see that it has the red and
green dots on. When I create a custom color in a Word document, I open the drop-
down menu on the color button, select “more colors” and then “custom colors” and then
select the amounts of red, green, and blue that I want to mix to create a custom color.
Try that and see what colors you can create by mixing red, green, and blue, or just point
your browser to the following page where you can experiment with various mixtures of
red, green, and blue light:
       Three different types of cones are found in the human retina. One is
maximally responsive to short wavelength light (violet-blue), one to medium
wavelength light (yellowish green) and one to longer wavelength light (towards
the red end of the visual spectrum). Gary‟s Figure 8.12 (below) illustrates this well.
Long wavelength (red) light stimulates the "red" cones moderately and the other cones
not at all. Your brain recognizes this pattern of stimulation as red light. Pure yellow
(573 nanometer) light stimulates the "red" cone a lot, the "green" cone moderately, and
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the "blue" cone not at all. Your brain senses this pattern as yellow. If both red and
green pure wavelengths hit your retina at the same time and place they would
produce this same pattern of stimulation of the "red" and "green" cones, and you would
see yellow even though no 573 nanometer light entered your eye. Some colors (the
nonspectral colors) can only be created by mixing two different wavelengths of light.
Purple, for example, can only be created by mixing long wavelength (red) with short
wavelength (blue) light. There is no pure wavelength of light that produces the
sensation of purpleness. The meaning of “purple” has, by the way, changed across the
years. In earlier times “purple” referred to a crimson color, similar to that of blood and
red raspberry juice. You can more information on the history of “purple” in the
document The Color Purpur.

        After leaving the cones, color information is coded into red-green, blue-yellow,
black-white opponent processes. For example, some ganglion cells are excited by
input from the red cones and inhibited by input from the green cones. Others are
excited by input from the green cones and inhibited by input from the red cones. This
produces a red-green opponent process. Other ganglion cells are excited by input from
the blue cones, and inhibited by input from the green and red cones (or vice versa),
producing a blue-yellow opponent process. Afterimages nicely illustrate these
opponent processes.
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       Color-blindness may be caused by a missing visual pigment (dysfunctional
cones) or a defective opponent process. A trichromat is one with normal color vision
(she can produce any color she can see by mixing three basic colors). A dichromat
can produce any color he can see by mixing only two basic colors. Such a person's
Umwelt (sensory world) is missing the colors for which a normal person needs a third
basic color to produce. A monochromat has no color vision.
        The most common type of color-blindness is red-green color-blindness, in which
reds and greens are confused. The gene that is defective or missing is, in persons with
normal color vision, found on the X chromosome, so a woman would have to have two
defective red-green genes (one on each X chromosome) to be red-green color-blind.
Only one normal red-green gene (on either X) is needed to have normal red-green
vision. Men have only one shot at getting a normal red-green gene (since they have
only one X chromosome), so red-green color-blindness is most often found in men. One
very interesting case is a woman who had normal color vision in one eye but red-green
color-blindness in the other eye. She was likely a chimera (an organism with different
genetic compositions in different parts of the body). Because she had normal color
vision in one eye, she could close that normal eye and tell us what it is like to have
red-green color-blindness. She described the red-green color-blind world as shades of
blues, yellows, and grays, with absolutely no sense of red or green.
       The web page at gives simulated examples of what
things look like to persons with different types of color-blindness. You can even load a
picture file of your own and view it through the simulated eyes of a color-blind person.
Object Identification
       One primary purpose of our visual system is to help us identify objects.
Accordingly, our visual system has evolved ways to organize visual information in
ways that help us identify objects. For example, our visual system exaggerates
differences in brightness (and color) at the boundary between one intensity of
brightness and an adjacent intensity of brightness. This helps us see where one object
ends and another begins.
        It is also important for use to be able to detect the orientation of objects relative
to one another and to their background. Hubel and Wiesel found columns of nerve
cells in the visual cortex that were specialized for the perception of objects of specific
orientations. For example, one column of cells fired maximally when the object being
viewed was oriented vertically, like the leftmost bar in the right margin, while an
adjacent column of cells would respond maximally to an object slanted a few degrees
clockwise, like the rightmost bar in the margin, et cetera.
       Hubel and Wiesel also found cells specialized to
detect the spatial frequency of repetitive stimuli, which
contributes to our ability to perceive the visual texture
of objects. For example, consider the patterns I drew to
the right -- the pattern on the left has a lower spatial frequency than does the pattern on
the right.
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       Surface Interpolation. Apparently our brains conserve neural resources by
paying little attention to areas where the surface is uniform. When stimulation from an
area within a uniform surface is blocked (for example, because the light is falling on the
blind spot), our brains fills in the missing area with a surface identical to that around it.
You should perform the little experiment described in the caption of Figure 8.4 in your
textbook (page 278) to demonstrate surface interpolation.
The Primary Visual Cortex and Beyond
        The primary visual cortex is at the very rear of the occipital lobe. Within the
primary visual cortex different areas’ activity is associated with what type of task
the person is doing -- some areas are more active when attending to color, others when
attending to shape, an so on.
        See What? The lower portions of the occipital and temporal lobes are
involved with the identification of forms and objects. Persons with damage to these
parts of the brain (from stroke, for example), may suffer from a visual agnosia, an
inability to interpret visual information regarding forms or objects. Persons with visual
form agnosia can see the basic elements of a form but cannot recognize the shape
of the form. Those with visual object agnosia can identify the shapes of objects and
even draw what they have seen, but they cannot identify the object by name. This
deficit is not strictly one of language, however, since they can identify by name objects
which they have touched. Persons afflicted with prosopagnosia loose the ability to
identify persons by face. Apparently we have a brain area in the lower temporal lobe
that is specifically adapted to identify human faces. Gray mentions the case of a
shepherd who, after a stroke, lost the ability to identify humans by their faces but still
could identify individual sheep by sight.
         Where Is It? Upper parts of the occipital and parietal lobes are involved with
the location of objects in space. Damage here can cause a person to loose the
ability to judge distances, follow moving objects, move around obstacles, or learn and
remember the way from one place to another. Apparently the neurons in this part of the
brain are able to take into account the shapes of objects when guiding our path
through space where there are obstacles we need to avoid and when guiding our hand
movements, as when catching an object thrown to us -- but they do this without
providing conscious awareness of the shapes of the objects. While persons with
visual form agnosia are not consciously aware of the shapes of objects, their brains
do take into account the shapes of objects when dealing with them. This is one piece
of evidence that our brains can use information of which we are not consciously
aware to guide our actions.
Pattern and Object Recognition
       Recognizing an object, such as your hand, may require two different sorts of
perceptual processes. One is a “bottom-up” or “data driven” process, that is, one
where the information is flowing from the object to the perceptual system. One
example of such a process is the detection of basic visual features present in the
pattern of light bouncing off of the object and into your eyes. “Top-down” or
“concept-driven” processes are also needed. These processes bring information
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down from “higher” cognitive centers so that the perceptual mechanism can try to
organize the basic features to match those associated with your memory of what a
hand looks like. If there is a match, then you see a hand.
        When you look at a scene, your nervous system uses parallel processing to
very quickly detect primitive features such as the colors and slants of lines -- all of the
primitive features are detected simultaneously. Next your brain uses serial processing
to integrate the primitive features, which leads to your perception of organized wholes.
This takes more time, since you process information from only one area of the scene at
a time. You can demonstrate this to yourself by running the “Feature Integration”
Activity which can be found with the other Chapter 8 materials on the CD that came
with your textbook (Focus on Research).
       Geons. Irving Biederman proposed that there are 36 basic three-
dimensional shapes from which we construct our perception of objects. A few of
these are illustrated in Gray‟s Figure 8.31 (below). Each of these basic shapes is
recognized by its elementary features, and the basic shapes can be combined in
various ways to produce our perception of whole

        Distinctive Features. How is it that we know to which features to attend to be
able to discriminate one object from another similar object. For example, how is it that
you know how to discriminate a man from a woman? Usually we have little difficulty
doing this (especially with naked people), even though these two classes of objects are
very similar in shape. Eleanor Gibson argued that we learn such distinctive
features. Gray noted that we often are not consciously aware of the features that we
use to make such discriminations. His example was that of discriminating male from
female newborn chicks. Experienced chicken-sexers can do this easily, but typically
cannot tell you how it is that they do it, other than that it involves looking at the chicks‟
cloacae, even though the difference is supposed to be easy to describe if it can be
brought to a conscious level. [If you just have to know how to sex a baby chick, Dr. Gray
explains the procedure in the document at:
       Context. We also use the context of an object to help us recognize it. Even
somewhat familiar shapes may not be immediately recognized when we encounter
them out of their usual context -- for example, I may recognize you quickly when I see
you in our classroom or outside in the hallway, but not so quickly recognize you if I see
you at the Wal-Mart.
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Perceptual Grouping

        Gestalt psychologists argued that our perceptual system has been constructed to
detect meaningful patterns, perceptual wholes (Gestalten) in our stimulus world. The
principles of perceptual grouping emphasized by the Gestalt psychologists (and
illustrated in Gray‟s Figure 8.26, above) include the following:
      Proximity -- we group together things that are close to each other.
      Similarity -- we group together things are similar to one another.
      Closure -- we close up gaps in borders to make complete shapes.
      Good Continuation -- in (d) above, we see line segment „a‟ continuing on to „b‟
       rather than turning off to „c‟ or „d‟.
      Common Movement -- elements that move together are grouped together.
      Good Form -- we tend to see simple, regular forms rather than complex,
       irregular forms. In (f) above, we see the leftmost object as a single cross, but are
       likely to see the middle object as two rectangles, one atop the other.
      Figure and Background -- we tend to organize stimuli into a
       background and a figure, with an inclination to make the
       surrounding part the background and the surrounded part the
       foreground. Some figures may produce some ambiguity with
       respect to foreground and background, such as that in Gray‟s
       Figure 8.28, at the right.

Depth Perception
      The images that are cast on the retina of your eye are two-dimensional. How
then do you perceive the third dimension, depth.
       Binocular cues. These cues require that you use both eyes. The primary
binocular cue is retinal disparity or binocular parallax, the difference in the view on
your left versus your right eye caused by your eyes being apart from one another. The
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farther apart an animal‟s eyes, the powerful this cue should be. Next time you look at a
rabbit, notice how far apart its eyes are. Try this little demonstration:
 Close one eye and hold up one finger about a foot or so in front of your nose.
 Line the finger up with a more distant object and then close the open eye while
    opening the closed eye.
 The finger should seem to jump due to the retinal disparity being greater for the
    close finger than for the distant object. Your brain assumes that objects with greater
    retinal disparity are closer than are objects with less retinal disparity.
 If you will now open both eyes, keeping the finger a foot or so in front of your nose,
    and focusing on a distant object rather than on your finger, you will see two fingers
    (as viewed by each of your two eyes).
 Now, focus on the finger and bring it slowly to your nose, keeping it in focus. Your
    eyes turn inwards to maintain the focus. Feedback stimulation from the muscles that
    turn your eyes is used by the brain to judge depth, but this cue, binocular
    convergence, is much less important than is retinal disparity. Greater convergence
    is necessary to focus on close objects than on not so close objects.
  Monocular Cues. These cues require only one eye. Try this demonstration of
motion parallax:
   Close one eye and hold up one finger about a foot or two in front of your nose.
   Line up the finger with a distant object.
   Now move your head back and forth, left, right.
   Note that the finger seems to move more rapidly than does the distant object.
     The brain interprets greater parallax (apparent change in position) during your
     head movement as due to greater closeness.
   Several monocular cues to depth can be incorporated into two-dimensional pictures.
These include:
    Occlusion -- when one object blocks the view of the other object, we infer that
      the blocked object is more distant.
    Relative Size of Retinal Image -- with objects for which we have experience
      regarding how large they usually are, we assume that an object whose image is
      smaller is more distant, ceteris paribus.
    Linear Perspective -- you stand on the railroad tracks looking at the horizon
      towards which they run. You know they are parallel, but the image they cast on
      your retina is one where the lines converge (get closer to one another) as they
      approach the horizon. Your brain interprets increasing convergence of parallel
      lines as indicating greater depth.
    Texture Gradient -- you can see very fine texture in objects that are close to
      you, but the amount of texture you can see decreases as objects become more
      distant from you. For example, standing in a field of wheat, you can see the
      individual blades and even irregularities in them for those that are up close, but
      farther away the fine texture fades, and even farther you can no longer see
      individual blades. Your brain uses decreased ability to see fine texture to infer
      greater depth.
    Closeness to the Horizon -- objects that appear to be close to the horizon are
      inferred to be more distant than objects far from the horizon.
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      Shading -- When light comes primarily from one direction, as from sunshine,
       shadows cast by objects yield information about depth. Figure 8.39 on page 312
       of your textbook can be used to illustrate this. Give it a try.
Perceptual Constancies
        Our perceptual system makes adjustments so that our perception of certain
attributes remains constant under circumstances where the stimulus changes but the
object of perception does not. We discussed brightness constancy when covering
psychophysics earlier.
        Size Constancy. Within limits, your brain takes into account apparent depth
when judging the size of objects that differ in how far they are for you. For example, if
you take two people of about the same size and ask one to stand 20 feet away and the
other only 5 feet away, the retinal image of the more distant person will be smaller than
that of the close person, but you see the two as about the same size.
       Shape Constancy. Take a quarter and hold it about a foot in front of your eyes
with the edge facing up. The image cast on your retina is circular and you see the coin
as being circular. Now rotate it slowly, moving the upper edge downwards and the
lower edge upwards. The image cast on your retina is no longer circular, but you do not
perceive a change in the shape of the quarter.
        Color Constancy. We tend to see colors as constant even when the
wavelength composition of the light illuminating them changes. For example, you are
foraging for strawberries. In the noonday sun, you see the ripe ones as red, the not
quite ripe ones as pink , and the newly set ones as green. You come back to the
strawberry patch at dusk. The composition of the sunlight is now rosy-red rather than
noonday white, and this shifts the wavelength composition of the light reflected by the
berries (towards the red end of the visual spectrum), but you still see the ripe ones as
red, the not quite ripe ones as pink, and the just set ones as green. How is this
       Edwin Land was responsible for developing Polaroid cameras, motivated to do
so when, during the Christmas holidays of 1943, his three-year-old daughter
complained about not being able immediately to see the photographs Edwin had just
taken. After developing black and white instant photograph, Land moved on to color
instant photography. Some of the research he conducted in the process of developing
instant color photography suggests how our perceptual apparatus achieves color
        Land‟s experimental materials included Mondrian-like painting. Piet Mondrian
(1872 - 1944) was a Dutch painter whose paintings typically consisted of straight lines
and rectangular patches. Apparently Mondrian was obsessed with order and tidiness.
It has been reported that something as minor as a lack of perfect symmetry in a table
setting would cause Mondrian great distress. For Land, the attraction to Mondrian‟s
work was, in part, that some of it contained many patches of different colors. At the
right is an example of the Mondrian-like stimulus that Land used in his research.
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       Land started by measuring the wavelengths of light
reflected by differently colored patches, for example, orange and
green, when illuminated by white light. Then he illuminated the
Mondrian with three projectors, each with different color filters.
By adjusting the intensities of the projectors, he changed the
wavelength composition of the light reflected from a green patch so that it exactly
matched the wavelength composition of the light had been reflected from the orange
patch under white light. This green patch should now look orange, right -- but it
doesn‟t, it still looks green. Why?
      The explanation is that the color of an object is not absolutely determined by
the composition of the wavelengths of light reflected by it. The brain compares
the wavelengths of light reflected off on an object to that reflected off of other
objects and infers the color.
         Back at the berry patch, in the rosy-red illumination present at dusk, there is
still more long-wavelength (red) light reflected off of the ripe berries than off of the not
quite ripe berries, and more long wavelength light reflected off of the not yet ripe berries
than off of the just set berries, so those berries still look red, pink, and green to us, just
like in the white noonday sun.
Unconscious-Inference Versus Direct-Perception
         Gray discusses two different global interpretations of the perceptual phenomena
we have been discussing. One perspective is the unconscious-inference perspective.
By this perspective, the brain somehow very quickly does a number of
computations to infer attributes of the distal object from information provided in
the proximal stimulus. For example, the proximal visual stimulus is two-dimensional
as it is cast on the retina, but we somehow infer depth from information in that image
(and, top-down, from knowledge we have acquired interpreting such stimuli). Hermann
von Helmholtz (recall that Wilhelm Wundt was one of Helmholtz‟s assistants) is given
credit for fathering this perspective.
         A different perspective is the direct-perception perspective of James J.
Gibson. Gibson argued that the information needed to perceive attributes such as
depth in the distal stimulus is already present in the proximal stimulus, and that
the perceptual system does not need to download stored data and computational rules
to infer attributes such as depth. I am not convinced that one of these perspectives is
superior to the other. It seems to me that both require the nervous system to process a
lot of information quickly and automatically to be able to infer attributes such as depth.
Links of possible interest.
      Anterior Ischemic Optic Neuropathy -- a visual problem I am dealing with since
       Jan. 2004.
      Rotating Snakes of Akiyoshi Kitaoka -- I don‟t know much about visual illusions,
       but find some of them entertaining.
Revised April, 2008.
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