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detects smaller objects with age (discussed above), it
is likely that it can also resolve smaller differences
between the two eyes. Detecting differences between
the two eyes is fundamental to depth perception. We
also know that ocular dominance columns in the kitten
visual cortex continue to mature after eye opening
(Chapter 9), during the period when binocular vision
is improving. During the same period, individual
neurons respond more selectively to visual stimuli.
For example, individual cortex neurons respond to a
smaller range of bar orientations during development
in cats and ferrets (Bonds, 1979; Chapman and Stryker,
1993). Thus, some interesting candidate mechanisms
have been identified, but we have yet to design the
experiments that test their relationship to perception.
Color vision is another fascinating perceptual in
that helps us categorize objects. Interestingly, human
infants seem to use color in a different way than do
adults. When 2- to 4-month-old infants are stimulated
with a special stimulus that requires subjects to use
color information to detect motion, they are found to
perform better than adults when both groups were
compared to a reference stimulus in which only lumi-
nance information was needed to perceive motion
(Dobkins and Anderson, 2002). These results suggest
that the visual pathways that carry information about
an object's color have a relatively strong input to
motion processing areas of the visual cortex at first,
and this is reduced during postnatal maturation.
Even though acuity improves dramatically in
human infants during the first two years, they can fail to
make proper use of visual information. When 18- to 30-
month-old children were provided with a large object,
followed by exposure to a miniature replica, they often
failed to understand the concept of size. For example,
children were first permitted to use a child-sized chair
on which they could sit comfortably. They were then
escorted from the room, and when they returned a
miniature chair replica had replaced the original object.
In many instances, children would attempt to sit on
the miniature chair; such “scale errors” reached a
maximum at about 2 years of age (DeLoache et al.,
2004). The children could easily discriminate between
objects of different size and would choose to sit in the
large chair when given a choice. The results suggest that
visual information about object identity is not being
integrated with information about its size.
FIGURE 10.14 Investigating the process of depth perception in
human infants. The testing device, called a visual cliff , consists of a
sheet of plexiglas that covers a high-contrast checkerboard pattern.
On one side of the device, the cloth is placed immediately beneath
the plexiglas, and on the other side it is placed 4 feet below. The
majority of infants would not crawl onto the seemingly unsupported
surface, even when their mothers beckoned them from the other
side. These results suggest that infants perceive depth by 6 months
of age. (Gibson and Walk, 1960)
the eyes must be rotated away from one another (diver-
gence) to look at a distant object. Cruel as it sounds, one
of the simplest ways to determine whether depth per-
ception is present in young animals is to ask whether
they are willing to crawl off a “cliff.” To do this safely,
an infant is placed on an elevated glass surface that is
patterned on one half and clear on the other. If an infant
is willing to crawl out over the clear surface, off the
“perceptual cliff,” then one assumes poor depth per-
ception (Figure 10.14). By the time that they crawl, most
infants do avoid the “cliff,” indicating that depth per-
ception is present (Walk and Gibson, 1961). To deter-
mine whether infants can perceive depth before they
crawl, 1- to 4-month-old subjects were equipped with a
heart rate monitor and suspended either above the
shallow side or the deep side. Interestingly, the heart
rate was lower when the infants were suspended above
the deep side, suggesting that they were interested but
not fearful. An accelerated heart rate was measured
after the infants began to crawl (Campos et al., 1970).
More precise measurements of depth perception
obtained in nonhuman species show a rather sudden
improvement. For example, binocular perception in
cats goes from being rather poor to almost adult-like
between 4 and 6 weeks postnatal (Timney, 1981).
Why does binocular vision improve with age? Do
neurons in the cortex suddenly become selective for
binocular stimulation? In fact, several neural mecha-
nisms may contribute to the maturation of binocular
vision (Daw, 1995). First, since the visual system
ACUTE HEARING
As is true of the visual system, the auditory system
displays improved sensitivity and discrimination as an
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