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Behavioral scientists tend to concentrate on two cri-
teria: absolute sensitivity and discrimination. Absolute
sensitivity is a measure of the minimum stimulus
amplitude that can be detected: the softest touch, the
finest line, the quietest sound. Discrimination is a
measure of our ability to perceive a difference between
two similar stimuli: sky-blue versus turquoise, middle
C versus C sharp, margarine versus butter. Below, we
explore how developing animals first comprehend
their sensory world.
Eye (theory)
High acuity
Human infants show clear evidence of being able to
see at birth. For example, they stare for longer periods
of time at a familiar face, such as their mother.
However, their visual skills appear to be very poor as
compared to those of an adult. Visual acuity, or the
ability to detect fine detail, is almost entirely absent at
birth (Figure 10.13). One measure of visual acuity is the
number of black and white lines that can be observed
per degree of visual space. (The “rule of thumb” states
that, at arms distance, your thumb occupies about one
degree of visual space.) Adults can see about 30 black
and white lines per degree, but babies can only see
about one. In the more common language of an eye
doctor, the baby sees at 20 feet what a normal adult can
see at 600 feet, and adult sensitivity is reached between
3 and 5 years of age (Birch et al., 1983). In principle,
this level of acuity would permit an infant to distin-
guish the fingers of a hand, but their actual abilities
remain somewhat of a mystery. Our best ideas come
from “preferential looking” studies which tell us what
babies prefer to look at, given a choice (e.g., faces,
curved lines, complex patterns), but not what they see.
The modest visual acuity of primates at birth is
partly due to an immature retina. Photoreceptors are
relatively short and wide at birth, meaning that less
light is absorbed and a greater piece of visual real
estate is viewed (Yuodelis and Hendrickson, 1986).
Thalamic and cortical neurons may also impose limits
on visual acuity. If one compares the theoretical acuity
of the retina (based on the density of cone photore-
ceptors) with the acuity of single cortex neurons, then
the cortex neuron is found to be worse than expected
in developing primates (Jacobs and Blakemore, 1988;
Kiorpes and Movshon, 2004). Furthermore, the
animal's behavioral acuity is worse than that of indi-
vidual cortical neuron at first (Figure 10.13). Such
results suggest that the development of accurate con-
nections (Chapters 5 and 6) may only create a minimal
operating system, and optimal performance is
V1 cortex
(mean & max)
(acuity range)
Low acuity
Age (weeks)
FIGURE 10.13 Development of acuity along the visual pathway
in primates. A. These two images show how a visual scene appear
to an adult (left) and a neonatal primate (right). The “infant view”
is spatially lowpass filtered (blurred) and reduced in contrast to
reflect the theoretical limitations of the retina. B. Acuity is measured
by determining the number of visible bars per degree (left). The the-
oretical acuity of the eye is linked to the spacing of cones within the
fovea (blue circles and shading). The behavioral acuity of individ-
ual animals is quite poor at birth and continues to mature until about
50 weeks postnatal (red circles and shading). This data suggests that
acuity development is not limited by properties of the retina. Single
neurons in the primary visual cortex respond to a broad range of
spatial frequencies. (Green circle is the mean value recorded, and
green bar extends to maximum value recorded at that age.) Behav-
ioral acuity appears to track cortical neuron development until about
8 weeks, but there is a disparity in performance after this age. There-
fore, it is possible that maturation of higher visual cortical areas is
necessary for adult-like performance to emerge. (From Kiorpes and
Movshon, 2004)
acquired through detailed changes in synaptic archi-
tecture or function (Chapter 9).
Binocular vision involves the coordinated use of
both eyes to judge the distance of an object, and this
requires both sensory and motor development. To look
at an object close up, the eyes must be rotated toward
one another (convergence), so that the visual image
activates the correct portion of each retina. Similarly,
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