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purchased tool or joystick. When the level of noise or
illumination changes abruptly, our eyes, ears, and
nervous system adjust to maintain the fidelity of the
signal. Similarly, we generally accept the notion that
our rearing environment influences much of our adult
behavior (see Chapter 10). To take an obvious example,
we produce the language to which we were exposed
as infants, whether it was Hindi, American Sign Lan-
guage, or Spanish. But how much is the developing
nervous system actually altered by the environment?
Are synapse formation and elimination influenced
directly by a use-dependent process?
An early approach to this problem, and one that is
still regularly employed, involved the elimination of
sensory structures. Denervation studies demonstrate
how neuron growth and survival depend on intact
connections during development (Chapter 7). The next
experimental step was to change nervous system activ-
ity to find out whether improperly used synapses
became weak or lost entirely. With this goal in mind,
Wiesel and Hubel (1963a, 1965) began to explore the
effects of monocular and binocular deprivation on the
development of visual coding properties in the CNS.
Their results clearly showed that synaptic activity
influenced the maintenance or elimination of neural
connections during development.
Before we can understand the functional changes
brought about by visual deprivation, it is necessary to
review some basic properties of the visual cortex. As
described above, each neuron in Layer IV of the visual
cortex receives projections that are largely driven by
one eye or the other (Figure 9.5). When visual stimuli
are delivered to the appropriate eye, Layer IV neurons
respond with a burst of action potentials. Cortical
neurons that respond to only one eye are referred to as
monocular. The majority of cortical neurons, particu-
larly those laying outside of Layer IV, are activated by
both eyes,and are referred to as binocular. Thus, when
an extracellular electrode passes through the visual
cortex, recording from many neurons in succession,
most cells are found to be binocular (i.e., most neurons
are recorded outside of Layer IV). Hubel and Wiesel
(1962) divided the cortex neurons into seven groups,
based on the relative ability of each eye to evoke a
response. For example, if a neuron was driven solely
by the contralateral eye, then it was assigned to group
one. If it was driven equivalently by each eye, then it
was assigned to group four, and so forth. This data can
be conveniently represented as a histogram of ocular
dominance (Figure 9.8). Judged by its continued use
during the past 35 years, ocular dominance histograms
provide a sensitive measure of the innervation pattern
in the visual cortex.
A series of experiments were performed in which
light-evoked activity was decreased by keeping the
eyelid closed, referred to as visual deprivation (Wiesel
and Hubel, 1963a, 1963b, 1965). This manipulation
does not damage the retina, and LGN neurons remain
responsive to visual stimulation after the eyelid is
reopened. At first, a single eyelid was kept closed for
a few months, and recordings were made from the
visual cortex after the eye was reopened. The effect
was unmistakable: Most cortical neurons no longer
responded to stimulation of the deprived eye (Figure
Control
V
A
B
Test
C
Rearing
Ocular Dominance Histograms
Cortex
Binocular
60
Monocular
Binocular
IV
50
40
LGN
30
20
Eyes
10
0
1
2
3 4
binocular
Ocular Dominance
5
6
7
monocular
monocular
FIGURE 9.8 Response properties of visual cortex neurons in normally reared cats. A. The visual system
received normal stimulation until the time of recording. B. Single neuron recordings were made with an extra-
cellular electrode that passed tangentially through the cortex. Neurons respond to only one eye in Layer IV
(monocular). In Layers I-III and V-VI, neurons respond to both eyes (binocular) due to convergent connections.
In normal cats, the terminal stripes from each eye-specific layer of the LGN occupy a similar amount of space.
C. Each neuron was characterized as responding to a single eye (monocular), or responding to both eyes (binoc-
ular). In normal animals, most visual cortical neurons are binocular. (Adapted from Hubel and Wiesel, 1962)
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