Healthcare and Medicine Reference
In-Depth Information
9.9), although they continued to respond to the unma-
nipulated eye.
Reasoning that disuse must have weakened the
synapses from the deprived eye, Wiesel and Hubel
recorded from binocularly deprived kittens, expecting
to see a total absence of visually evoked activity. It came
as a great surprise, then, that most cortical neurons
remained responsive to stimuli through both eyes. That
is, the ocular dominance histogram obtained from
binocularly deprived animals resembled that of normal
animals (Figure 9.10). “It was as if the expected ill
effects from closing one eye had been averted by
closing the other” (Wiesel and Hubel, 1965). However,
many neurons displayed abnormal responses, and a
large fraction of neurons were completely unrespon-
sive to light, as originally predicted (Sherman and
Spear, 1982).
The total amount of evoked activity does not nec-
essarily predict whether a synapse will be strong or
weak. Rather, differences in the amount of synaptic
activity seem to determine the strength of a connec-
tion. This idea came to be known as the competition
hypothesis . Under this proposal, retinal synapses in the
LGN should not be affected by deprivation because
LGN neurons are monocular and receive afferents
that are either uniformly active or uniformly deprived
of light. Similarly, binocular deprivation evens the
playing field in the cortex because all afferents should
have a similar low level of activity. Monocular depri-
vation creates a situation in which cortical neurons
receive a set of active afferents from the open eye and
a group of afferents with lowered activity from the
closed eye, placing the latter at a disadvantage.
In a pivotal test of the competition hypothesis,
kittens were raised with an artificial strabismus (cf.
misalignment of the eyes), produced by surgically
manipulating one of the extraocular muscles (Hubel
and Wiesel, 1965). This manipulation mimics a clinical
condition in humans, called amblyopia , which com-
monly results in the suppression of vision through one
of the eyes, presumably to avoid double vision. In stra-
bismic kittens, visual stimuli activate different posi-
tions on the two retinas, and cortical neurons are rarely
activated by both eyes at the same time. Following
several months of strabismus, recordings were once
again made from the visual cortex. This time, both eyes
effectively activated neurons in the cortex, but most
cortical neurons respond to stimulation of one eye or
the other (Figure 9.11). Few binocular neurons were
observed. Therefore, an equivalent amount of activity
in the two pathways is not sufficient to explain the
results. Instead, it seems that the timing of synaptic
activity must somehow be involved in allowing inputs
to remain active on cortical neurons. Synapses from
each eye must be active at nearly the same instant if
both are to keep strong, functional contacts with the
same postsynaptic neuron. Small disparities in the
timing of synaptic activity may determine the strength
of a synapse, an idea that has since been tested in tissue
culture (below).
The ocular dominance columns (stripes) formed by
geniculate terminals have served as an important
anatomical model of synapse elimination. In normal
animals, when 3 H-proline is injected into one eye, there
is a periodic variation in silver grain density in Layer
IV of the cortex. Labeled and nonlabeled regions are
Monocular Deprivation
Ocular Dominance Histograms
3 4
Ocular Dominance
FIGURE 9.9 Response properties of visual cortex neurons in monocularly deprived cats. A. The visual
system received normal stimulation through one eye, and the other eye was kept closed until time of record-
ing. B. The terminal stripes from the deprived eye became much narrower, and the visual response of neurons
outside of Layer IV was more responsive to the open eye. C. In monocularly deprived cats, the vast major-
ity of cortical neurons responded to the open eye only. (Adapted from Wiesel and Hubel, 1963a)
Search Pocayo ::

Custom Search