Although critical periods for language and other distinctively human behaviors are in some ways the most compelling examples, it is difficult if not impossible to study the underlying changes in brain circuits. A much deeper understanding of the changes in circuitry that accompany critical periods has come from studies of the developing visual system. In an extraordinarily influential series of experiments, David Hubel and Torsten Wiesel found that depriving an experimental animal of normal visual experience during a restricted period of early postnatal life irreversibly alters neuronal connections (and functions) in the visual cortex. These observations provided the first evidence that the brain translates the effects of early experience (that is, patterns of neural activity) into permanently altered wiring.

To understand these experiments and their implications, it is important to review the organization and development of the mammalian visual system. Information from the two eyes is first integrated in the primary visual (striate) cortex, where most afferents from the lateral geniculate nucleus of the thalamus terminate (see Chapter 12). In some mammals—carnivores, anthropoid primates, and humans—the afferent terminals form an alternating series of eye-specific domains in cortical layer IV called ocular dominance columns (Figure 24.3). As already noted in Chapter 12, ocular dominance columns can be visualized by injecting tracers, such as radioactive proline, into one eye; the tracer is then transported along the visual pathway to specifically label the geniculocortical terminals (i.e., synaptic terminals in the visual cortex) corresponding to that eye (Box C). In the adult macaque monkey, the domains representing the two eyes are stripes of about equal width (0.5 mm) that occupy roughly equal areas of layer IV of the primary visual cortex. Electrical recordings confirm that the cells within layer IV of macaques respond strongly or exclusively to stimulation of either the left or the right eye, while neurons in layers above and below layer IV integrate inputs from the left and right eyes and respond to visual stimuli presented to either eye. Ocular dominance is thus apparent in two related phenomena: the degree to which individual cortical neurons are driven by stimulation of one eye or the other, and domains (stripes) in cortical layer IV in which the majority of neurons are driven exclusively by one eye or the other. The clarity of these patterns of connectivity and the precision by which experience via the two eyes can be manipulated led to a series of experiments that greatly clarified the neurobiological processes underlying critical periods.