What's Critical for the Critical Period in Visual Cortex?

regions representing the closed eye, and expansion of Durham, North Carolina 27710 those representing the open eye (Figure 1B). While in normal primates and cats, the thalamic inputs representing the two eyes parse cortical layer 4 into alternating, equal-sized stripes, eye closure during the critical period During a brief period in postnatal life, sensory experiences indelibly shape the behavior of many vertebrate reduced the cortical territory of the closed eye to species. Salmon learn their natal rivers, birds learn their shrunken broken stripes, with its former territory now father's songs, and humans acquire language based invaded by inputs representing the other eye (Hubel et on particular sensory experiences during such " critical al., 1977). Behaviorally, animals monocularly deprived periods. " Early ethological investigations of critical periods focused on the behavioral consequences of early sensory experiences, but how and where such experiences were permanently etched into brain circuits was unknown. The notion that critical periods actually represented heightened epochs of brain plasticity, and that experience could produce permanent, large-scale changes in neuronal circuits emerged from Hubel and Wiesel's investigations in the cat and monkey visual cortex beginning in the mid-1960s (Hubel and Wiesel, 1970). Despite over three decades of subsequent experimentation, passionate disagreement remains over the cellular and molecular mechanisms initiating and eventually terminating this brief period of remarkable plasticity. For years, the battle was waged using pharmacological manipulations in the visual system of cats and primates. Recently, however, genetic manipulations in the mouse have emerged as a powerful new tool for dissecting the molecular underpinnings of critical periods. Two recent reports, using very different genetic manipulations , highlight the development of inhibitory circuitry as a potent modulator of the critical period, although in the best tradition of this contentious area, they reach rather different conclusions. In one case transgenic over-expression of a neurotrophin has made it possible, for the first time, to prematurely close the critical period (Huang et al., 1999); in another other case, critical period plasticity was disrupted in a knockout mouse and subse-and the mice engineered to obtain them offer a wealth In cats (and primates) the projections from the ipsilateral (red) and of new opportunities to delve more precisely into the contralateral (green) portions of the two retinas segregate into ana-molecular mechanisms regulating this unique time in tomically discrete layers in the visual thalamus (LGN). LGN neurons vertebrate brain development. from each layer project in turn …