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The orientation selectivity of cells in ferret primary visual cortex was studied during normal development and in animals deprived of vision or of visual cortical activity. In normal animals from the age when visual responses were first recorded (postnatal day 23) through postnatal week 5, only about 25% of cells showed orientation-selective responses. By postnatal week 7, cortical responses had matured to an adult-like state, with approximately 75% of cells clearly selective for orientation. This development of orientation selectivity was not merely a reflection of the development of cortical cell responsiveness: at all ages studied, there was no correlation between responsiveness and selectivity. Infusion of TTX into visual cortex to silence neuronal activity completely blocked the maturation of orientation selectivity. Visual deprivation by bilateral lid suture impaired but did not completely block the normal development of orientation selectivity. We conclude that the maturation of orientation-selective responses in ferret primary visual cortex requires cortical neuronal activity, and that normal development requires visually driven activity.

Recordings from single units in kitten primary visual cortex show that a reversible blockade of the discharge activities of cortical neurons and geniculocortical afferent terminals by intracortical infusion of the sodium channel blocker tetrodotoxin (TTX) completely prevented the ocular dominance shift that would normally be seen after monocular deprivation. The blockade of cortical plasticity, like the blockade of discharge activity, was reversible, and plasticity was restored following recovery from the effects of TTX. These results extend previous work suggesting involvement of electrical activity at the level of the cortex in the phenomenon of cortical plasticity by demonstrating an absolute requirement for discharge activities in the primary visual cortex.

We have investigated the role of the N-methyl-D-aspartate (NMDA) receptor, a subtype of glutamate receptor, in the responses of cells in adult cat visual cortex. After intracortical infusion of the NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (DL-APV) for one day, iontophoretic responses to NMDA, to kainate, and to quisqualate revealed a receptor blockade specific to NMDA receptors and extending several millimeters from the cannula. In this region, neuronal responses to visual stimulation were profoundly suppressed, in a manner strongly correlated with the degree of NMDA receptor blockade. Neither NMDA receptor blockade nor activity suppression was caused by the inactive stereoisomer L-APV. Hence, we conclude that NMDA receptors make a major contribution to normal excitatory transmission in adult visual cortex.