Developmental inhibitory gate controls the relay of activity to the superficial layers of the visual cortex

A developmental reduction in the radial transmission of synaptic activity h as been proposed to underlie the end of the critical period for experience- dependent modification in layers II/III of the visual cortex. Using paired- pulse stimulation, we investigated in visual cortical slices how the propag ation of synaptic activity to the superficial layers changes during develop ment and how this process is affected by sensory experience. The results ca n be summarized as follows. (1) Layers II/III responses to repetitive stimu lation of the white matter become increasingly depressed between the third and sixth week of postnatal development, a time course that parallels the e nd of the critical period. (2) Paired-pulse depression is reduced after dar k rearing and also by blocking inhibitory synaptic transmission. (3) Paired -pulse depression and its regulation by age and sensory experience is more pronounced when stimulation is applied to the white matter than when applie d to layer IV. Together, these results are consistent with the idea that th e maturation of intracortical inhibition reduces the capability of the cort ex to relay incoming high-frequency patterns of activity to the supragranul ar layers.