Cortical specification: microcircuits, perceptual identity, and an overall perspective.

The microcircuitry within a cortical area and its perceptual identity are both specified relatively late in development. We have asked whether and how the pattern of input activity during development influences these features of cortex. Routing visual projections to the auditory pathway in ferrets leads to visual activation of the developing auditory cortex, causing auditory cortex to receive patterns of input activity very different from normal. Visual inputs respecify, in instructive fashion, the microcircuitry within primary auditory cortex and alter the perceptual identity of the area so that its activation is identified with visual stimuli. Several other features of primary auditory cortex remain unaltered, however, by the change in input modality. In general, the pattern of input activity is one of several environmental cues that influence the developing cortex and interact with its intrinsic developmental program. It is clear from our (as well as other) experiments that appropriate environmental signals must be present at the appropriate time in development in order to influence cortical specification.