Intrinsic and extrinsic factors that shape neocortical specification

Increasing evidence points to the importance of intrinsic molecular cues in specifying the regional identity of mammalian neocortex. Few such cues, ho wever, have been found to be restricted to individual functionally defined cortical areas before the arrival of afferent information. In contrast, tha lamocortical axons are specifically targeted to individual cortical areas, raising the possibility that they can instruct some aspects of cortical are al identity. Cortical structure and function can be altered by modifying th e source or pattern of activity in thalamocortical afferents. In particular , studies of crossmodal plasticity have shown that in many respects, one se nsory cortical area can substitute far another after a switch of input moda lity during development. Afferent inputs might therefore direct the formati on of their own processing circuitry, a possibility that has important impl ications for brain development, plasticity and evolution.