The signs of aphasia

Much debate in 19th century neurology centered around the question of whether there is functional specialization within the neocortex. Today, this is no longer a contentious issue: functional specialization in the adult neocortex is well established. Nonetheless, a form of this old debate rages on in research on neocortical development. At issue is whether neocortical regionalization arises from properties intrinsic to the neocortex itself (Rakic, 1988), or whether it develops in response to extrinsic factors, such as pat-terns of thalamocortical input (O’Leary, 1989). Over the last decade, evidence has accumulated on both sides of the fence. Many studies, for example, have demonstrated a fair degree of neocortical reorganization in response to a variety of sensory-input manipulations (Katz and Shatz, 1996; Pons, Garraghty, Ommaya et al., 1991; Sadato, Pascual-Leone, Grafman et al., 1996; Schlaggar and OLeary, 1991; Sur, Garraghty and Roe, 1988), suggesting that extrinsic factors exert an influence on neocortical organization. Other studies, however, have shown that some regional specific features of neocortex (e.g. gene expression) emerge independent of extra-neocortical influence (Afimatsu, Miyamoto, Nihonmatsu et al., 1992; Miyashita-Lin, Hevner, Wassarman et al., 1999). Taken together, this collection of work seems