Cross-Modal Plasticity in Sensory Cortex

The evolution of the mammalian brain has involved marked degrees of encephalization, and this trend is particularly spectacular in the neocortex (see Jenson, Finlay, this volume). An important question in understanding neocortical evolution is how this expansion may be exploited by structures which form afferent connections with the expanded cortical populations. For example, what happens when additional cortical processing circuitry becomes available to sensory inputs as a result of mutation or duplication? Does the newly acquired circuitry replicate the existing mode(s) of information processing, or does it process sensory input in a new way? The latter change would be more likely to increase the animal’s behavioral repertoire and hence reproductive “fitness”. Certainly in the visual system, the number of separable visual cortical areas increases from hedgehogs to rats to cats and monkeys (Kaas et al., 1970; see Kaas, 1987 for review), and there is a large body of evidence that the different areas perform different transformations on their sensory input. Whether this segregation of function derives from differences inherent in cortical circuitry, or from parcellation of subtypes of afferent input (or both) is unknown from either an evolutionary or developmental perspective.