Cortical heterogeneity: implications for visual processing and polysensory integration.

Recent studies have revealed substantial variation in pyramidal cell structure in different cortical areas. Moreover, cell morphology has been shown to vary in a systematic fashion such that cells in visual association areas are larger and more spinous than those in the primary visual area. Various aspects of these structural differences appear to be important in influencing neuronal function. At the cellular level, differences in the branching patterns in the dendritic arbour may allow for varying degrees of non-linear compartmentalisation. Differences in total dendritic length and spine number may determine the number of inputs integrated by individual cells. Variations in spine density and geometry may affect cooperativity of inputs and shunting inhibition, and the tangential dimension of the dendritic arbours may determine sampling strategies within cortex. At the systems level, regional variation in pyramidal cell structure may determine the degree of recurrent excitation through reentrant circuits influencing the discharge properties of individual neurones and the functional signature of the circuits they compose. The ability of pyramidal neurones in visual areas of the parietal and temporal lobes to integrate large numbers of excitatory inputs may also facilitate cortical binding. Here I summarise what I consider to be among the most salient, and testable, aspects of an inter-relationship between morphological and functional heterogeneity in visual cortex.