A note on the termination of commissural fibers in the neocortex.

The general arrangement of fibers in the corpus callosum shows a strongly symmetrical (homotopic) pattern characteristic of a true commissure. Connections between non-symmetrical (heterotopic) areas also have been demonstrated for many regions, both by anatomical studies using the Marchi technique13,14,16,24,26,29 and by electrophysiological methods6,18. The contribution of recently developed silver techniques, notably the suppressive Nauta-Gygax method23, has been to emphasize the extent of non-homotopic connections3,8,9,19,21. Some clearly delineated areas of neocortex appear not to receive any commissural fibers1,2,6,8,9,17,18,21,28; outstanding examples of such areas are ‘visual cortex’ or area 17, and the hand and foot portions of somatosensory cortex. It is generally presumed that each cortical area contributes fibers to the corpus callosum, and the sparse histological evidence available indicates that the cells of origin reside in cortical layers III through VI5,12,25.

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