Cellular aspects of callosal connections and their development

Detailed visualization, three-dimensional reconstruction, and quantification of individual callosal axons interconnecting the visual areas 17 and 18 of the cat was undertaken in order to clarify the structural basis for interhemispheric interaction. These studies have generated the notion of macro- vs micro-organization of callosal connections. The first refers to the global distribution of callosal connections in the hemisphere as well as to the pattern of area-to-area connections. The latter refers to the fine radial and tangential distributions of individual callosal axons. A discrete disjunctive, 'columnar' pattern of termination of callosal axons, previously unknown for the visual areas, was found. The consequence of caliber and distribution of callosal axons and their branches on the dynamic properties of interhemispheric interactions were analyzed by computer simulations. These studies suggested that callosal axons could synchronize activity within and between the hemispheres in ways relevant for the 'binding' of perceptual features. These new concepts prompted a reexamination of the normal development of callosal connections. The central issue is whether intrinsic developmental programs, or else cellular interactions open to environmental information specify the morphological substrate of interhemispheric interactions. The answer to this question is still incomplete. In development, transient, widespread arbors of callosal axons, which could provide the basis for plastic changes of callosal connections were found in the white matter and the deep cortical layers. On the other hand, growth into the cortex and synaptogenesis of callosal axons appear to be highly, topographically specific albeit not necessarily independent of visual experience.