CELLULAR ASPECTS OF CALLOSAL CONNECTIONS AND THEIR DEVELOPMENT

Detailed visualization, three-dimensional reconstruction, and quantifi cation of individual callosal axons interconnecting the visual areas 1 7 and 18 of the cat was undertaken in order to clarify the structural basis for interhemispheric interaction. These studies have generated t he notion of macro- vs micro-organization of callosal connections. The first refers to the global distribution of callosal connections in th e hemisphere as well as to the pattern of area-to-area connections. Th e latter refers to the fine radial and tangential distributions of ind ividual 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 a xons and their branches on the dynamic properties of interhemispheric interactions were analyzed by computer simulations. These studies sugg ested that callosal axons could synchronize activity within and betwee n the hemispheres in ways relevant for the 'binding' of perceptual fea tures. These new concepts prompted a reexamination of the normal devel opment of callosal connections. The central issue is whether intrinsic developmental programs, or else cellular interactions open to environ mental information specify the morphological substrate of interhemisph eric interactions. The answer to this question is still incomplete. In development, transient, widespread arbors of callosal axons, which co uld 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 appe ar to be highly, topographically specific albeit not necessarily indep endent of visual experience.