OVERALL PATTERN OF CALLOSAL CONNECTIONS IN VISUAL-CORTEX OF NORMAL AND ENUCLEATED CATS

The effect of neonatal bilateral enucleation on the overall distributi on of callosal connections in striate and extrastriate visual cortex o f the cat was studied using tangential sections from the physically un folded and flattened cortex. Callosal neurons were labeled by administ ering the anatomical tracer horseradish peroxidase directly to the tra nsected corpus callosum. The pattern of callosal connections in binocu larly enucleated cats showed both consistent differences and consisten t similarities with the pattern in normal cats. In agreement with prev ious studies, it was found that callosal labeling at the 17/18 border of enucleated cats was considerably sparser than in normal cats. Moreo ver, we found that the strip containing the majority of labeled cells at the 17/18 border was narrower than in normal cats. In both normal a nd enucleated cats, scattered cells were distributed on either side of the 17/18 callosal strip, well into areas 17 and 18. In much of extra striate cortex, the pattern of callosal connectivity in enucleated cat s looked surprisingly normal. Details of the callosal pattern that wer e consistently found in normal cats could also be recognized in binocu larly enucleated cats, such as two to four bridges of labeling spannin g areas 18 and 19. Also, four zones that were free of callosal connect ivity in area 7, on the banks of the suprasylvian sulcus, and in the p osterior suprasylvian sulcus were found in both normal and enucleated cats. Finally, as in normal cats, dense cell labeling occurred on the crown of the suprasylvian gyrus at its posterior end, from which it ex tended laterally across both banks of the suprasylvian sulcus and into the fundus of this sulcus. The results of this study suggest that, al though the stabilization of callosal connections at the 17/18 border r egion appears to depend on visual input, this input plays a less promi nent role in the stabilization of callosal connections in extrastriate visual cortex. (C) 1995 Wiley-Liss, Inc.