Functional specificity of callosal connections in tree shrew striate cortex

Although callosal connections have been shown to link extensive regions of primary visual cortex, the distribution of these connections with respect t o the map of visual space and the map of orientation preference remains unc lear. Here we combine optical imaging of intrinsic signals with injection o f fluorescent microspheres to assess the functional specificity of callosal connections in the tree shrew. By imaging both hemispheres simultaneously while presenting a series of spatially restricted stimuli, we find that a s ubstantial region of visual space is represented bilaterally. Each hemisphe re includes a representation of the ipsilateral visual field that is highly compressed relative to that of the contralateral visual field and is most extensive in the lower visual field, where similar to 30 degrees of central visual space are represented bilaterally. Callosal connections extend thro ughout the region of bilateral representation but terminate in a spatially restricted manner that links visuotopically corresponding sites in the two hemispheres. In contrast, callosal connections appear to terminate without regard for the map of orientation preference, showing little sign of the or ientation-specific modular and axial specificity that is characteristic of long-range horizontal connections. By coordinating the activity in the two hemispheres in a way that preserves nearest neighbor relationships, callosa l connections may best be viewed as elements of local circuits that operate within a single bilateral representation of visual space.