2 RULES FOR CALLOSAL CONNECTIVITY IN STRIATE CORTEX OF THE RAT

In the rat, callosal cells occupy lateral as well as medial portions o f striate cortex. In the region of the border between areas 17 and 18, which contains a representation of the vertical meridian of the visua l field, cells projecting through the corpus callosum are concentrated throughout the depth of the cortex. In contrast, in medial portion of striate cortex, where peripheral portions of the visual field are rep resented, callosal cells are preferentially found in infragranular lay ers. These differences in topography and laminar distribution suggest that these callosal regions, referred to as medial and lateral callosa l regions in the present study, subserve different functions. We explo red this possibility by analyzing the patterns of callosal linkages in these two callosal regions. We charted the location of retrogradely l abeled cells within striate cortex of one hemisphere after placing res tricted injections of one or more fluorescent tracers into selected si tes in the contralateral striate cortex. We found the medial and later al callosal regions have distinctly different topographic organization s. Injections into medial striate cortex of one hemisphere produced la beled cells predominantly in mirror-symmetric loci in medial portions of contralateral striate cortex. The arrangement of these connections suggests that they mediate direct interactions between cortical region s representing visual fields located symmetrically on opposite sides o f the vertical meridian of the visual field. In contrast, the mapping in the lateral callosal region is reversed: injections into the 17/18a border produced labeled fields located medial to the contralateral 17 /18a border, while injections slightly medial to the 17/18a border pro duced labeled fields located at the contralateral 17/18a border. Assum ing that lateral striate cortex represents a portion of the ipsilatera l visual field, this reversal in the callosal mapping suggests that ca llosal fibers in the lateral callosal region interlink cortical loci r epresenting similar portions of the visual field. (C) 1995 Wiley-Liss, Inc.