Overall patterns of eye-specific retino-geniculo-cortical projections to layers III, IV and VI in primary visual cortex of the prosimian galago, and correlation with cytochrome oxidase blobs.

Abstract

Studies in galago have not provided a comprehensive description of the organization of eye specific retino-geniculate-cortical projections to the recipient layers in V1. Here we demonstrate the overall patterns of ocular dominance domains in layers III, IV and VI revealed after injecting the transneuronal tracer wheat germ agglutinin conjugated to horseradish peroxidase (WGA- HRP) into one eye. We also correlate these patterns with the array of cytochrome oxidase (CO) blobs in tangential sections through the unfolded and flattened cortex. In layer IV, we observed for the first time that eye-specific domains form an interconnected pattern of bands 200-250 um wide arranged such that they do not show orientation bias and do not meet the V1 border at right angles, as is the case in macaques. We also observed distinct patterns of ocular dominance patches in layer III and layer VI. The patches in layer III, likely corresponding to patches of K LGN input described previously, align with layer IV ocular dominance columns (ODCs) of the same eye dominance. Moreover, the layer III patches overlap partially with virtually all CO blobs in both hemispheres, implying that CO blobs receive K LGN input from both eyes. Finally, we found that CO blobs straddle the border between neighboring layer IV ODCs. These results, together with studies showing that a high percentage of cells in CO blobs are monocular, suggest that CO blobs consist of ipsilateral and contralateral subregions that are in register with underlying layer IV ocular dominance columns of the same eye dominance. In macaques and humans, CO blobs are centered on ODCs in layer IV. Our finding that CO blobs in galago straddle the border of neighboring layer IV ODCs suggests that this may represent an alternative way by which visual information is processed by eye specific modular architecture in mammalian V1.