THE MODULAR ORGANIZATION OF PROJECTIONS FROM AREA-V1 AND AREA-V2 TO AREA-V4 AND TEO IN MACAQUES

In addition to the major anatomical pathways from V1 into the temporal lobe, there are other smaller, ''bypass'' routes that are poorly unde rstood. To investigate the direct projection from V1 to V4 (bypassing V2) and from V2 to TEO (bypassing V4), we injected the foveal and para foveal representations of V4 and TEO with different retrograde tracers in five hemispheres of four macaques and analyzed the distributions o f labeled neurons in V1 and V2 using flattened preparations of the cor tex. In V1, labeled neurons were seen after injections in V4 but not T EO. The V4-projecting neurons were located in the foveal representatio n of V1, in both cytochrome oxidase (CO)-rich blobs and CO-poor interb lob regions. In V2, TEO-projecting neurons were intermingled with V4-p rojecting neurons, although the former were far sparser than the latte r. Across the cases, 6-19% of the TEO-projecting neurons were double l abeled, that is, also projected to area V4. Both V4- and TEO-projectin g neurons formed bands that ran orthogonal to the V1/V2 border, and bo th were located in CO-rich thin stripes and CO-poor interstripe region s. In some cases, a continuous band of V4-projecting neurons was also found along the V1/V2 border in the foveal representation of V2. The r esults indicate that the pathways from V1 to V4 and from V2 to TEO inv olve anatomical subcompartments thought to be concerned with both colo r and form. These ''bypass'' routes may allow coarse information about color and form to arrive rapidly in the temporal lobe. The bypass rou te from V2 to TEO might explain the partial sparing of color and form vision that is seen after lesions of V4. By analogy, given the bypass route from the foveal representation of V1 to V4, lesions of V2 affect ing the foveal visual field would also be insufficient to block color and form vision.