V1 partially solves the stereo aperture problem.

If a bar stimulus extends beyond a cell's receptive field, then alterations in binocular disparity parallel to the bar's orientation leave the portion of the stimulus within the cell's receptive field unchanged. This makes it hard for the cell to respond correctly to the bar's disparity. Likening the cell's receptive field to an aperture through which the cell views the world, this issue has been called the "aperture problem" and is a specific form of the more general stereo correspondence problem. We found no cells in macaque primary visual cortex (V1) that, when faced with this situation, were sensitive to the disparity of the bar. However, we did find a number of cells that showed sensitivity to parallel disparity shifts, but these cells responded only to the ends of the long bar. The ability to respond selectively to such tracking features could be the first step towards solving the stereo aperture problem. The second step would require either that the disparity information that in V1 is associated only with the ends of the stimulus be associated with the rest of the stimulus or that subsequent stages of visual processing respond preferentially to the end-selective cells. As this second step does not appear to occur in V1, we conclude that V1 only partially solves the stereo aperture problem.