Reciprocal interactions between occlusion and motion computations.

The "aperture problem" refers to the inherent ambiguity of the motion generated by an untextured contour moving within an aperture. The limited spatial extent of the receptive fields of neurons in cortical areas like V1 and MT render them susceptible to this problem. Most psychophysical experiments have probed how the visual system overcomes the aperture problem by presenting moving contours behind one or more simulated apertures. The assumption has been that the computational ambiguities that arise in resolving these displays are equivalent to the computational problems created by receptive fields that sample a small region of visual space. Evidence is presented here that challenges this view. We demonstrate that a fundamental computational difference in the interpretation of contour terminators arises in these two variants of the aperture problem. When the aperture is a receptive field, and a moving contour extends beyond its boundaries, the contour "terminators" delimit the boundaries of the receptive field, not the ends of the contour. In contrast, when a moving contour is viewed through a simulated aperture, the contour terminators are generated by the occluding edges of the aperture. In a series of experiments, we show that reciprocal interactions arise between computations of occlusion and those of motion direction and integration. Our results demonstrate that the visual system solves the aperture problem by decomposing moving contours into moving segments, and unpaired terminators that arise from the accretion and deletion of contours behind occluding edges, generating both coherent motion and illusory occluding surfaces.