The aperture problem is a problem of underdetermination that arises in motion perception called (Hildreth, 1983). A motion detector’s task is often to detect the movement of a contour. However, if the motion detector is of limited order if it is an aperture or window that is much smaller than the contour that it observes -- the motion detector can only be sensitive to the component of the contour’s motion that is perpendicular to the edge of the contour. It is completely blind to any motion parallel to the contour. This is because movement in this direction will not change the appearance of anything within the aperture. As a result, the motion detector is unable to detect the true movement of the contour. Hildreth was able to show that if there were cooperative interactions amongst motion detectors that implemented a natural constraint (smoothness of motion, or the relative velocity principle), then this problem of underdetermination could be solved. That is, each motion detector delivers a motion signal that is a) consistent with the component of motion that it can see, and b) is most consistent with motion detected by neighboring detectors. In this case, the entire set of detectors will deliver true motion readings.