Figure 1.

A schematic of two aperture problems and the computations that they entail. (a) The ambiguity in motion speed and direction generated when an untextured contour is either sampled by a receptive field smaller than the contour’s length or when a contour moves behind an aperture in the environment. Two discrete time frames are presented (t₁ and t₂), which may be regarded as either the temporal sampling limit of the visual system or a limit on the presentation rate of the display device (e.g., a computer monitor). (b) The unique computational problem that arises when a contour moves behind an occluding surface in the environment. Three possible interpretations of motion direction are shown (arrows and solid black contour segments). Note that regions of the contour will either be accreted (dashed segments) or deleted (dotted segments) in all possible directions of motion and therefore will not have corresponding elements in the two views. (Also see † footnote)