Visual search of expansion and contraction.

The perception of expansion/contraction in human subjects was examined with a visual search paradigm. When searching for a target defined by two-dimensional expansion among distractors defined by two-dimensional contraction, the time needed to find the target did not vary as the number of distractors was increased. However, for a target defined by two-dimensional contraction among distractors defined by two-dimensional expansion, the search time increased as a function of the number of distractors in the display. A similar search asymmetry remained between one-dimensional expansion and one-dimensional contraction, even though one-dimensional expansion was searched in a serial manner. This asymmetry between expansion and contraction reflects a basic characteristic of higher-order motion information processing.