Direction biasing by brief apparent motion stimuli.

The perceived direction of a motion step (probe stimulus) can be influenced by an earlier motion step or a brief motion sweep containing a series of steps (biasing stimulus). Depending upon experimental conditions, the biasing of the direction of the probe step (a phase shift of 180 degrees +/-Phi) by a biasing stimulus which precedes it by approximately 250 ms can either increase (positive filter biasing) or decrease (negative filter biasing) the tendency to see the probe move in the biasing direction as computed with a motion filter with a biphasic temporal impulse response. In a series of experiments it was found that biasing motions traversing 90 degrees of phase angle in fewer than six steps in less than 100 ms produced positive filter biasing. Also, biasing of the probe direction could be dissociated from the consciously reported direction of the biasing stimulus, and it did not occur when the probe preceded rather than followed the biasing stimulus. A biasing sweep containing more than six steps traversing 90 degrees or a sweep traversing 270 degrees produced negative filter biasing. Perceptual fusion of the steps of the sweep was not a necessary condition for obtaining negative filter biasing. In general, the negative filter biasing effects were found to be the most pervasive for the conditions investigated, and they are suggestive of a direction-specific, adaptation-like (gain-control) process in first-order motion filters. The exception to the negative biasing rule was found only with biasing stimuli which were short in duration or distance spanned.