Previous studies have indicated that the formation of coherent patterns for multielement motion displays depends on global cooperative interactions among large ensembles of spatially distributed motion detectors. These interactions enhance certain motion directions and suppress others. It is reported here that perceiving one element moving between two nearby locations likewise is subject to cooperative influences (possibly facilitating and inhibiting interactions within a local ensemble of overlapping detectors). Thresholds depending on luminance contrast were measured for a generalized singleelement apparent-motion stimulus, and evidence for spontaneous switching and hysteresis effects indicated that motion perception near the 50% threshold was bistable. That is, for conditions in which motion and nonmotion were perceived half the time, the two percepts were distinct; when one was perceived, it clearly was discriminable from the other. These results indicated that (1) single-element apparent-motion thresholds depended on the immediately preceding state of the ensemble of motion detectors responding to the stimulus, and (2) the stimulus activation of individual motion detectors always might be influenced by recurrent, cooperative interactions resulting from the detectors' being embedded within interconnected ensembles.Previous studies have indicated that the formation of coherent patterns for multielement motion displays depends on global cooperative interactions among ensembles of spatially distributed motion detectors that enhance certain motion directions and suppress others. Chang and Julesz (1983) and Williams and Sekuler (1984) have shown that all the dots in a random cinematogram appear to move coherently when a limited number ofmotion directions are stimulated consistently by a relatively small percentage of the moving elements, and Petersik (1990) has demonstrated that motion perceived in one part of a random cinematogram can increase the likelihood of motion being perceived in a neighboring part of the cinematogram. Nawrot and Sekuler (1990) and Hock and Balz (1994) have provided evidence that, depending on the distance, global spatial interactions can either facilitate or inhibit the perception of motion in a particular direction.In this article, we report the results of experiments involving the perception of single-element apparent motion over small spatial displacements. When motion is not perceived, two stationary elements are perceived instead (the perception of nonmotion was sometimes, but not always, accompanied by the perception of flicker). The key finding is that the motion perceived for singleelement apparent motion is like the perception of globalWe are grateful to Gregor Schaner, Kathleen Eastman, Martin Giese, and three anonymous reviewers for their careful reading of an earlier version of the manuscript and their valuable suggestions. Correspondence and requests for materials should be sent to H. Hock, Department of Psychology, Florida Atlantic University, Boca Raton, FL 33431 (e-mail: ...

Adaptation was used to probe the perceiver's activation state when either motion or nonmotion percepts are formed for bistable, single-element apparent motion stimuli. Although adaptation was not observed in every instance, when it was observed its effect was to increase the probability of both motion-to-nonmotion and nonmotion-to-motion switches, the time scale of adaptation corresponding to neurophysiological observations for directionally selective cortical cells (Giaschi et al. 1993). This susceptibility to de-stabilizing adaptation effects indicated that the nonmotion percept was not the result of inadequate stimulation producing subthreshold levels of motion detector activation; if that were the case, activation-dependent adaptation would have decreased the nonmotion-to-motion switching rate by reducing activation further below threshold. Above-threshold activation levels are therefore associated with both nonmotion and motion perceptual states, and the failure to perceive motion despite the presence of adequate motion detector stimulation can be attributed to inhibitory competition between detectors activated by motion-specifying stimulus information and detectors activated to similar levels by motion-independent stimulus information, consistent with the dynamical quality of single-element apparent motion.