Biasing the Percept of Ambiguous Motion Stimuli

Abstract

Ambiguous visual stimuli evoke unstable percepts that spontaneously alternate between different interpretations over time. In a recent report (Leopold et al., 2002) the percept of an ambiguous stimulus was stabilized, even frozen, onto a particular interpretation when the stimulus was presented intermittently rather than continuously. Here, we report that intermittent viewing can also destabilize the percept of an ambiguous stimulus — reliably inducing a perceptual alternation with every stimulus onset. The stimulus used here was a structure-from-motion rotating cylinder presented 5 deg in the periphery. This 2-D pattern of moving dots yields a strong percept of a 3-D cylinder whose direction of rotation spontaneously reverses between two opposing interpretations (in this case, up or down) during continuous viewing. The cylinder stimulus was shown for on-times of 1 sec, separated by blank-times in which only the fixation point was present. The duration of the blank-time (0, 200, 400, 800, 1600, or 3200ms) was varied in separate blocks. After blank-times of 200ms the perceived direction of rotation typically reversed with every stimulus onset, while for gap times of 3200ms the percept was more likely to stay the same (replicating the previously reported freezing effect). For intermediate blank-times, there was a smooth transition between the “reversing” and “freezing” phenomena. This pattern was highly consistent across subjects (n=10) and also occurred when the stimulus was presented foveally. These phenomena may reveal the involvement of two different visual memory systems operating over different time scales. Further experiments revealed that both the reversing and freezing effects can be evoked even when the stimulus changes location (moving to the opposite visual hemifield) with each onset. This suggests that perceptual reversals are not driven simply by local sites of motion adaptation, but may reflect competition between global object interpretations.