Abstract
Numerous studies have shown that perception of the current stimulus is affected by the preceding stimuli. This sequential effect was shown with various visual features, including orientation of Gabor and direction of pointing arrow. In motion direction estimation, the feature responsible for the sequential effect is ambiguous. The direction, which is the task-relevant feature, or the orientation, a feature that is implicitly activated, can determine the effect. Here we examined whether the sequential effect in motion perception is determined by the direction or the orientation of the preceding motion. Subjects were asked to report the perceived direction of a random-dot motion stimulus (coherence: 40%, duration: 500 ms, direction: randomly chosen from 0-360°). In line with past studies, subjects’ responses were systematically attracted to the motion direction on the previous trial when the current motion direction was in the vicinity of the previous motion direction. Interestingly, when the current motion direction was instead in the vicinity of the opposite direction of the previous motion direction, the responses were biased toward the opposite direction of the previous motion direction. The magnitudes of biases toward the two different directions were comparable. Furthermore, response error on the current trial was positively correlated with response error on the previous trial when the previous and current motion directions were similar, and they were also positively correlated when the previous and current motion directions were opposite to each other. To rule out possible contributions of motion streaks in random-dot motion, we ran a control experiment using a streak-free, moving texture stimulus generated by bandpass filtering random noise in the spatiotemporal frequency domain. The results showed consistent sequential effects of motion orientation. Our findings show that the sequential effects in motion perception mostly occur based on the orientation of the preceding motion, rather than the direction of it.