Science & Technology, Social Sciences, Life Sciences & Biomedicine, Ophthalmology, Psychology, Psychology, Experimental, Experimental Psychology, 1701 Psychology, 1702 Cognitive Sciences
Ternus motion displays are well known to evoke two qualitatively different motion percepts. Group motion is typically reported for interstimulus intervals (ISIs) larger than a critical value of about 50 ms, while ISIs less than that produce the alternative percept of element motion. According to Braddick's two-process theory of motion processing, element motion occurs at brief and zero ISIs because low-level motion detectors signal the stationarity of the middle element(s). Breitmeyer and Ritter's account similarly points to the fact that the correspondence problem is trivially solved in favour of element motion at a zero ISI. Contrary to what one would expect based on these theories, we found that group motion can be evoked with a zero ISI: using multiple-frame stimuli moving in a single direction, group motion occurred even when the middle stimulus element(s) had no temporal gap. Additionally, we found that group motion turned into element motion again when we introduced an offset - onset asynchrony between the outer elements. A possible explanation for this finding is that only in this case the stimulus is compatible with the perceptual inference that all elements are moving at the same speed.