Speed and direction of locally-paired dot patterns

Phenomenal transparency in random-dot kinematograms is abolished when two m otion directions are 'locally-balanced' by pairing limited-lifetime dots at each location [Qian, Andersen and Adelson (1994). Journal of Neuroscience, 14, 7357-7366]. Qian et al. also report that locally-paired stimuli appear as directionless flicker when the paired dots differ in their directions b y 90 degrees or more. They attribute this to local inhibition between motio n detectors more than 45 degrees apart. We investigated perceived motion in such displays, by requiring subjects to make direction and speed judgement s with locally-paired stimuli containing two directions 60, 90 or 120 degre es apart. Subjects perceived coherent motion in these displays and made rel iable direction judgements, indicating that the two motions are combined ra ther than interfering destructively. Our results show that the judged motio n of locally-paired stimuli is in the vector-average direction of the two c omponents. This vector-averaging rule also applies when the two sets of com ponent dots differ in their velocity. Similarly, speed judgements comply wi th a vector-averaging rule for a range of speeds as well as for mixed-speed stimuli. These results suggest that the abolition of transparency does not necessarily imply abolition of a global motion percept. The local interact ion abolishing transparency is not exclusively inhibitory, at least for dir ections up to 120 degrees apart, but generates a Vector combination of the superimposed motions. (C) 2000 Elsevier Science Ltd. All rights reserved.