Fast random motion biases judgments of visible and occluded motion speed.

Human sensitivity to speed differences is very high, and relatively high when one has to compare the speed of an object that disappears behind an occluder with a standard. Nevertheless, different speed illusions (by contrast, adaptation, dynamic visual noise) affect proper speed judgment for both visible and occluded moving objects. In the present study, we asked whether an illusion due to non-directional motion noise (random dynamic visual noise, rDVN) intervenes at the level of speed encoding, thus affecting speed discrimination, or at the level of speed decoding by non-sensory decision-making mechanisms, indexed by speed overestimation of visible and invisible motion. In Experiment 1, participants performing a temporal two-Alternative Forced Choice task, judged the speed of a target moving in front of the rDVN or a static visual noise (SVN). In Experiment 2 and 3, the target disappeared behind the rDVN/SVN, and participants reported whether the target reappeared early or late (Experiment 2), or the time to contact (TTC) with the end of the occluded trajectory (Experiment 3). In Experiment 1 and 2, we found that rDVN affected the point of subjective equality (pse) of the individual's psychometric function in a way indicating speed overestimation, while not affecting speed discrimination threshold (just noticeable differences, jnd). In Experiment 3 the rDVN reduced the TTC. Though not entirely consistent, our results suggest that a similar speed decoding mechanism, which read-out motion information to form a perceptual decision, operates regarding of whether motion is visible or invisible.