WHAT IS NOISE FOR THE MOTION SYSTEM

Motion coherence thresholds in random-dot patterns have been widely ad opted as a measure of performance in visual motion processing, However , there has been diversity in the type of ''noise'' in which a coheren t motion signal has to be detected. Here we compare coherence threshol ds for three ways of creating motion noise: dots replotted in random p ositions in each new frame; dots with a set displacement but following a random walk from frame to frame; or dots moving in random direction s which remain constant for a given dot over a sequence of displacemen ts. In each case, the signal dots may either remain the same throughou t the display sequence, or the signal dots may be re-selected afresh o n each frame (''different''). With our display (3 deg square, 120 msec exposure, velocity = 5 or 10 deg sec(-1)), all these different noise conditions yielded similar thresholds around 5-8%, There were some sma ll but systematic differences between conditions. Thresholds in random -direction displays were consistently higher than those in random-walk or random-position displays, especially at the lower velocity, Howeve r, this effect is much smaller than would be expected from the increas ed standard error of the noise mean in random direction, perhaps becau se the motion system integrates information most effectively over a lo cal region of space and/or time. Subjects'' performance could not be e xplained by a strategy of identifying individual signal dots with exte nded trajectories, The similarity between random-walk and random-posit ion thresholds implies that subjects do not exploit the marked differe nces in speed distribution between signal and noise dots in the latter case. The practical message for the design and interpretation of expe riments using coherence thresholds is that the results are not much af fected by the choice of noise, at least within the range of stimuli te sted here. Copyright (C) 1996 Elsevier Science Ltd.