Ambiguity in the perception of moving stimuli is resolved in favour of thecardinal axes

The aim of this study was to determine whether there is a link between the statistical properties of natural scenes and our perception of moving surfa ces. Accordingly, we devised an ambiguous moving stimulus that could be per ceived as moving in one of three directions of motion. The stimulus was a c ircular patch containing three square-wave drifting gratings. One grating w as always either horizontal or vertical; the other two had component direct ions of drift at 120 degrees to the first (and to each other), producing fo ur possible stimulus geometries. These were presented in a pseudorandom seq uence. In brief presentations, subjects always perceived two of the grating s to cohere and move as a pattern in one direction, and the third grating t o move independently in the opposite direction (its component direction). A lthough there were three equally plausible axes (one cardinal and two obliq ue) along which the coherent and independent motions could occur, subjects routinely saw motion along one of the cardinal axes. Thus, the visual syste m preferentially combines the two oblique gratings to form a pattern that d rifts in the opposite direction to the cardinal grating. It was only when t he contrast of one of the oblique gratings was changed that an oblique axis of motion was perceived. This perceptual anisotropy can be related to natu rally occurring bias in the visual environment, notably the predominance of horizontal and vertical contours in our visual world. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.