Abstract
Asymmetries in global motion perception have been noted for many directions of motion; yet, consistent directional asymmetries have not emerged. At VSS 2004 we reported preliminary direction discrimination results based on a slow speed of motion. We now extend those findings to a faster speed of motion. We investigated the effect of direction, speed and visual field location on global motion processing in 40 university students using random dot kinematograms. Coherence thresholds for direction discrimination using a 2 AFC paradigm were obtained for horizontal (left or right) and vertical motion (up or down), at slow (1 deg/s) and fast (8 deg/s) speeds in the full-field, and four hemifields (left, right, top, bottom). At the fast speed, horizontal coherence thresholds were lower than vertical thresholds, an effect found previously (e.g. Raymond, 1994). This effect was qualified by an interaction with location such that horizontal thresholds were significantly lower than vertical thresholds when motion was presented in the full-field and the top and bottom hemifields, but not when motion was in just the right or left hemifields. Conversely, there was no difference between horizontal and vertical thresholds and no effect of visual field location when the speed of motion was slow. Further, there was a trend for coherence thresholds to be lower for faster motion than for slower motion. Taken together, these results suggest that there are different mechanisms involved in processing slow and fast global motion, and that direction discrimination may be dependent upon speed-tuned mechanisms. This is consistent with past global motion research in which fast-moving noise dots did not impair extraction of motion signals carried by slow-moving dots (Edwards et al., 1998). Edwards and his colleagues suggested that global motion extraction occurs within speed-tuned systems; our results suggest differences in direction discrimination within these speed-tuned systems.