Interactions between Global and Local Motions during Object Tracking
63.318, Wednesday, May 15, 8:30 am - 12:30 pm, Royal Ballroom 6-8
Bennett Bertenthal1, Sam Harding1, Rob de Ruyter2; 1Department of Psychological and Brain Sciences, Indiana University, 2Department of Physics, Indiana University
Object tracking was studied with a streaming-bouncing paradigm. In a typical display, two discs approach each other from the left top and bottom edges of the screen until they coincide, then change direction by 90 deg, and continue to the right top and bottom. If the discs share identical surface features, the perception of the event is ambiguous and could be perceived as streaming (discs pass through each other) or bouncing. If the discs are not identical, distinctive surface features could disambiguate this percept. The discs were comprised of random textures generated by applying an isotropic Gaussian filter to white spatial contrast noise, and subsequent normalization to a defined contrast variance. For one disc, the local direction of texture motion was either the same or opposite its direction of global motion, whereas the local direction of the other disc was oriented (+/-) 0, 15, 30, or 45 deg relative to the first disc. The two discs intersected via occlusion or transparency. Participants fixated a cross in the center of the screen throughout the trial and reported whether the two discs "streamed" or "bounced." There were a total of 192 trials with 24 per intersection by texture motion condition. Consistent with previous studies, there was a strong bias to perceive the discs streaming although this response decreased as the difference in the direction of the local motions increased. The perception of streaming was significantly greater when the local motions moved in the same as opposed to the opposite direction of the discs. In previous studies with occlusion at the intersection and static textures, streaming was reported at a rate of 20%, whereas it was typically above 50% in this study. Taken together, these results suggest that the perception of local motions interact with the perception of global motions while tracking objects.