Local, short-range and global, long-range motion percepts of bistable stroboscopic motion

Two competitive percepts are produced from a bistable stroboscopic motion display; end-to-end motion responses and group motion responses. The motion display was viewed via a four-channel Gerbrands Model T-4A tachistoscope. Six undergraduate subjects were instructed to make decisions about which of the two percepts occurred. Within subject group motion responses for several issues were evaluated through analysis of variance. (1) Spatial perturbations involved moving the two inner elements either toward the direction of motion (in-phase), against the direction of motion (anti-phase), or a mixed condition where one was in-phase and the other anti-phase. As expected, all conditions produced more group motion than the other conditions. (2) Additional collinear elements were placed above the standard display either with perfect overlap, in-phase, anti-phase, or over the outer two elements of the standard display. All conditions produced more end-to-end motion responses. (3) Increasing the vertical separation between additional collinear elements and the standard display should reduce the vertical cohesion and return to the horizontal-local grouping effects. These hypotheses were supported with increases of group motion responses with increases of vertical separation. (4) Horizontal misalignments between the standard display and additional elements were also expected to reduce vertical-figural cohesion. The results were that both small and large misalignments produced more group motion percepts.