Structure-from-motion and biological motion perception influences on binocular rivalry

Abstract

Previously, we showed in psychophysical experiments that rivalry between biological motion patterns under binocular conditions is different from low-level rivalry (Beintema, Halfwerk & van Wezel VSS 2004). Biological motion patterns evoked slower alternation and less suppression than less-recognizable inverted and scrambled versions, but only at less-than-natural gait speed. We used this stimulus in an event-related fmri study to investigate the influence of higher level processing on binocular rivalry. We tested rivalling upright vs rivalling inverted walker conditions at less-than-natural gait speeds. Clear centers of activation differences were found in the Fusiform Gyrus, Middle Temporal Gyrus and Inferior Temporal Gyrus, but also in the Parietal and Frontal Lobes. Furthermore, psychophysical 2AFC experiments under monocular viewing conditions showed that a walking figure is perceived to be rotating about the vertical at low speed (as low as 1/8 of natural gait speed), especially when its recognizability is decreased. The opposite trend was found for simulated rotation of a frozen figure, which was perceived as non-rotating at higher speeds (up to twice natural gait speed), especially when it was most recognizable. Interestingly as well, structures could be perceived as rotating and being non-rigid, suggesting two independent perceptual processes. The psychophysics suggest that binocular rivalry is not only influenced by biological motion perception, but also by structure-from-motion perception. The latter might explain activity differences found in the regions that could correspond to MT+ and the more posterior located KO/LO.