Dynamics of enage a trois in moving plaid ambiguous perception

Abstract

The perception of ambiguous moving rectangular plaids with transparent intersections is tristable rather than bistable. Not only does it alternate between coherent and transparent motion, but also for transparent motion which grating is perceived in front is ambiguous and alternates (Hupé & Juillard, SFN 2009). The dynamics of perceptual tristability can inform us about how the visual system deals conjointly with two computational challenges among the most important in perceptual organization: motion integration vs. segmentation and depth ordering. Twenty-six subjects reported continuously the three possible percepts of red/green plaids displayed for 1 minute (for transparent motion they had to indicate whether the red or the green grating was in front). The sequence between the three possible transitions was neither random nor hierarchical (as observed in multistable binocular rivalry by Suzuki & Grabowecky, 2002). Rather, switches between two transparency states were typically interleaved by a coherent percept. Moreover, the duration of that coherent percept determined whether switching to the opposite depth ordering should occur (for short coherent percept duration) or not. The preferential status of the coherent interpretation in this threesome may explain why the first percept is systematically coherent and lasts longer than subsequent coherent percepts, even for parameters that strongly favor the transparent interpretation (Hupé & Rubin, 2003. Such a behavior is not typical in bistable perception, but it was also observed for auditory streaming: Pressnitzer & Hupé, 2006). I tested this hypothesis by making the plaid perception bistable, introducing either occlusion or stereo cues to remove the ambiguity of depth ordering. Both manipulations resulted in the first percept (when coherent) having now the same duration as subsequent coherent percepts. Interestingly, the preference for coherency (first percept bias) was affected by stereo but not occlusion cues, meaning that the first percept bias for coherency and its longer duration are two independent phenomena.