Humans perceive binocular rivalry and fusion in a tristable dynamic state

Abstract

Human vision combines inputs from the two eyes into one percept. Small differences "fuse" together, whereas larger differences are seen "rivalrously" from one eye at a time. These outcomes are typically treated as mutually exclusive processes, with paradigms targeting one or the other and fusion being unreported in most rivalry studies. Is fusion truly a default, stable state that only breaks into rivalry for non-fusible stimuli? Or are monocular and fused percepts three sub-states of one dynamical system? To determine whether fusion and rivalry are separate processes, we measured human perception of Gabor patches with a range of interocular orientation disparities. Observers (10female, 5 male) reported rivalrous,fused, and uncertain percepts overtime. Wefound a dynamic "tristable" zone spanning from 25-35° of orientation disparity where fused, left-eye-, or right-eye-dominant percepts could all occur. The temporal characteristics of fusion and non-fusion periods during tristability matched other bistable processes. We tested statistical models with fusion as a higher-level bistable process alternating with rivalry against ourfindings. None ofthesefit our data, but a simple bistable model extended to havethree states reproduced many of our observations. We concludethat rivalry and fusion are multistable substates capable of direct competition, rather than separate bistable processes.