Existing neural explanations of spontaneous percept switching under steady viewing of an ambiguous stimulus do not fit the fact that stimulus interruptions cause the same percept to reappear across many ON/OFF cycles. We present a simple neural model that explains the observed behavior and predicts several more complicated percept sequences, without invoking any "high-level" decision making or memory. Percept choice at stimulus onset, which differs fundamentally from standard percept switching, depends crucially on a hitherto neglected interaction between local "shunting" adaptation and a near-threshold neural baseline. Stimulus ON/OFF timing then controls the generation of repeating, alternating, or more complex choice sequences. Our model also explains "priming" versus "habituation" effects on percept choice, reinterprets recent neurophysiological data, and predicts the emergence of hysteresis at the level of percept sequences, with occasional noise-induced sequence "hopping."

When our two eyes view incongruent images, we experience binocular rivalry: An ongoing cycle of dominance periods of either image and transition periods when both are visible. Two key forces underlying this process are adaptation of and inhibition between the images' neural representations. Models based on these factors meet the constraints posed by data on dominance periods, but these are not very stringent. We extensively studied contrast dependence of dominance and transition durations and that of the occurrence of return transitions: Occasions when an eye loses and regains dominance without intervening dominance of the other eye. We found that dominance durations and the incidence of return transitions depend similarly on contrast; transition durations show a different dependence. Regarding dominance durations, we show that the widely accepted rule known as Levelt's second proposition is only valid in a limited contrast range; outside this range, the opposite of the proposition is true. Our data refute current models, based solely on adaptation and inhibition, as these cannot explain the long and reversible transitions that we find. These features indicate that noise is a crucial force in rivalry, frequently dominating the deterministic forces.

When sensory input allows for multiple, competing perceptual interpretations, observers’ perception can fluctuate over time: bistable perception. Imaging studies in humans have revealed transient responses in a right-lateralized network in the frontal-parietal cortex (rFPC) around the time of perceptual transitions between interpretations, potentially reflecting the neural initiation of transitions. We investigated the role of this activity in male human observers, with specific interest in its relation to the temporal structure of transitions, which can be either instantaneous or prolonged by periods during which observers experience a mix of both perceptual interpretations. Using both bistable apparent motion and binocular rivalry we show that transition-related rFPC activity is larger for transitions that last longer, suggesting that rFPC remains active as long as a transition lasts. We also replicate earlier findings that rFPC activity during binocular rivalry transitions exceeds activity during yoked transitions that are simulated using video replay. However, we show that this established finding only holds when perceptual transitions are replayed as instantaneous events. When replay, instead, depicts transitions with the actual durations reported during rivalry, yoked transitions and genuine rivalry transitions elicit equal activity. Together, our results are consistent with the view that at least a component of rFPC activation during bistable perception reflects a response to perceptual transitions, both real and yoked, rather than their cause. This component of activity could reflect the change in sensory experience and task demand that occurs during transitions, which fits well with the known role of these areas in attention and decision-making.

No abstract