Binocular rivalry (BR) is observed when the two eyes receive conflicting information, leading to perceptual switches between the eyes' images. Previous computational models and empirical evidence were inconsistent in their account of the effect of moderate attention withdrawal on the switch frequency of BR. One concern with the empirical work in question, however, is that the observers reported BR dominance while performing a secondary attention task: a dual task design that might have produced unreliable data. To avoid this potential concern, we reexamined the effect of moderate attention withdrawal on binocular rivalry by using optokinetic nystagmus to track perceptual switches of a task-irrelevant rivalry stimulus (foveally presented dot fields moving in opposite directions in the two eyes), while observers performed an auditory attention task.Our results showed that switch frequency decreased as a function of increasing attention load, confirming the existing empirical finding with our new method that does not share the original method's potential shortcomings. Furthermore, our results showed an increased proportion of non-exclusive percepts as a function of increasing attention load, coupled with the decreased switch frequency. This suggests that attention may modulate the switch frequency of BR by modulating perceptual grouping. Specifically, attention may modulate switch frequency of BR by binding the dominant rivalry percept across space; hence the increased proportion of piecemeal rivalry accompanying attention withdrawal. We developed a mathematical model that implements this proposed mechanism of altered binding across space, to examine whether it can account for the empirical patterns. Attention implemented as an overall gain modulation of lateral facilitation can account for the observed change in the proportion of exclusive percepts with attention load, but still the predicted effect on switch frequency is in the wrong direction; the same shortcoming we observed in existing, non-spatial, models. Other aspects of attentional modulation might help fitting the model to both aspects of empirical findings.
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