The pupil response to perceptual switches: What happens when you ignore them.

The pupil has been found to dilate after switches in bistable perception, prompting the suggestion that norepinephrine-based neuromodulation plays a causal role in those switches. However, the pupil dilates in response to task-relevant events in general, and, in existing work, perceptual switches were typically task-relevant (e.g., they had to be reported). As such, observed switch-related dilations may have reflected nonspecific task relevance rather than switch-specific processes. Here, we measured pupil responses to perceptual switches that were task-irrelevant. Observers viewed a rotating structure-from-motion sphere consisting of equilateral triangles that inverted at semi-random intervals. In separate conditions, observers either reported perceptual switches (rendering them task-relevant) or reported changes in the triangles' orientation (rendering the switches task-irrelevant). We then used observers' optokinetic nystagmus to infer perceptual switch moments, even when observers did not report them. Control analyses confirm the reliability of this method. We found that task-relevant switches were followed by pupil dilations, but task-irrelevant ones were not. These results suggest that pupil-associated neuromodulation, although closely linked to task-relevant events, may not have any specific tie with perceptual bistability. These results are consistent with results we recently reported for binocular rivalry, indicating commonality across distinct forms of perceptual bistability.