Perceptual switches during structure-from-motion do not elicit pupil response

During perceptual bistability, the observer’s percept typically switches unpredictably between two alternative interpretations despite the physical properties of the stimulus staying the same. Such perceptual switches have been suggested to be linked to norepinephrine-based neuromodulation in the brain, as pupil dilations have been found to occur in association with perceptual switches. However, the pupil dilates in response to task-relevant events in general, and in existing work perceptual switches were explicitly reported and thus task-relevant. As such, observed switch-related dilations may have reflected non-specific task relevance rather than switch-specific processes. Here we measured pupil responses to perceptual switches that were not task-relevant. Observers viewed a structure-from-motion rotating sphere consisting of equilateral triangles that inverted at semi-random intervals (approximately every 3 seconds). In separate conditions observers either reported perceptual switches (rendering them task-relevant) or reported changes in the triangles' orientation (rendering the perceptual switches task-irrelevant). We tracked participants' eyes and used an algorithm based on optokinetic nystagmus (OKN) to infer perceptual switch moments, even when observers did not report them. Control analyses confirm the reliability of this algorithm. We found that task-relevant switches elicited pupil-dilations, but task-irrelevant ones did not. These results suggest that pupil-associated neuromodulation, while 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, and thereby indicate that similar results hold across distinct forms of perceptual bistability.