Pupil and gaze during binocular rivalry

The pupil provides a rich, non-invasive measure of the neural bases of
perception and cognition, and has been of particular value in uncovering
the role of arousal-linked neuromodulation, which alters cortical
processing as well as pupil size. But pupil size is subject to a multitude
of influences, which complicates unique interpretation. We measured pupils
of observers experiencing perceptual multistability -- an ever-changing
subjective percept in the face of unchanging but inconclusive sensory
input. In separate conditions the endogenously generated perceptual
changes were either task-relevant or not, allowing a separation between
perception-related and task-related pupil signals. Perceptual changes were
marked by a complex pupil response that could be decomposed into two
components: a dilation tied to task execution and plausibly indicative of
an arousal-linked noradrenaline surge, and an overlapping constriction
tied to the perceptual transient and plausibly a marker of altered visual
cortical representation. Constriction, but not dilation, amplitude
systematically depended on the time interval between perceptual changes,
possibly providing an overt index of neural adaptation. These results show
that the pupil provides a simultaneous reading on interacting but
dissociable neural processes during perceptual multistability, and suggest
that arousal-linked neuromodulation shapes action but not perception in
these circumstances.