Existing results reveal a transient pupil dilation accompanying perceptual switches, suggestive of a noradrenaline surge ( Einhäuser et al., 2008 Hupé et al., 2009 Kloosterman et al., 2015b de Hollander et al., 2018). Like previous researchers, we focused on pupil size changes that accompany switches between alternative percepts, because pupil dilations can non-invasively convey noradrenaline release ( Murphy et al., 2014 Joshi et al., 2016 de Gee et al., 2017). We used a new combination of experimental methods to evaluate this idea. Such stimuli cause perception to alternate between different interpretations of the sensory data, and recent work suggests that the noradrenergic arousal system associated with the brainstem's locus coeruleus impacts perception in this situation, perhaps by altering the response gain of visual cortical neurons involved ( Einhäuser et al., 2008 Sara and Bouret, 2012 Laeng et al., 2012 Kloosterman et al., 2015a Pfeffer et al., 2018). Recent work has used multistable visual stimuli to examine neuromodulatory influences on visual processing. The brainstem's neuromodulatory systems can profoundly influence cognitive functions by altering neural response properties within the cortical circuits that mediate those functions ( Aston-Jones and Cohen, 2005 Sara and Bouret, 2012 Lee and Dan, 2012 Pfeffer et al., 2018). These results show that the pupil provides a simultaneous reading on interacting but dissociable neural processes during perceptual multistability, and suggest that arousal-linked neuromodulator release shapes action but not perception in these circumstances. Constriction, but not dilation, amplitude systematically depended on the time interval between perceptual changes, possibly providing an overt index of neural adaptation. 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. 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.
We measured pupils of observers experiencing perceptual multistability-an ever-changing subjective percept in the face of unchanging but inconclusive sensory input. But pupil size is subject to a multitude of influences, which complicates unique interpretation. 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 both cortical processing and pupil size.
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