Pupillometry to index visual perception and its inter-individual differences

The pupil is primarily regulated by prevailing light levels, but is also modulated by perceptual and attentional factors. We have investigated multiple cases where pupil constriction reflects attention to light, light detection during eye-movements (reflecting saccadic suppression) and illusory perception of light stimuli. This clearly indicates that cortical visual processing feeds into the pupillary control system, producing slight but consistent constrictions and dilations. We explored the possibility to exploit these subtle pupil size changes to index visual processing and specifically perceptual styles. We measured pupil-size in typical adult humans viewing a bistable-rotating cylinder, constructed so the luminance of the front surface changes with perceived direction of rotation. In some participants, pupil diameter oscillated in phase with the ambiguous perception, more dilated when the black surface was in front. Importantly, the magnitude of oscillation predicts autistic traits of participants, assessed by the Autism-Spectrum Quotient AQ. Further experiments suggest that these results are driven by differences in perceptual styles: high AQ participants focus on the front surface of the rotating cylinder, while those with low AQ distribute attention to both surfaces in a more global, holistic style. This is the first evidence that pupillometry reliably tracks inter-individual differences in perceptual styles; it does so quickly and objectively, without interfering with spontaneous perceptual strategies.