Pupil responds spontaneously to visuospatial regularity.

Beyond the light reflex, the pupil responds to various high-level cognitive processes. Multiple statistical regularities of stimuli have been found to modulate the pupillary response. However, most studies have used auditory or visual temporal sequences as stimuli, and it is unknown whether the pupil size is modulated by statistical regularity in the spatial arrangement of stimuli. In three experiments, we created perceived regular and irregular stimuli, matching physical regularity, to investigate the effect of spatial regularity on pupillary responses during passive viewing. Experiments using orientation (Experiments 1 and 2) and size (Experiment 3) as stimuli consistently showed that perceived irregular stimuli elicited more pupil constriction than regular stimuli. Furthermore, this effect was independent of the luminance of the stimuli. In conclusion, our study revealed that the pupil responds spontaneously to perceived visuospatial regularity, extending the stimulus regularity that influences the pupillary response into the visuospatial domain.