The pupil responds spontaneously to perceived numerosity
Castaldi E., Pomé A., Cicchini G.M., Burr D., Binda P.
Although luminance is the main determinant of pupil size, the amplitude of the pupillary light response is also modulated by stimulus appearance and attention. Here we ask whether
perceived numerosity modulates the pupillary light response. Participants passively observed
arrays of black or white dots of matched physical luminance but different physical or illusory
numerosity. In half the patterns, pairs of dots were connected by lines to create dumbbell-like shapes, inducing an illusory underestimation of perceived numerosity; in the other half,
connectors were either displaced or removed. Constriction to white arrays and dilation to
black were stronger for patterns with higher perceived numerosity, either physical or illusory,
with the strength of the pupillary light response scaling with the perceived numerosity of the
arrays. Our results show that even without an explicit task, numerosity modulates a simple
automatic reflex, suggesting that numerosity is a spontaneously encoded visual feature.