The pupil responds spontaneously to perceived numerosity

Elisa Castaldi,David Burr,Antonella Pomè,Guido Marco Cicchini,Paola Binda

doi:10.1038/s41467-021-26261-4

Abstract

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.

Highlights

Luminance is the main determinant of pupil size, the amplitude of the pupillary light response is modulated by stimulus appearance and attention
We focused on the sustained response, considering pupil difference traces to estimate the impact of numerosity on the net pupillary response to luminance. β-Weights for the sustained predictor varied across conditions, reinforcing the results obtained by taking the simple mean of the response over the stimulus window. β-Weights were highest for 24 isolated dots, lowest for 18 connected dots (experiment 1: t(15) = 3.8, p = 0.002, Cohen’s d = 0.9, log10BF = 1.4; experiment 2: t(12) = 2.2, p = 0.047, Cohen’s d = 0.6, log10BF = 0.2) and intermediate for 18 isolated or 24 connected dots
In this study, we investigated whether pupil size is spontaneously modulated by numerosity, for stimuli of identical luminance

Summary

Introduction

Luminance is the main determinant of pupil size, the amplitude of the pupillary light response is modulated by stimulus appearance and attention. Visual processing of numerosity is rapid, with numerosity-specific signals emerging in the occipital cortex only 75 ms after stimulus onset[11], and humans can saccade rapidly to the more numerous target, as quickly as 190 ms[12], implicating primitive, possibly subcortical, circuitry that quickly transforms numerosity information into an oculomotor response This and other literature point to numerosity being a salient perceptual feature, eliciting a spontaneous perceptual response. Pupils constrict more to passively observed white arrays and dilate more to black arrays for more numerous (luminance matched) stimuli, whether perceived numerosity was manipulated by dot number or by exploiting a grouping-based illusion. This suggests an implicit association between numerosity and perceptual strength, which can be read out—objectively and quantitatively—from the pupil

Methods

Results

Conclusion