Pupils Respond to Differing Perceived Quantities

Article: More than light detectors: the magic of your eyes' pupils 
Source: University of Sydney (Australia)
Published: October 25, 2021 
Article: The Pupil in Your Eye Can Perceive Numerical Information, Not Just Light
Source: ScienceAlert
Published: October 27, 2021

Experiments tested participants' pupillary responses to differing
perceived quantity (left) and stimulus connectedness (right)

Although luminance is the main determinant of pupil size, other factors can also affect pupillary response. These include automatic responses to emotions and cognitive engagement. Scientists in Australia, with colleagues in Italy, were interested in studying whether pupils also have a physiological response to stimulus appearance and attention, that is, whether perceived numerosity modulates the pupillary light response. The study recruited 16 volunteers with normal vision who participated in a pupillometry experiment, 14 of whom participated in a numerosity discrimination experiment, and 13 of whom participated in a second pupillometry and psychophysics experiment. The perception aspects of the experiments involved tasks such as passively observing visual stimuli (black or white dots) of matched luminance but differed in their level of connectedness, whether separated dots or joined by a line, which has the illusory effect of decreasing the total number of perceived objects. The researchers also tested whether varying the relative size of the dots and the thickness of the line had any effect. The authors report, "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." One of the researchers adds that the spontaneous ability to perceive quantity, an innate "number sense," is shared among most species, with evolutionary implications such as in the perception food items or of predators.

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Castaldi E, Pomè A, Cicchini GM, et al. "The pupil responds spontaneously to perceived numerosity." Nature Communications.  12:5944. 12 October 2021. https://doi.org/10.1038/s41467-021-26261-4