It is common knowledge that pupils dilate or contract in size depending on the light available in the surroundings. Now, scientists have found that even the number of objects in one’s line of sight influences how much the pupils respond. The response, moreover, is a spontaneous reaction that we develop soon after birth, before understanding numbers as a concept.
The study, published this month in Nature Communications, observed how participants’ pupils responded to “numerosity,” or a “number sense” — a quantitative characteristic in objects they were perceiving.
This happens in real time, which means that the areas in the brain that regulate pupil size also receive signals from “higher-level” areas — something researchers hadn’t known was possible before. Scientists are still not sure how exactly this happens, but one hypothesis is that the prefrontal cortex is the higher-level area which also influences visual perception.
The researchers used white and black dots; in one set of dots, connecting lines between the dots formed “dumbbell” like shapes, leading to an illusion of “underestimated” numerosity; in another set, the dots were unconnected. What they found was significant: pupils responded not only to light (constricted for white dots, dilated for black) but also, correspondingly, to the perceived numerosity of the dots. In other words, because there seemed to be fewer objects (dumbbells) in one set than the other (unconnected dots), participants’ eyes adjusted accordingly. The more the number of perceived objects, the more the pupils dilated.
“The results of our study suggest that patterns perceived as more numerous are spontaneously represented as perceptually stronger and consequently evoke stronger pupillary responses,” the paper states.
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“When we look around, we spontaneously perceive the form, size, movement and color of a scene… Equally spontaneously, we perceive the number of items before us. This ability, shared with most other animals, is an evolutionary fundamental: it reveals immediately important quantities, such as how many apples there are on the tree, or how many enemies are attacking,” David Burr, a psychologist from the University of Sydney in Australia, noted.
Further, researchers found that humans were more sensitive to numerosity than changes to area and densities. It is important to note here that pupil responses are completely involuntary, which makes this particular finding all the more fascinating because it implies that our “number sense” perhaps predates our own understanding of numbers.
“The current results are also in line with other studies suggesting that numerosity is a salient visual feature that is difficult to ignore, and can drive automatic oculomotor orientation responses,” they add.
The implications of the study, beyond allowing a better understanding of numerical cognition, are also significant because they could potentially allow earlier identification of dyscalculia in children — a mathematical learning dysfunction, according to neuroscientist Elisa Castaldi from the University of Florence in Italy.
