The pupil in your eye can perceive numerical information, not just light

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You may know that the pupil size in our eyes changes depending on the level of light in our surroundings, but there’s more to the story: Scientists have now found that the pupil size also changes depending on how many objects we observe.

The more objects there are in a scene, the larger the pupil becomes, as if it could better accommodate everything it needs to look at. This “perceived multiplicity” is a simple and automatic reflex, as the new research shows.

In a new study, researchers observed the pupil size of 16 participants while they looked at images of dots. In some images, the dots were connected in a dumbbell shape – creating the illusion that there were fewer objects – and the pupil size shrank.

How the student reacts to different objects and patterns. (Castaldi et al., Nature Communications, 2021)

“This result shows that numerical information is inextricably linked with perception”, says psychologist and neuroscientist Elisa Castaldi from the University of Florence in Italy.

“This could have important practical implications. For example, this ability is impaired in dyscalculia, a dysfunction of math learning, so our experiment could be useful in the early detection of this condition in very young children.”

Although the number of black or white dots in the viewed images did not change, the perceived number of objects changed due to the connecting lines. Participants were asked to passively look at these images without paying particular attention to the total number of elements and without solving a specific task.

Where this reaction comes from is likely related to the need for survival – most species are thought to have a dedicated “number sense” that enables them to spot enemies in the wild, find food, get home and much more.

When it comes to people, weighing numbers seems like something that pops up right away a few hours after the birth – Even if you are bad at math you have a built-in ability to judge numbers and it seems that our students’ expansion is part of a response to it.

“When we look around, we spontaneously perceive the shape, size, movement and color of a scene”, says the psychologist David Burr from the University of Sydney in Australia and the University of Florence.

“We perceive the number of objects in front of us just as spontaneously. This ability, shared with most other animals, is an evolutionary foundation: It immediately shows important parameters, such as how many apples are hanging on the tree or how many enemies are attacking. “.”

Previous research had pointed out that pupil size is not only influenced by light: visual illusions with brightness, size and context also have an influence, which supports the idea that this enlargement in our eyes is at least partially controlled by signals higher up in the brain.

The researchers want to further investigate why this happens and what else could affect the pupil size – such as the movement that the eye needs to take in everything that appears in a scene.

And there is a lot more to discover here too. Our eyes seems to be more sensitive on the number of elements we study rather than their spacing or arrangement, which is another response that can be analyzed in future studies.

“The latest research from our laboratory shows that pupil size is also regulated by cognitive and perceptual factors.” says physiologist Paola Binda from the University of Pisa in Italy.

The study was published in Nature communication.


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