Your pupils can count… sort of
The pupils of your eyes might be able to count, in a manner of speaking. Recent research suggests pupil size can change based on the number of objects an individual observes in their visual field. Pupils are holes located in the centre of the eye which change size to regulate the amount of light that enters the eye based on how much is available in the environment. The response of our pupils to light is a basic sensory response, but pupils have been found to change size based on factors other than light, such as arousal.
Researchers hypothesised that pupils may also change size based on the number of objects that a person sees in their environment. It’s thought that most species have a certain “number sense”, said study coauthor David Burr, a professor at the University of Sydney and the University of Florence. Previous research has suggested that humans may develop a ‘crude number discrimination’ as soon as a few hours after birth.
“When we look around, we spontaneously perceive the form, size, movement and colour of a scene. Equally spontaneously, we perceive the number of items before us,” Burr said. “This ability, shared with most other animals, is an evolutionary fundamental. It immediately reveals important quantities, such as how many apples there are on the tree, or how many enemies are attacking.”
To figure out if this ability lies within the pupils, the researchers presented 16 adult participants with images of dots on a monitor in an otherwise quiet, dark room. As the participants stared at the screen, without being asked to count the dots, a machine measured their pupil sizes. The images contained either 18 or 24 black or white dots, and each dot was either separate or coupled up with a small line, giving the person the illusion that there were fewer dots on the monitor than there actually were.
The researchers found that the size of the participants’ pupils changed depending on how many dots they perceived. The participants’ pupils dilated, or expanded, when they perceived a greater number of dots and constricted when they perceived fewer dots.
The findings suggest that the pupil is equipped with some mechanism that can sense quantity. “This result shows that numerical information is intrinsically related to perception,” said Elisa Castaldi, a postdoctoral researcher at the University of Pisa. “This could have important practical implications.”
For example, similar methods could be employed to detect dyscalculia, a learning disability that makes people have trouble doing maths, in very young children. “It’s very simple: subjects simply look at a screen without making any active response, and their pupillary response is measured remotely,” Castaldi added.