The eyes tell the story – or more precisely, the size of a person’s pupils may be a way to measure someone’s interest in a particular topic, according to new research.
Physicians have analyzed the size and reactivity of the pupil to help diagnose illness for centuries. Perhaps less well known is the actual measuring of the diameter of the pupil to show what a person is paying attention to.
In more recent times, “pupillometry”has been used in social psychology, clinical psychology, and on humans, animals, children and infants.
The pupil changes size in reaction to light. In a dark room, your pupils open wide to let in more light; as soon as you step outside into the sunlight, the pupils shrink to pinpricks.
The variability prevents the sensitive retina, found at the back of the eye from being overwhelmed by bright light.
Something similar happens in response to psychological stimuli, according to study co-author Bruno Laeng, Ph.D., of the University of Oslo. When someone sees something they want to pay closer attention to, the pupil enlarges. It’s not clear why this happens, Laeng said.
“One idea is that, by essentially enlarging the field of the visual input, it’s beneficial to visual exploration,” he said.
However it works, psychological scientists can use the fact that people’s pupils widen when they see something they’re interested in. Laeng and his colleagues have used pupil size to study people who had damage to the hippocampus, which usually causes very severe amnesia.
Normally, if you show one of these patients a series of pictures, then take a short break, then show them another series of pictures, they don’t know which ones they’ve seen before and which ones are new.
But Laeng measured patients’ pupils while they did this test and found that the patients did actually respond differently to the pictures they had seen before. “In a way, this is good news, because it shows that some of the brains of these patients, unknown to themselves, is actually capable of making the distinction,” he said.
Researchers believe pupil measurement may be an effective method to study the attention of babies. Tiny infants can’t tell you what they’re paying attention to.
“Developmental psychologists have used all kinds of methods to get this information without using language,” Laeng said. Seeing what babies are interested in can give clues to what they’re able to recognize — different shapes or sounds, for example.
A researcher might show a child two images side by side and see which one they look at for longer. Measuring the size of a baby’s pupils could do the same without needing a comparison.
This technique is really an extension of eye-tracking to determine what an individual is looking at. Laeng and his coauthors hope to convince other psychological scientists to use this method in future studies.
The paper is published in Perspectives on Psychological Science.