ScienceDaily (Jan. 27, 2012) — The eyes are the window into the soul — or at least the mind, according to a new paper published in Perspectives on Psychological Science, a journal of the Association for Psychological Science. Measuring the diameter of the pupil, the part of the eye that changes size to let in more light, can show what a person is paying attention to. Pupillometry, as it’s called, has been used in social psychology, clinical psychology, humans, animals, children, infants — and it should be used even more, the authors say.
The pupil is best known for changing 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. This keeps the retina at the back of the eye from being overwhelmed by bright light. Something similar happens in response to psychological stimuli, says Bruno Laeng of the University of Oslo, who cowrote the paper with Sylvain Sirois of Université du Québec à Trois-Rivières and Gustaf Gredebäck of Uppsala University in Sweden. When someone sees something they want to pay closer attention to, the pupil enlarges. It’s not clear why this happens, Laeng says. “One idea is that, by essentially enlarging the field of the visual input, it’s beneficial to visual exploration,” he says.