It is the light we think we see that counts. Optical illusions designed to seem brighter than they are make your pupils constrict a little more. This suggests that we have evolved systems for anticipating dazzling light to protect our eyes.
Our pupils' fast response to light appears to occur even without input from the brain. For example, it is seen in people with damage to the visual cortex. Appearances can be deceptive, though.
Bruno Laeng of the University of Oslo in Norway measured tiny changes in pupil size as volunteers viewed various illusions that were all identical in brightness, though did not look so. If light levels alone dictated pupil size, they would have reacted identically whichever image a person viewed. Instead, people's pupils constricted more when they viewed the illusions designed to appear brightest.
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"What's surprising is that even something as simple as how bright we think our environment is will be affected by our expectations," says Stuart Peirson of the University of Oxford, who was not involved in the study.
Previous studies show that the brain controls pupil size in other situations: our pupils dilate when we make decisions, for instance.
Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1118298109
