The pupils of the human eye shrink in response to brightness, even when the glow is merely an optical illusion.

Credit: US NATL ACAD. SCI.

Bruno Laeng and Tor Endestad at the University of Oslo used infrared eye trackers to monitor the pupils of participants looking at illusions of lightness or brightness. These graphic designs give the impression of having brighter or whiter components (pictured right) than similar images of equal luminance (left). The pupils rapidly constricted when people glanced at the 'brighter' image, then slowly readjusted to the picture's true light intensity.

The authors suggest that the physical response to the illusions may be an adaptation designed to protect the eye's sensitive light-absorbing cells from potentially damaging levels of light.

Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1118298109 (2012)