Ambiguous images seem to flicker between two alternatives, as if the brain cannot quite make up its mind how to perceive them. The Necker cube, for instance, sometimes looks as if it is pointing into the page and sometimes appears to point out. In an experiment on six volunteers with different kinds of ambiguous visual and auditory stimuli, Christof Koch of the California Institute of Technology and his colleagues found that the pupils dilated around the time that perception shifted. The extent of the momentary dilation, which could be as much as one millimeter, also correlated with how long that particular perception lasted. (Pupils span about two millimeters under bright light.) Because the neurotransmitter norepinephrine controls the pupils, the compound may also play a role in rapid, unconscious decision making. Take a look at the study in the February 5 Proceedings of the National Academy of Sciences USA.
This article was originally published with the title "Eye on the Illusion" in Scientific American 298, 4, 36 (April 2008)