Eye on the illusion

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 volunteers with different kinds of ambiguous visual and auditory stimuli, Christ of 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. Because the neurotransmitter norepinephrine controls the pupils, the compound may also play a role in rapid, unconscious decision making.