The eye pupil adjusts to imaginary light.

If a mental image is a rerepresentation of a perception, then properties such as luminance or brightness should also be conjured up in the image. We monitored pupil diameters with an infrared eye tracker while participants first saw and then generated mental images of shapes that varied in luminance or complexity, while looking at an empty gray background. Participants also imagined familiar scenarios (e.g., a "sunny sky" or a "dark room") while looking at the same neutral screen. In all experiments, participants' eye pupils dilated or constricted, respectively, in response to dark and bright imagined objects and scenarios. Shape complexity increased mental effort and pupillary sizes independently of shapes' luminance. Because the participants were unable to voluntarily constrict their eyes' pupils, the observed pupillary adjustments to imaginary light present a strong case for accounts of mental imagery as a process based on brain states similar to those that arise during perception.