Mental Imagery Can Change Pupil Size
December 3, 2013

Just Thinking About Light Or Dark Can Change Your Pupil Size

[ Watch the Video: Thinking About Light Can Change Your Pupil Size ]

Brett Smith for - Your Universe Online

Animals involuntarily constrict or dilate their pupils if they are sensing either too much or not enough light entering the eye, respectively. Now, a new study from a pair of Norwegian researchers has found that simply imagining a scene that is either too brightly or dimly lit can affect the size of your pupils.

"Visual imagery is a private and subjective experience which is not accompanied by strongly felt or visible physiological changes," explained study author Bruno Laeng, a psychological scientist from the University of Oslo. "It is a particularly difficult topic to research, as years of controversy about the nature of mental imagery testifies."

Along with his University of Oslo colleague Unni Sulutvedt, Laeng performed a series of experiments to determine if they could access subjective mental imagery by tracking the size of pupils. The Norwegian team started by having participants look at a display while triangles of different degrees of brightness were shown. Later, participants were asked to actively imagine those triangles.

The researchers wrote in their report in the journal Psychological Science that participants' pupils would vary in size according to an original triangle's brightness. For example, when imagining brighter triangles, participants’ pupils were smaller and when imagining darker triangles, participants’ pupils were dilated.

In another series of experiments, participants' pupils also dilated or constricted when they were told to picture a dark room or a face in the sun compared with a face in the shade. Additional experiments demonstrated that the results were not due to voluntary changes in pupil size or the mental effort spent imagining scenes, the researchers said.

"Because humans cannot voluntarily constrict the eyes' pupils, the presence of pupillary adjustments to imaginary light presents a strong case for mental imagery as a process based on brain states similar to those which arise during actual perception," Laeng said.

The research team suggested that their findings could be applied to future studies involving the mental experiences of animals, babies, and even patients with neurological problems.

While humans have circular pupils, some animals – such as cats and crocodiles – have vertically oriented slit or oval pupils. Biologists have suggested that slit pupils are an ideal feature for nocturnal animals because they are more effective at keeping out light during the daytime. The constriction of a circular pupil has been found to be less complete than that of a slit pupil, which has two additional muscles dedicated to closing itself.

A 2010 study in the Journal of Evolutionary Biology found that the pupil shape of Australian snakes is related to the time of day in which they are most active. While nocturnal snakes tended to have vertical-slit pupils, snakes that were active both at night and during the day tended to have circular pupils, such as the Egyptian cobra. However, the study team also found that the snakes’ foraging behavior was a much stronger indicator of pupil shape – with ambush foragers being nocturnal and active foragers being diurnal.

The researchers from that study suggested that vertical slit pupils may also aid in camouflaging a snake, by breaking up the familiar outline of the predator’s eye.