It is common knowledge that our pupils adjust in size when exposed to light or dark enviornments. But new research published in the journal Psychological Science suggests that the size of our pupils also changes when we imagine these surroundings, even when our eyes are not directly exposed to light and dark.

Researchers from the University of Oslo in Norway say their findings may be useful in studying the mental experiences of patients who suffer from severe neurological disorders.

To reach their findings, the investigators monitored pupil diameters of study participants by using an infrared eye tracker throughout a series of experiments.

The first experiment required participants to look at a screen that showed triangles with various brightness levels. They were then asked to actively imagine the triangles.

Results of this experiment revealed that the subjects' pupils varied in size, depending on the brightness of the triangle they were imagining. When they imagined brighter triangles, their pupils were smaller but enlarged as they imagined darker triangles.

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Experiments showed that pupil size adjusts when we actively imagine light or dark images or settings, suggesting that pupil size is linked to a subjective sense of brightness.

Another experiment required participants to imagine different scenery that had various levels of brightness. This included a sunny sky, a dark room or a face in the sun, compared with a face in the shade. Results showed that subjects' pupils changed in diameter with each scene.

Evidence that pupil size 'is not only a light reflex response'

The investigators say their findings demonstrate that the size of our pupils is not only a "mechanistic" response, but that our pupils change in size in relation to a subjective sense of brightness.

Explaining the findings, Bruno Laeng, of the University of Oslo and study author, says:

"We believe that the first take-home message is that the eye pupillary response is not simply a light reflex servomechanism, but it adjusts to the subjective sense of luminance.

The study also shows that the eye pupils dilated or constricted respectively to 'dark' versus 'bright' imagined objects or scenarios, as if preparing to experience these scenarios. We also believe that the findings support the idea that the luminance of an object is stored in the memory trace together with the object shape."

Laeng says although their results will be of interest to psychologists and philosophers, they should also interest neuroscientists.

"That is, because humans cannot voluntarily constrict the eyes' pupils, the presence pupillary adjustments to imaginary light present a very strong case for accounts of mental imagery as a process based on 'brain states' similar to those which arise during perception," he explains.

"One could also think of clinical application of it as a manner to probe private experiences in animals, small babies and neurological patients."

The researchers note that they plan to continue with further research looking at the application of pupillometry to the study of consciousness and imagery.

Medical News Today recently reported on a study suggesting that 50% of people can see in the dark.