Bringing up visual images in the mind, like a sunny day or a night-time sky, has a matching effect on the size of our pupils. In fact, it’s as if we were actually seeing the image, according to new research from the University of Oslo.

By the way, in case you ever wondered why pupils look black, it’s because light rays entering the pupil are absorbed by the tissues inside the eye, either directly, or after diffuse reflections within the eye that don’t exit the narrow pupil.

These new findings suggest that the size of our pupils is not just a mechanical response, but that it also adjusts to our subjective sense of brightness.

“Visual imagery is a private and subjective experience which is not accompanied by strongly felt or visible physiological changes,” said lead researcher Bruno Laeng. “It is a particularly difficult topic to research, as years of controversy about the nature of mental imagery testifies."

Monitoring Subjective Mental Imagery

Laeng, with co-author Unni Sulutvedt, conducted a battery of experiments to see whether they could tap into subjective mental imagery by monitoring pupil size using an eye-tracking device.

Participants were first asked to look at a screen while triangles of different levels of brightness appeared.

When later asked to actively imagine those triangles, the participants' pupils varied in size according to the triangle’s original brightness.

When imagining brighter triangles, their pupils were smaller. But when imagining darker triangles, their pupils were larger.

In a further experiment series, the researchers observed that participants' pupils changed in diameter also when imagining a sunny sky, a dark room, or a face in the sun compared with a face in the shade, as if in preparation for experiencing the various scenes.

Mental Image Processes

Not only that, but the experiments demonstrated that these results aren’t due to voluntary changes in pupil size or differences in the mental effort required to imagine scenes.

“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,” says Laeng.

Laeng and Sulutvedt advocate that their work may have further applications. It could potentially allow science to probe the mental experiences of animals, babies, and even patients with severe neurological disorders.


B. Laeng, U. Sulutvedt. The Eye Pupil Adjusts to Imaginary Light. Psychological Science, 2013; DOI:10.1177/0956797613503556

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