Pupils are the dark opening in the center of the eye that allow light in, where it hits the retina, which then sends the image to the brain. Based on ambient light levels, pupils dilate and contract in order to let more or less light in, respectively.
Results from an experiment conducted by the University of Oslo in Norway have shown that pupil adjust not only in response to changes in light intensity, but also to what's going on the in the viewer’s imagination.
According to the study, imagining a brightly lit scene will cause the pupils to open, allowing more light to enter the eye, and imagining a dark environment has the opposite effect.
The experiment tested this through several methods. In one test, participants were asked to look at a series of coloured triangles on a screen, each lit by different levels of light. When asked to mentally picture the triangles that they had seen, their pupils reacted as if the light had once again changed.
The study, which was published in the journal Psychological Science, shows that similar results were seen when asking participants to imagine a sunny sky, a dark room, or faces in the sun or shade.
These findings give scientists hope that better testing can be developed for non-communicative subjects, such as animals, babies, and people with severe neurological disorders or injuries.