You might want to grab on to the edge of your desk before looking at the image above. If you're like most people, the black oval will appear to expand, as if you're falling into a hole. At least that's what 86% of the people who took part in a new study – "The Eye Pupil Adjusts to Illusorily Expanding Holes" by Laeng, Nabil, and Kitaoka, published by Frontiers in Human Neuroscience — thought after this illusion tricked their pupils into actually dilating.
"The 'expanding hole' is a highly dynamic illusion: The circular smear or shadow gradient of the central black hole evokes a marked impression of optic flow, as if the observer were heading forward into a hole or tunnel," said the study's lead author and University of Oslo professor Dr. Bruno Laeng.
"Here we show based on the new 'expanding hole' illusion that that the pupil reacts to how we perceive light—even if this 'light' is imaginary like in the illusion—and not just to the amount of light energy that actually enters the eye.
The illusion of the expanding hole prompts a corresponding dilation of the pupil, as it would happen if darkness really increased," said Laeng.
From Neuroscience News.com:
In the new study, published in Frontiers in Human Neuroscience, Laeng and colleagues show that the "expanding hole" illusion is so good at deceiving our brain that it even prompts a dilation reflex of the pupils to let in more light, just as would happen if we were really moving into a dark area. …
The illusion appeared most effective when the hole was black. Fourteen percent of participants didn't perceive any illusory expansion when the hole was black, while 20% didn't if the hole was in color. Among those who did perceive an expansion, the subjective strength of the illusion differed markedly.
The researchers also found that black holes promoted strong reflex dilations of the participants' pupils, while colored holes prompted their pupils to constrict. For black holes, but not for colored holes, the stronger individual participants subjectively rated their perception of the illusion, the more their pupil diameter tended to change.