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An entirely new optical illusion tricks the majority of people into thinking that a dark “black hole” region in the center of a still image is rapidly expanding, as if the observer were moving toward it. Researchers now suspect the image is literally deceiving brain To think that the observer is moving to a dark place, such as a cave or tunnel.
The illusion consists of a large black oval surrounded by a dark halo on a white background filled with smaller black ovals. Normally, when a person stares at an image, the dark elliptical region will appear to be expanding outward for a few seconds, which is why the design is called an “expanding aperture.”
In a new study, researchers found that 86% of the 50 participants who viewed an optical illusion reported seeing increased darkness. The team suspects that the illusion plays on the brain’s perception of changing light levels.
“The widening gap is a very dynamic illusion,” lead researcher Bruno Ling, a psychologist at the University of Oslo in Norway, said in a statement. Ling added that the illusion tricks the mind into seeing a change in brightness that isn’t really there, “as if the observer was looking forward into a hole or tunnel.”
The illusion hijacks a natural reaction in the brain that predicts when light is about to change, the researchers said. The dark area in the center of the image mimics the entrance to a cave or tunnel, and the surrounding pattern gives the observer the impression that they are moving toward that cave or tunnel. When the brain registers a potential change in light intensity, such as walking in a cave, it can cause the pupils to constrict or dilate to prepare you for the upcoming disturbance beforehand.
An illusion is so good at fooling the brain that it also causes the pupils to dilate as if they were actually traveling to a darker place. The researchers used special cameras to track the observers’ eye movements while they viewed the illusion, and the scientists found that their subjects’ pupils dilate just like the dark area of the illusion seemed to expand in their mind. Those who saw a larger dark hole showed more amplitude than those who saw a less intense “black hole,” the researchers said.
“The illusion of an expanding aperture results in a similar dilation of the pupil, as it would if it really got darker,” Ling said. This explains that “the pupil reacts to how we perceive light, even if that light is imaginary.”
The researchers also exposed the observers to versions of the illusions in which the color of the ellipse was changed. When this happened, the effect of phantom dilatation was reduced and the observer’s pupil dilation became less noticeable. And when the colors were inverted (placing white ovals on a black background), the pupils contracted rather than dilated, as if they were heading toward a bright light.
Researchers have no idea why some people looking into the expanding aperture can’t see the dark area moving. The team hopes to test the illusion on other animals and see if they can learn more about how these visual systems differ from those in humans, to solve this mystery.
The new study was published online May 30 in the journal Frontiers in human neuroscience (Opens in a new tab).
Originally published on Live Science.