Expanding hole: Most people's brains are fooled by this optical illusion - here's why

Expanding hole: Most people's brains are fooled by this optical illusion - here's why
Photo credit: Frontiers in Human Neuroscience

Take a look at this image: does it look like the black hole is expanding, as if you're moving into a dark environment or falling into a hole? If so, you're not alone.

The illusion, known as the 'expanding hole', is powerful enough to trick even our reflexes - deceiving our pupils to expand in anticipation of an expected decrease in light.

A new study has found that the 'expanding hole', an illusion which is new to science, tricks the brains of approximately 86 percent of people.

Dr Bruno Laeng, a professor at the Department of Psychology of the University of Oslo in Norway and the study's first author, said 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," he said.

Optical illusions aren't mere gimmicks: researchers in the field of psychosociology study the illusions to better understand the complex processes used by the body's visual system to anticipate and make sense of the visual world - in a far more roundabout way than a photometer device, which simply registers the amount of photonic energy.

In the new study, published in Frontiers in Human Neuroscience, Laeng and his colleagues found the illusion is so good at deceiving our brain, the black hole prompts our pupils to dilate to let in more light - just as they would if we were entering a darkened area.

The illusion indicates that the pupillary light reflex, which controls the width of the pupil in anticipation of expected changes in light, depends on perception and the perceived environment - not necessarily physical reality. 

"Here we show based on the new 'expanding hole' illusion 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 would happen if darkness really increased," Laeng explained.

The illusion
The illusion is new to the field of science. Photo credit: Frontiers in Human Neuroscience

Laeng and colleagues also explored how the colour of the hole - black, blue, cyan, green, magenta, red, yellow or white - and the surrounding dots affect how strongly we mentally and physiologically react to the illusion. 

On a screen, they presented variations of the 'expanding hole' to 50 women and men with normal vision, asking them to rate subjectively how strongly they perceived the illusion. While participants gazed at the image, the researchers measured their eye movements and their pupils' unconscious constrictions and dilations. As controls, the participants were shown 'scrambled' versions of the expanding hole, with equal luminance and colours, but without any pattern.

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 percent didn't if the hole was in colour. 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 coloured holes prompted their pupils to constrict. For black holes, but not for coloured holes, the stronger individual participants subjectively rated their perception of the illusion, the more their pupil diameter tended to change.

The researchers don't yet know why a minority seem insusceptible to the 'expanding hole' illusion. They also don't know whether other vertebrate species or even nonvertebrate animals with camera eyes such as octopuses might perceive the same illusion as we do.

"Our results show that pupils' dilation or contraction reflex is not a closed-loop mechanism, like a photocell opening a door, impervious to any other information than the actual amount of light stimulating the photoreceptor," said Laeng.

"Rather, the eye adjusts to perceived and even imagined light, not simply to physical energy. Future studies could reveal other types of physiological or bodily changes that can 'throw light' onto how illusions work."