Looking at this image, do you notice an expansion of the black center as if you are walking into a dark tunnel? Researchers report that 86 percent of people believe the image has this effect. Her closer investigation of this perceptual phenomenon shows that the optical illusion even causes a reaction in the eye: although there is no real change in brightness, the pupils dilate to let in more light, as if it were actually getting darker. This makes it clear that the pupils not only adapt passively to the incident light, but are controlled in a complex way, the scientists say.
We see movement where there isn’t actually any: cogs that seem to be turning, areas that are rocking in the corner of the eye… Although they are actually static, certain patterns in images trigger impressions of movement or change. These optical illusions can amaze us – but they are more than a mere curiosity or gimmick: the perceptual phenomena can give scientists insights into how humans react to visual stimuli and form an image of their environment. In this context, the researchers led by Bruno Laeng from the University of Oslo are now looking at an interesting form of optical illusion that has not yet been scientifically described: the “expanding hole”.
Apparently expanding darkness
The dynamic illusion is created by a circular patch of shadow that fades towards black towards the center and is depicted on a background of small dots. “For most people, this image evokes a strong impression of optical flow – as if the observer is driving into a dark hole or a tunnel,” says Laeng. As part of their study, he and his colleagues examined more closely which factors lead to the perception phenomenon and how the eyes react to the optical illusion. They presented 50 subjects with normal vision with different variants of the expanding hole and asked them to subjectively rate how strongly they perceived the illusion of movement. While the participants looked at the picture, the researchers also recorded the unconscious narrowing or widening of the pupils using an eye tracker.
First of all, it turned out that the effect was strongest in the standard model with a shadow that runs centrally into black. 86 percent of the subjects perceived the illusory expansion in this version. However, about 80 percent also saw the effect when the hole had a white or colored shadow gradient with a bright center. In this case, the impression is created as if you are moving into an area that is becoming brighter. However, the subjective descriptions of the subjects showed that the strength of the illusion varied greatly between individuals in all cases.
Pupils react as they do to changes in light
As the team further reports, perception of the illusion was reflected in eye responses in an interesting way: the researchers found that black holes caused participants’ pupils to dilate, while the colored holes caused pupils to constrict. In the case of the black holes, Laeng and his colleagues were also able to show that the pupil diameter of the participants expanded the more the more intensively they subjectively assessed their perception of the illusion. What is emerging is that the optical illusion tricks the brain into triggering a change in the pupils to adjust for expected changes in light, just as it happens when we actually move into areas of new light.
“Here, using the ‘expanding hole’ illusion, we show that the pupil responds not only to the amount of light energy that actually enters the eye, but also to how we perceive light – even when that ‘light’ is as in the illusion is only imaginary,” says Laeng. “Pupil dilation or constriction is therefore not a mechanism like a photocell that triggers a door opening, for example. Rather, the eye adjusts to the perceived and even imaginary light, not just to the physical effects,” says the scientist.
So far, however, an interesting question remains: Why does a minority seem reluctant to be deceived? The scientists say that there is further research potential. According to them, it would also be interesting to clarify to what extent different animal species perceive the same illusion as we do. “Future studies could uncover other types of physiological or physical changes that can ‘shed light’ on how deception works,” says Laeng.
Source: Frontiers, professional article: Frontiers in Human Neuroscience, doi: 10.3389/fnhum.2022.877249