Scientists create an optical illusion that feels like an expanding black hole

Interestingly, the illusion worked perfectly even when the size or color scheme of the image was changed during the experiments. “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,” Professor Kitaoka told The Sun while explaining how a human eye perceives the image.

So how the black hole illusion works?

The researchers presented different versions of the black hole image in front of the test subjects and tracked the movement of their eyes. The pupil (the tiny black circle in the human eye that regulates the amount of light that falls on the eye’s retina) was found to be dilating when the eyes focused on the black hole image. Also, over 86 percent of subjects felt the illusion most captivating when the color scheme was black.

However, when different colors of holes were used, about 20 percent of individuals couldn’t perceive the illusion. Interestingly, when the subjects were shown white-colored holes instead of black, their pupils underwent contraction. Moreover, during the experiments, the researchers didn’t change the external light settings, but the pupil still didn’t react the same way to the different colored images.      

These findings suggest that the pupil in our eye reacts to light and explains how we perceive it. Bruno Laeng, the lead author of the study and professor of neuropsychology at the University of Oslo, told IE, “we have learned that the pupillary light reflex is not a closed-loop servomechanism (like a photocell opening a door) which is impervious to any other information that the actual amount of light stimulating the photoreceptors. The eye adjusts to perceived (even imagined) light, not simply to physical energy.”

The expanding hole image makes the eyes feel like the black hole in the image is growing and darkness is increasing in the surrounding. This false perception makes the pupil expand, whereas no light-related change has occurred in reality. According to the researchers, pupil dilation might also be a result of our brain predicting a future image as “an illusory expansion” of the large black hole region in the original image. 

Why do optical illusions matter?

The ability of optical illusions to trick our brain, eyesight, and other bodily functions has been discussed in some previous studies as well. For instance, a 2021 study involving 54 subjects reveals that false self-motion sensations can occur if an optical illusion leads to a conflict between a person’s visual and vestibular stimuli perception. Scientists believe that optical illusions are much more than just some time pass brain puzzles.

They serve as essential applications for studying how the human brain functions and responds to different kinds of stimuli. “The study of visual illusions can help us understand some of the basic mechanisms of vision. In other words, the illusions may uncover some of the ‘tricks’ the perceptual system is using to make sense of the visual world and ‘uncover’ some of the underlying assumptions or strategies used by the brain. Our visual input is often ambiguous and consistent with different interpretations,” said Professor Laeng

The study conducted by Kitaoka, Lang, and his colleagues also highlight how an image’s perception affects our visual neural network. Moreover, some previously published research suggests that optical illusions can also serve as essential tools for investigating brain disorders.

Interestingly, the researchers will now test the ‘expanding hole’ illusion on animals. The current study is published in the journal Frontiers in Human Neuroscience. 

Abstract: 

Some static patterns evoke the perception of an illusory expanding central region or “hole.” We asked observers to rate the magnitudes of illusory motion or expansion of black holes, and these predicted the degree of dilation of the pupil, measured with an eye tracker. In contrast, when the “holes” were colored (including white), i.e., emitted light, these patterns constricted the pupils, but the subjective expansions were also weaker compared with the black holes. The change rates of pupil diameters were significantly related to the illusory motion phenomenology only with the black holes. These findings can be accounted for within a perceiving-the-present account of visual illusions, where both the illusory motion and the pupillary adjustments represent compensatory mechanisms to the perception of the next moment, based on shared experiences with the ecological regularities of light.