Looking at an image and seeing that something just isn’t quite right is always an intriguing experience. From past experience, we expect to see one thing, but often upon immediate observation we see something else quite different. Optical illusions demonstrate to us directly that reality is created by our perceptions of the environment and these perceptions are processed in our brain. So, maybe reality is just all in our heads?

“Reality is merely an illusion, albeit a very persistent one” – Albert Einstein
(a popular misquotation extracted from “For us believing physicists, the distinction between past, present and future is only a stubborn illusion.”  Einstein: His Life and Universe by Walter Isaacson (2008), p. 540)

Classic examples of optical illusions include the floor tiling at the Basilica of St. John Lateran in Rome and the “flashing” grid illusion first reported by Ludimar Hermann in 1870. The twentieth century artist M. C. Escher took the phenomena to an artistic level and created some of the most popular and aesthetically interesting illusions, and many more optical illusions may be viewed with an image search.

Rotating Snakes illusion, Copyright A.Kitaoka 2003

In 2003, Akiyoshi Kitaoka, a professor of psychology at the Ritsumeikan University in Kyoto, Japan, designed a new visual phenomenon called the peripheral drift illusion, or “Rotating Snakes” (read the original report, PDF). In this design, an apparent motion of the image is seen in the observer’s peripheral vision. The effect is strongest when the image contains clearly graduating sections of repetitive diminishing or increasing brightness and these sections follow fragmented or curved edges. A variety of examples of the design can be previewed on Kitoaka’s website of Rotating Snakes.

This visual phenomena has fascinated scientists with the challenge to explain how our brains process this image. It was not until quite recently that an answer may have been experimentally discovered (“Microsaccades and Blinks Trigger Illusory Rotation in the “Rotating Snakes” Illusion”, Otero-Millan, et al. The Journal of Neuroscience, 25 April 2012, 32(17): 6043-6051; doi: 10.1523/​JNEUROSCI.5823-11.2012, Read the abstract). Researchers from the Laboratory of Visual Neuroscience at the Barrow Neurological Institute in Arizona, lead by  Dr. Susana Martinez-Conde, presented “Rotating Snake” images to participants while recording their eye motion with high-resolution. Previously, it had been presumed that the eyes were drifting during observation to create the apparent motion. However, they instead found that when the observers acknowledged motion in the images, their eyes were undergoing small rapid movements called microsaccades. These mini-eye movements represent small jumps in a person’s gaze position that help to refresh the input on retinal receptors during the intentional fixation on an image (“Toward a model of microsaccade generation: The case of microsaccadic inhibition” Rolfs, et al. Journal of Vision, August 6, 2008 vol. 8 no. 11 article 5 doi: 10.1167/8.11.5, Read the full-text PDF).

It is quite amazing to gaze at an image that you consciously know is static, yet you unquestionably see an apparent animation. Your understanding of reality conflicts directly with your observation of reality. For a quick personal experiment to see if I could control this reality distortion, I was able to temporarily pause the motion with a very focused attempt to stare only at one corner of the Rotating Snake image. As I let my focus shift just bit, the rotation immediately re-appeared. It is only a guess as to whether I was inhibiting the microsaccades of my eyes, or if I was positioning the image in some “peripheral blind spot” where the retinal receptors taking input from the eye motions couldn’t receive the input. Nevertheless, I do still feel quite grounded in reality; however, I am reminded to maintain an appreciation of questioning what I directly perceive around me as my brain will continue to work in ways that is beyond my conscious control.


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