An optical illusion designed by researchers to test how contrast deceives the brain appears to show a diamond moving across the screen, twitching up and down and left to right, without ever physically changing location.
Dubbed the “Perceptual Diamond”, the illusion “produces motion continuously and unambiguously” to trick the viewer into thinking it is moving around the screen, yet it remains steady and slightly illuminated. Rather, its motion is mimicked by changing the contrast between the edges of strips around the diamond’s edges and the background. Shifts in contrasts around the edges, like in this illusion, can create the perception of motion.
The Perpetual Diamond illusion provides no clues as to its orientation or direction until it is animated, generating movement through contrast signals alone, wrote the authors in i-Perception.
"We often take the perception of motion for granted because we assume that motion corresponds to objects shifting location in the real world," explained study author Arthur Shapiro, from the American University in Washington DC, in an email to IFLScience. "However, the brain has many processes that can lead to the perception of motion, and there are many types of images that can stimulate these processes."
Depending on the combination of illuminated edges, the diamond will appear to move in different directions. For example, if the two top edges blink between black and white and the two bottom edges do the opposite, the diamond appears to continuously move upward.
The authors say that it is a tool for understanding “spatial contrast, temporal contrast, contrast gain, and color contrast” rather than how color or motion, in particular, might influence disconnects between visual and perceived reality. It is different than previous optical illusions of this sort in that the stimulus, the Perpetual Diamond, neither moves nor changes in its brightness or texture but still appears to move in all four directions.
"There are a lot of ways of measuring how we see," said Shapiro, who worked with co-author Oliver Flynn on the illusion. "The most common is an eye chart, which tells us how well we can see fine detail in high contrast. But there are other issues: how well do we see the contrast, how well do we adapt to different lights and lighting environments, and how well do we see different levels of brightness? What aspects of visual function should we measure in order to assess unhealthy visual systems or assess whether treatments for eye disease are effective?"
The Perpetual Diamond is useful because it allows for fast and easy measurements of contrast and acuity, said Shapiro, adding that his students have been using the illusion to assess vision in patients with macular degeneration, glaucoma, and other vision conditions.
“If an illusion is conceived of as a perception that differs from reality (whatever those two terms are believed to mean), then the experience of motion in the Perpetual Diamond is not an illusion: As the contrast modulation creates motion energy, the perception of motion corresponds to a property physically present in the stimulus,” wrote the authors. “The best perceptual story seems to be that the diamond moves and yet remains in the same location.
Fun tip: Shapiro says to do the distance check! Walk away from the screen, and see how far you can go before the effect disappears. Observers in the study were able to correctly identify the seeming direction of motion even when the edges were extremely thin.
"To me, that is probably the most remarkable aspect of the phenomenon," he said.