An Exploration of Optical Illusions: Part 2

Last week, we explored the science of the infamous dress that rocked the Internet – blue and black, white and gold? Today we will continue this theme as we investigate the broader world of optical illusions.

The vision scientist Michael Bach offers a working definition of illusions: they are essentially “a mismatch between the immediate visual impression and the actual properties of the object.” They generally involve a manipulation of shapes, lines, colors, and sizes in order to produce the fictitious perception. Many scientists agree that the term “optical illusions” is misleading. This is because “your perception of an illusion has more to do with how your brain works – and less to do with the optics of your eye.” As a more scientifically accurate alternative, they suggest calling them visual illusions instead.

There are myriad categories of visual illusions, some contrived by man, still others occurring in the natural world. Artistotle was one of the first to describe these natural illusions: “He noticed that if you watch a waterfall and shift your gaze to static rocks, the rocks appear to move in the opposite direction of the flow of water, an effect we now call ‘motion aftereffect’ or the waterfall illusion.” Scientists propose a number of ideas for why this occurs, though none of these ideas are paired with certitude. The most commonly accepted hypothesis for this phenomenon has to do with a sort of neuronal fatigue. That is to say, “Tracking the flow of the water seems to ‘wear out’ certain neurons in the brain as they adapt to the motion. When you then shift your gaze to the rocks, other competing neurons over-compensate, causing the illusion of movement in the other direction.”

Other famous visual illusions demonstrate the importance of depth perception – in other words, “Our ability to determine distances between objects and see the world in three dimensions.” “The Ebbinghaus illusion, for example, revealed that our brain makes judgments about size using adjacent objects – and this can be manipulated…. [Similarly,] the Ponzo illusion illustrated that context is also fundamental for depth perception.” These illusions are pictured below.













Two prominent scientists offer diverging theories to explain how visual illusions work. Mark Changizi’s hypothesis is rooted in our experience of time. According to Changizi, visual illusions occur “due to the brain’s attempt to ‘see’ the future. They occur during the slight time lag after light reaches the retina in your eye, but before your brain translates it into a visual perception… When the brain attempts to generate a perception, it basically is taking a guess at the near future by trying to fast-forward a tenth of a second. As a result of this ‘neural delay,’ you might not be perceiving an image as it actually is, but as you expect it might soon be.”

Susana Martinez-Conde offers a different explanation, based on the notion that although our brain tries to maximize efficiency, it is still a limited and occasionally fallible organ. She says, “’So in some cases, illusions may be due to the brain’s need to take shortcuts.’ Simply put, the brain might need to quickly give more importance to some features in a visual scene than others.”

Regardless of the school of thought to which you subscribe, it’s no question that the human visual system is highly evolved and extremely complex. However, this does not mean it is entirely exempt from error.








– By Betty Vine



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