You know ‘em, you love ‘em: Suddenly, pictures bulge out in three dimensions, or static dots begin to swirl, or you see colors that aren’t really there. Tricking your eye is as easy as the well-known “finger sausage” maneuver: Hold out your arms, point your index fingers toward each other, then move them slowly together while gazing at a loosely focused spot behind them. Suddenly, a floating blimp appears between the two fingers. It isn’t there. But there it is. (A phenomenon known to neuroscientists as “cognitive impenetrability.”)

Neuroscientists who specialize in visual perception have long recognized that everything we see is, in essence, an optical illusion. But the illusions we are referring to here are cases where the brain’s subjective perception of a visual image is out of kilter with the physical world. That’s part of their charm. It would not be outlandish to suggest that brain science received an early push from the effort to decode the painterly visual tricks that the 17th and 18th Century masters were able to achieve by brushstroke. The illusions produced by skilled painters were proof that what we see is not always what is there to be seen. Some early painters delighted in a style called trompe l’oeil, French for “trick the eye.” (The practice lives on in Holland, where urinals in men’s public bathrooms often sport a lifelike graphic of a housefly, which, when aimed at — and who does not — makes life easier for restroom custodians.)

However, there is more to this category of illusions than mere amusement. It was Aristotle who first noted that the act of staring at a waterfall for several minutes would cause nearby objects to appear to float upwards. A related illusion, the “flash lag effect,” may affect the outcome of sporting events, for example by influencing a judge’s perception of where a ball touches the ground.  Visual illusions are even employed in highway design, in cases where traffic engineers call for road stripes to be painted closer and closer together as a curve becomes sharper. The idea is that drivers will be fooled by the illusion into thinking that they are speeding up, and will slow down as a result.

Optical illusions represent one of those fertile fields where science meets art. Escher’s “Ascending and Descending” artwork, and the “impossible staircase” drawing by mathematician Roger Penrose were conceived within two years of each other.  Illusions come in a variety of categories: motion, luminance, contrast, color, 3D interpretation, and gestalt effects, for starters. One common set of illusions involves size constancy—comparing two squares and picking the larger one, for example. This illusions stems from the innate tendency of the visual system to “multiply retinal (or angular) size with assumed distance, enabling us to estimate size independent of geometrical perspective,” according to ophthalmologists Michael Bach and Charlotte Poloschek in Advances in Clinical Neuroscience and Rehabilitation.  We possess this ability from birth, and only notice it when it fails, as when the moon appears larger near the horizon.

In fact, optical illusions are not really “optical” at all. In general, they do not result from physical properties of the eye, or the abnormal activation of rods and cones. Optical illusions are truly formed in the “mind’s eye.” They are visual illusions. Consider, as an easy example, the results of a hard blow to the head. Those lights you see aren’t “optical” at all. No actual spots or streaks of light enter your eyes. Instead, the brain incorrectly interprets one of the results of the blow — the mechanical activation of neurons in the eye — as “light.”

In an fMRI study of subjects observing a classic illusion called “Rotating Snakes,” in which a static picture induces the illusion of smooth motion when stared at, researchers determined that the illusion activated motion sensitive areas of the primary visual cortex — but that the effect appeared to be dependent upon “some component of eye movements.” When people moved their eyes while looking at the illusion, the motion detection areas of the visual cortex lit up. No eye movement, no illusion.

Is there a possible evolutionary explanation for these gaps in our visual intelligence? Neurobiologist Mark Changizi at Rensselaer Polytechnic Institute told ABC News that visual illusions are one way the brain attempts to “see into the future” as a means of enhancing survivability. Visual illusions are produced in the split-second interval between light reaching the retina, and the brain’s translation of that retinal image into a visual perception, “Illusions occur when the brain attempts to perceive the future,” says Changizi, “and those perceptions don’t match reality.”

Another theory takes as its starting point the limitations on the amount of input a brain can effectively process — only so many neuronal connections; only so many ways to transmit data. But the potential optical information out there for the processing is practically infinite. Visual illusions may be one result of the way the brain attempts to take shortcuts; to delimit the vast amount of incoming perceptions. The brain’s visual processing structures make inferences, fill in the blanks, and resort to default estimations based on normal rules of perception. The results can sometimes be erroneous.

As it turns out, you can trust your eyes. Your eyes are fine. However, you cannot trust the primary visual cortex in your brain. When it comes to what the eye sees, humans must accept the fact that the brain is prone to leaping to conclusions. It makes inferences, fills in gaps, and draws conclusions — all of which, in the case of visual illusions, turn out to be incorrect.

References

Bach, M., Poloschek, C.M. (2006). Optical Illusions. Advances in Clinical Neuroscience and Rehabilitation, 6, 20–21.

Kuriki I, Ashida H, Murakami I, & Kitaoka A (2008). Functional brain imaging of the Rotating Snakes illusion by fMRI. Journal of vision, 8 (10), 16-10 PMID: 19146358