See Inside April/May/June 2009

Illusions in Motion: The Power of Symmetry

Our brain's preference for symmetry influences how we perceive motion

THE AESTHETIC APPEAL of symmetry is obvious whether you are a child playing with a kaleidoscope or a Great Mogul emperor building the Taj Mahal as a monument to eternal love. That preference, as it turns out, greatly shapes how we see the world when items are in motion. More on that shortly.

In the natural, as opposed to man-made, world, symmetry—whether we see it in prey, predator, mate or mother—serves as an early-alert system, drawing your attention. Even an infant, who has blurry views of his or her surroundings because of not yet developed acuity, has an innate preference for symmetry. Babies stare longer at faces that have two eyes in the normal position than they do at those that have a cyclopean or a stacked-eyes configuration.

Symmetrical faces tend to be judged as more attractive than asymmetrical ones. Some researchers have postulated that this partiality may be because infestation with parasites can cause visible asymmetry in victims. As a parasite species evolves, it continuously tries to match its surface antigens to those of its host to evade immunological rejection. At the same time, there is a strong selection pressure on the host to be able to detect parasitic infestation and other abnormalities that might potentially reduce fitness and reproductive success. If parasitic infestation occurs sufficiently early in development, it can produce minor deviations from symmetry—hence the adaptive advantage of using asymmetry as a marker to avoid potential mates with poor health, weak genes or a challenged immune system.

Balance on the Whole
Early in the 20th century Gestalt ­psychologists started exploring the perceptual importance of symmetry. They rejected and attacked the prevailing atomistic, or reductionist, approaches to perception. The Gestaltists, beginning with Max Wertheimer, identified “laws” of perceptual organization and emphasized how the relation of all the elements in a scene, rather than the individual elements by themselves, will influence the final perception. For example, three collinear dots suggest a line, but three dots when misaligned will evoke an unmistakable perception of triangleness—even though the display lacks the shape’s hallmark of three lines and three vertices (a).

The illustration in b demonstrates one of the most basic Gestalt principles—organization of a scene into “figure” and “ground.” Even in novel, abstract images, a perceptual division exists between an object, or thing, and the background. Contours are seen to belong to the figure, which is seen to be lying in front of the shapeless ground. Here you see a black vase , but with some time and effort, you should be able to perceive an alternative percept of two white faces in profile in front of a dark ground.

Gestaltists identified many “laws” for determining what is seen as figure or ground in a display. In general, if contours are near each other, they will be perceived as belonging together, as being part of the same figure, a tendency termed the Law of Proximity. If contours are mirror-symmetric, they also will group together and define a figure, known as the Law of Symmetry.

So what happens when symmetry is pitted against proximity? Symmetry tends to dominate; that is, we usually see the shapes defined by mirror-symmetric contours as figure rather than as ground (c). Our brain is choosing symmetry to perceive objects.

Now we return to the idea of considering how symmetry can influence the processing of motion. Let us begin with apparent motion, the illusion of movement that you get when, for instance, two spatially separated spots are presented in rapid temporal succession (as in a string of Christmas bulbs that appear to jump back and forth). Even though the spots/bulbs themselves do not budge, your perception of motion between them is vivid. Because the same brain mechanisms appear to process real motion (your cat walking across the room) and apparent motion (holiday lights), apparent-motion displays provide a convenient tool for the study of motion perception.

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