WHAT IS ART? Probably as many definitions exist as do artists and art critics. Art is clearly an expression of our aesthetic response to beauty. But the word has so many connotations that it is best—from a scientific point of view—to confine ourselves to the neurology of aesthetics.

Aesthetic response varies from culture to culture. The sharp bouquet of Marmite is avidly sought after by the English but repulsive to most Americans. The same applies to visual preferences; we have personally found no special appeal in Picasso. Despite this diversity of styles, many have wondered whether there are some universal principles. Do we have an innate “grammar” of aesthetics analogous to the syntactic universals for languages proposed by linguist Noam Chomsky of the Massachusetts Institute of Technology?

The answer may be yes. We suggest that universal “laws” of aesthetics may cut across not only cultural boundaries but across species boundaries as well. Can it be a coincidence that we find birds and butterflies attractive even though they evolved to appeal to other birds and butterflies, not to us? Bowerbirds produce elegant bachelor pads (bowers) that would probably elicit favorable reviews from Manhattan art critics—as long as you auctioned them at Sotheby’s and did not reveal that they were created by birdbrains.

In 1994, in a whimsical mood, we came up with a somewhat arbitrary list of “laws” of aesthetics, of which we will describe six: grouping, symmetry, hypernormal stimuli, peak shift, isolation and perceptual problem solving. For each law, we will explain what function it might serve and what neural machinery mediates it.

[break] Pay Attention!

Let us consider grouping first. In a, you get the sense of your visual system struggling to discover and group together seemingly unrelated fragments of a single object, in this case a dalmatian. When the correct fragments click into place, we feel a gratifying “aha.” That enjoyable experience, we suggest, is based on direct messages sent to pleasure centers of the limbic system saying, in effect, “Here is something important: pay attention”—a minimal requirement for aesthetics. Fashion designers understand the principle of grouping. The salesclerk suggests a white tie with blue flecks to match the blue of your jacket.

Grouping evolved to defeat camouflage and more generally to detect objects in cluttered environments. Imagine a tiger hidden behind foliage (d, on page 77). All your eye receives are several yellowish tiger fragments. But your visual system assumes that all these fragments cannot be alike by coincidence, and so it groups them to assemble the object and pays attention. Little does the salesperson realize that he or she is tapping into this ancient biological principle in selecting your tie.

Evolution also had a hand in shaping the appeal of symmetry. In nature, most biological objects (prey, predator, mate) are symmetrical. It pays to have an early-warning alert system to draw your attention to symmetry, leading quickly to appropriate action. This attraction explains symmetry’s allure, whether for a child playing with a kaleidoscope or for Emperor Shah Jahan, who built the Taj Mahal (b) to immortalize his beautiful wife, Mumtaz. Symmetry may also be attractive because asymmetrical mates tend to be unhealthy, having had bad genes or parasites in their early development.

Let us turn now to a less obvious universal law, that of hypernormal stimuli. Ethologist Nikolaas Tinbergen of the University of Oxford noticed more than 50 years ago that newly hatched seagull chicks started begging for food by pecking at their mother’s beak, which is light brown with a red spot. A chick will peck equally fervently at a disembodied beak; no gull need be attached to it. This instinctive behavior arose because, over millions of years of evolution, the chick’s brain has “learned” that a long thing with a red spot means mother and food.

Tinbergen found that he could elicit pecking without a beak. A long stick with a red spot would do. The visual neurons in the chick’s brain are obviously not very fussy about the exact stimulus requirements. But he then made a remarkable discovery. If the chick viewed a long, thin piece of cardboard with three red stripes, it went berserk. The chick preferred this strange stimulus to a real beak. Without realizing it, Tinbergen had stumbled on what we call a “superbeak.” (He later shared the 1973 Nobel Prize in Physiology or Medicine for his work on animal behavior patterns.)

We do not know why this effect occurs, but it probably results from the way in which visual neurons encode sensory information. The way they are wired may cause them to respond more powerfully to an odd pattern, thereby sending a big “aha” jolt to the bird’s limbic system.

What has a superbeak got to do with art? If gull chicks had an art gallery, they would hang a long stick with stripes on the wall, and they would likewise adore it and pay dearly to own one. Art, similarly, stirs collectors to plunk down thousands of dollars for a painting without understanding why it is so compelling. Through trial and error and ingenuity, modern artists have discovered ways of tapping into idiosyncratic aspects of the brain’s primitive perceptual grammar, producing the equivalent for the human brain of what the striped stick is for the chick’s brain.

A related principle, called peak shift, plays a role in the appreciation of caricature or even good portraiture. Features that make a particular face (for example, George W. Bush’s) differ from the “average” of hundreds of male faces are amplified selectively so the result looks even more Bush-like than Bush himself. In 1998 philosopher William Hirstein of Elmhurst College and I (Ramachandran) suggested that cells in the monkey brain that are known to respond to individual faces (such as Joe, the alpha male) will do so even more vigorously to a caricature of the face than the original. This strong response has now been confirmed in experiments by Doris Tsao of Harvard University.

[break] Why Less Is More

We turn to the next two related principles: isolation and perceptual problem solving, or peekaboo. Any artist will tell you that sometimes in art “less is more”; a little doodle of a nude is much more beautiful than a full-color 3-D photograph of a naked woman. Why? Doesn’t this phenomenon contradict peak shift?

To resolve this particular contradiction, we need to recall that our brains have limited attentional resources—an attentional bottleneck results because only a single pattern of neural activity can exist at a time. Here is where isolation comes in. A cleverly contrived doodle or sketch (c) allows your visual system to spontaneously allocate all your attention to where it is needed—namely, to the nude’s contour or shape—without being distracted by all the other irrelevant clutter (color, texture, shading, and so on) that is not as critical as the beauty of her form conveyed by her outlines.

Evidence for this view comes from autistic children with savant skills such as Nadia. She produced astonishingly beautiful drawings, perhaps because, while most of her brain was functioning suboptimally, she may have had an island of “spared” cortical tissue in her parietal lobe, which is known to be involved in one’s sense of artistic proportion. Hence, she could spontaneously deploy all her attentional resources to this one spared “art module.” (Once she grew up and gained other social skills, her artistic skills vanished.) Bruce Miller of the University of California, San Francisco, has shown that even some adult patients who develop a degeneration of their frontal and temporal lobes (called frontotemporal dementia) suddenly develop artistic talents, possibly because they can now allocate all their attention to the parietal lobes.

A related “law” of aesthetics is peekaboo. In the ninth century A.D. Indian philosopher Abhinavagupta discovered this effect, which Austrian-British art historian Sir Ernst Gombrich rediscovered in the 20th century. An unclothed person who has only arms or part of a shoulder jutting out from behind a shower curtain or who is behind a diaphanous veil is much more alluring than a completely uncovered nude. Just as the thinking parts of our brains enjoy intellectual problem solving, the visual system seems to enjoy discovering a hidden object. Evolution has seen to it that the very act of searching for the hidden object is enjoyable, not just the final “aha” of recognition—lest you give up too early in the chase. Otherwise, we would not pursue a potential prey or mate glimpsed partially behind bushes or dense fog.

Every partial glimpse of an object (d) prompts a search—leading to a mini “aha”—that sends a message back to bias earlier stages of visual processing. This message in turn prompts a further search and—after several such iterations and mini “ahas”—we arrive at the final “aha!” of recognition. The clever fashion designer or artist tries to evoke as many such mini “ahas,” ambiguities, peak shifts and paradoxes as possible in the image.

We have barely touched on more elusive aspects of aesthetics such as “visual metaphor,” a pleasing resonance between the visual and symbolic elements of an image. Between the aesthetics of gull chicks and the sublime beauty of a Monet, we have a long journey ahead to truly understand visual processing in the brain. Meanwhile our studies have given us tantalizing glimpses of what the terrain might look like, inspiring us to continue our pursuit.