In the early 14th century, Pope Benedict sent one of his courtiers into Tuscany to fetch samples from the most masterful artists in the land. The courtier stopped by the workshop of the famed painter and architect Giotto di Bondone. Hearing the Pope's wishes, Giotto grabbed a sheet of paper and dipped his pen in ink. Then, bracing his arm against his side, he drew a perfect circle.

The courtier was appalled. "Am I to have no other drawing than this?" he demanded.

"This is enough and too much," Giotto replied coolly. "Send it with the others and see if it will be understood."

Giotto was spot on. Upon receiving the circle, the Pope and his council were awed and "saw that Giotto must surpass greatly all the other painters of his time," as later chronicled by art historian Giorgio Vasari.

Most people can quickly pick out an imperfect circle, but the ability to draw a perfect one freehanded seems to elude all but the legendary Giotto and similarly accomplished masters. This is because the seemingly complimentary tasks of recognizing imperfection and then correcting it to produce perfection actually have little to do with each other—at least so far as our brains are concerned. While the visual cortex handles image processing involved in detecting off-kilter spheres, completely different parts of the brain responsible for coordination and fine muscle control, combined with the complexity of the arm's structure, are to blame for our inability to draw a perfectly round sphere.

Recognizing imperfect circles, unlike drawing perfect ones, comes naturally to us and likely stems from our ability to pick up on subtlest differences in shapes. A survival asset honed over evolutionary time scales, this ability allows us to distinguish a friend's face from a stranger's; avoid poisonous mushrooms but harvest edible ones; and notice a venomous viper concealed among harmless vines. Seemingly across the board, we still retain those abilities today, and they translate into the art and circle-drawing world without any training. In trials with expert artists and non-experts, for example, both groups proved just as adept at spotting which drawings best represented an original image.

"The majority of us don't have a lot of practice in drawing, but we all do have a lot of practice in identifying complex stimuli," says Rebecca Chamberlain, a postdoctoral researcher in the laboratory of experimental psychology at the KU Leuven in Belgium. "Noticing subtle differences in shape and symmetry seems to be quite a fundamental property of our visual system."

Image: Only one of these shapes is a true circle, and you have no difficulty spotting it.

To pull off processing immense amounts of information—from the smallest nuances in an image to the convergent big picture—our visual cortex is organized in a hierarchical fashion, with each of its levels processing increasingly more complex features of an image and feeding the result to the level above. The primary visual cortex, the first brain area to process pixel-like input coming from the eyes, has a precise map of the retina and is highly specialized for detecting patterns, thanks to groups of neurons that each tune in to a certain visual feature. Some, for example, detect edges, such as a bright line on a dark background, while others pick up on small differences in orientation and color. In higher visual areas, others still respond to curvature, ultimately processing circles and other curved shapes.

All of this extremely detailed visual processing gives us enough information to be great circle critics, capable of detecting even the smallest distortions with remarkable precision. But when it comes to correcting those imperfect shapes, we struggle. For starters, circles don't give us much to go on; unlike a square or rectangle, they contain no corner anchors to guide our hand, only a single start and finish point. At the most basic level, though, our symmetrically challenged drawing ability stems from the fact that the joint motion coordination needed to sketch a circle is extremely complex. The brain prefers simplicity and will almost always opt for rotating a single joint, like the shoulder, to accomplish a specific movement of the arm. The rest of the arm then passively follows—without the pesky task of actually having to control an extra set of joints. "This is similar to a whip: you actively move the handle and the entire leash moves passively," says Natalia Dounskaia, an associate professor of kinesiology at Arizona State University. "During most arm movements, the shoulder is used as the handle and the elbow trails behind."

When drawing a circle, however, the brain must leave its comfort zone. "The circle is one of the hardest shapes to control," Dounskaia says. The arm's geometry forces the normally passive elbow to get involved so that together, coordinated shoulder and elbow rotations allow the hand to produce a circular shape, which also further requires incorporating sensory information. "The brain needs to deal with many, many parameters," Dounskaia says. "It's a lot of additional work."

Image: trying to draw a perfect circle is an all-too-familiar exercise in frustration.

All of that extra effort takes a toll, as evidenced by the fact that, when people are asked to speed up their circle drawing, the resulting figures precipitously decline in symmetry. Other studies have shown that, when asked to produce horizontal, straight lines—an easier shape than circles— people draw lines that tend to deviate diagonally up or down, and the effect is amplified when subjects are asked to simultaneously think about something like simple math problems. In both cases, deterioration of drawing quality seems to correspond with a slackening of the elbow as people let their brain revert to a simpler, shoulder-driven formula for coordination. "The brain doesn't have enough resources to focus on corrections of movement and do cognitive tasks at the same time," Dounskaia says.

There is variation, however, in how rubbish peoples' circles are. Those who are good at drawing in general, yet-to-be-published research has shown, are likewise better circle sketchers than those who are less artistically inclined, Chamberlain says. This likely has to do with the fact that artists seem to be better at detecting errors in their drawings as well as at correcting those perceptual anomalies as they go along. "It's very important, this idea that when you go wrong, you can see it and correct it," Chamberlain says. "A non-artist who goes wrong, not just in circle drawing but in any drawing, might be able to notice their errors but not know how to correct them."

For anyone determined enough, however, there is hope to at least approach circle-drawing perfection. Like most things in life, practice improves ability—as attested by the competitors at the World Freehand Circle Drawing Championship. Whether someone would choose to put him or herself through such an intense bout of training only so that their visuo-motor skills might reach the level of Giotto's, however, is another question. "I wouldn't say that drawing a perfect circle is impossible, but it's maybe something that most artists wouldn't spend their time doing," Chamberlain says. "I imagine it would eventually drive you to some kind of madness."