As Harold Cohen recalls it, his fascination with American Indian petroglyphs began in 1973, when, in a canyon in northern California, he stood gazing at a wall of rock and the pictures some long-dead artist had chisled there. Thirteen years later, Cohen, an artist who uses computers to explore the mysteries of creativity, is still struck by the memory: "It was an extraordinary sight--an escarpment rising from the floor of the desert, so you had a wall about 15 feet high that formed a kind of arc about 100 feet across." It looked like a theater, hewn by nature from the side of a cliff. Onstage were a number of petroglyphs, the ancient, primitive drawings that adorn rocks throughout the Southwest.
"There was a sense of purposefulness about the thing that impressed me enormously," he said. And so, as it did when confronted with a work of art, Cohen's mind began, almost automatically, to search for meaning. The symbols Cohen was trying to decipher were from a culture that had disappeared long ago, leaving no record, no history. There was no way to know what the artist had intended by these strange patterns.
For about a year, Cohen--a British-born artist, and now a UC San Diego professor of art--had been studying as a visiting scholar at Stanford University's artificial-intelligence lab. And his experience with the petroglyphs gave him an idea for how he might strip the process of image-making to its bare essentials and program a computer to create. Over time, he visited more petroglyph sites and, again and again, experienced that same compulsion to interpret. Why, he wondered, do we persist in regarding as meaningful images whose original meanings we cannot know?
Over the centuries, Cohen came to believe, a set of "representational strategies"--rules for image-making--have evolved in the brain. All people--whether they are ancient Indians, medieval Europeans, or modern-day Africans or Californians--share a similar set of wired-in procedures for making pictures. When we are moved by a thought, a feeling, an experience, our brains cause us to put down the same kinds of marks. Thus we can appreciate art, even when it comes from a culture distant in space and time. It doesn't really matter whether the artist's specific message comes across.
More important, Cohen believes that art--whether it's a 20th-Century abstract painting or primitive Indian rock drawing--compels us to begin interpreting the instant we sense those strategies at work. Art is a "meaning generator," not a medium for a message.
Now, if Cohen could figure out what those processes were, he could test his theory by embedding it in a program that would draw primitive images--drawings convincing enough to evoke in a viewer that same feeling of communion with an alien intelligence that he'd felt in looking at the petroglyphs. The viewer would sense a mind at work, and would try to discover the drawing's meaning. But the computer would merely possess a few syntactic rules of drawing.
To begin, Cohen gave his computer program--which he named Aaron--information about three concepts that he considered fundamental to drawing: the difference between inside and outside, between closed and open, and between figure and ground. This knowledge was contained in the form of some 300 "if-then" rules. To begin a drawing, an "expert," called Artwork, would pick a starting point at random. Then Artwork would call on another expert, Planner, to decide what kind of figure to draw--perhaps a curve, which a conventional program might have done by picking one from its memory and then reproducing it, rotating, shrinking, stretching, distorting it in various ways to make a unique figure. But that was not what a human artist would do, and Cohen wanted his program to model the human artistic process. Aaron would have to draw the figure from scratch.
To draw a curve, one must first know how to draw a line. So Planner would call on an expert that knew about lines. It would say, in effect, "Start at point A heading 5 degrees north and end at point B heading 175 degrees south." Now, to get from A to B there are any number of possible paths other than the obvious engineer's curves; so the line expert would call on another expert, Sectors, to break up the line-drawing process into a number of small steps. First Sectors would pick a destination point, a signpost, somewhere between A and B. Then another agent, Curves, would generate instructions that sent the pen veering toward the signpost. Once it was close to the mark, Sectors would stop and reconnoiter. It would see where the pen was and pick a second signpost that would take the line a little closer to its final destination. Then Curves would plot another rough trajectory. After this process had been repeated several times, the line would be complete. The result was a very spontaneous-looking curve.