The Eyes Have Trees

M. Changizi, et. al., The American Naturalist, vol. 167, no. 5. © 2006, University of Chicago Press.

Above: Changizi’s periodic table of letter topologies.

If a tree falls in the forest and lands next to another one, does a caveman invent the letter L? He might. According to Caltech postdoc Mark Changizi, a theoretical neurobiologist, letters and other commonly used symbols may have their particular shapes because “these are what we are good at seeing.”

In essence, he says, the basic elements of the Greek and Roman alphabets, plus the Chinese, Persian, and 96 other writing systems that have been used through the years, are visual repetitions of common sights, just as onomatopoeias such as “bow wow” are aural repetitions of common sounds. “Evolution has shaped our visual system to be good at seeing the structures we commonly encounter in nature, and culture has apparently selected our writing systems and visual signs to have these same shapes,” says Changizi, the lead author of a study published in The American Naturalist.

Engineers have known for some time that the best way to create a computer-vision system that recognizes objects is to identify where lines meet. In other words, a robot navigating a room sees the conglomeration of contours in a corner by its “Y” shape, and sees a wall because of its “L” junction with the floor. Says Changizi, “It struck me that these junctions are typically named with letters, such as ‘L,’ ‘T,’ ‘Y,’ ‘K,’ and ‘X,’ and that it may not be a coincidence that the shapes of these letters look like the things they really are in nature.”

So Changizi used topology to group letter and symbol shapes. An “L” can be turned into a “V,” for example, just by bending it, so they are topologically the same. Cutting line segments is not allowed, nor is changing the ways in which they intersect. He ended up with a catalog of 36 shapes made of two or three line segments, which he ranked according to how frequently they occured in three classes of images: pictures of things that ancestral humans would have seen millions of years ago, pictures across many cultures that he culled from National Geographic, and computer-generated architectural forms.

It turns out that the common shapes are precisely those that frequently show up in the letters of various writing systems, in company logos, and in symbolic systems such as musical notation. The forms not found as frequently in nature, by contrast, show up less often.

“It’s striking that symbols that are intended to be seen have high correlations to natural forms,” Changizi says. “Company logos, for example, are meant to be recognized, and we found that logos have a high correlation. Shorthand systems, which are meant to give a note-taker speed at the expense of a commonly recognizable system of symbols, do not. Figures that are intended to be ‘read’ seem to be selected because they are easy to see rather than easy to write. They’re for the eye.”

In addition to Changizi, the authors are Professor of Biology Shinsuke Shimojo and undergrads Qiong Zhang and Hao Ye (BS ’06). —RT