Footnote of the Month: June 2010

“Stereo Sue,” a chapter in The Mind’s Eye, is about Sue Barry, a neurobiologist who suddenly acquires stereopsis, and true three-dimensional vision, in her fifties.  After a lifetime of inferring depth by other monocular cues such as perspective and motion parallax, Sue is stunned by the beauty of her new sense of 3-D space and depth. While most of the the chapter is about Sue’s experiences, Dr. Sacks also points out how stereo vision is important for animals:

Stereopsis, as a biological strategy, is crucial to a diverse array of animals. Predators, in general, have forward-facing eyes, with much overlap of the two visual fields and, presumably, stereoscopic vision; prey animals, by contrast, tend to have eyes at the sides of their heads, which gives them panoramic vision, helping them spot danger even if it comes from behind. An astonishing strategy is found in cuttlefish, whose wide-set eyes normally permit a large degree of panoramic vision but can be rotated forward by a special muscular mechanism when the animal is about to attack, giving it the binocular vision it needs for shooting out its tentacles with deadly aim.

In primates like ourselves, forward-facing eyes have other functions. The huge, close-set eyes of lemurs serve to clarify the complexity of dark, dense close-up foliage, which, if the head is kept still, is almost impossible to sort out without stereoscopic vision—and in a jungle full of illusion and deceit, stereopsis is indispensable in breaking camouflage. On the more exuberant side, aerial acrobats like gibbons might find it very difficult to swing from branch to branch without the special powers conferred by stereoscopy. A one-eyed gibbon might not fare too well—and the same might be true of a one-eyed lemur or cuttlefish.

From The Mind’s Eye, Chapter 5.