To the brain, an object's distance seems to be an attribute as basic as its color or shape. Scientists had thought that neurons sensitive to distance would be found mainly in the "where" areas of the brain, which compute spatial relationships between objects. But in tomorrow's issue of Science, researchers report  finding distance-sensitive neurons in parts of the brain's visual system that specialize in determining what, not where, an object is. And in a surprising twist, they found that many of these neurons can determine distances without cues such as stereovision or context.
The researchers, John Allman and Allan Dobbins of the California Institute of Technology in Pasadena, recorded the activity of neurons in monkeys' brains as the animals looked at bars of light displayed on a computer screen at various distances. They identified some neurons that responded best to light bars that produce an image of a particular size on the retina--one kind of distance cue. But other neurons could detect a change in the distance of a bar even when its retinal size didn't change. Some of these neurons stopped registering distance when one eye was covered, suggesting that they depend on binocular cues. Others worked monocularly, but only when the monkey had a broad view of the room and the monitor. And some neurons continued to register distance when both context and binocularity were removed. Allman and Dobbins think those neurons may respond to cues such as the focus of the eye, which varies with distance, or the angle of gaze, which shifts inward toward the nose as an object gets nearer.
These depth-sensitive neurons turned up in the primary visual cortex, the first stop in the brain's visual system, and in a nearby area called V4, which is part of a processing path that specializes in the identity of objects. Neuroscientists say that these neurons could add the third dimension to visual maps in these brain areas, suggesting that the brain exploits distance information to help determine an object's identity, not just its position in space. "In retrospect," says Robert Desimone of the National Institute of Mental Health, "it makes perfect sense" that visual maps in the "what" stream would be three-dimensional. "But honestly," he adds, "I was surprised."