07/23/1998 07:00:00

Brain cells attuned to visual nearness and farness interact to allow judgments of size, research shows

PASADENA—Evolution has been benevolent to humans and other primates in providing us with eyes that can judge the size of nearby objects.

With a visual feature known as "size constancy," we can pretty accurately judge whether the furry thing walking across our field of view is the size of a mouse or the size of a lion, regardless of its distance and whether we recognize the object. Where survival of the species is concerned, the advantage of having size constancy is pretty obvious: it helps us identify dinner, but at the same time helps us stay off someone else's menu.

But the precise neurological nature of size constancy has never been well understood. If we are seeing our very first lion and the lion is walking away from us, then his image in our field of vision is getting smaller and smaller. Distance cues and stereoscopic vision are at play, but what is really happening in our brains? Is the third dimension added on at a late stage in visual processing, or are the images of lions at varying distances actually analyzed at the very first stage of visual perception?

New research from the California Institute of Technology shows that the latter is the case. Our brains need information for object and three-dimensional scaling, and this information is common to all visual cortical areas of the brain.

In the July 24 issue of Science, Caltech biology professor John Allman and his colleagues write that brain cells involved in vision tend to be apportioned to picking up farness or nearness. In working with rhesus monkeys trained to follow dots of varying size on a moving TV monitor, the researchers have found that the monkeys use their nearness and farness cells in tandem.

"The perception of depth is the product of the interaction of the two opposed tendencies, near and far," says Allman. "There are many systems in the body, and several in the visual system, which work by the precise counterbalancing of two opposed tendencies.

"For color perception, for example, you have opposition between black and white, red and green, and blue and yellow," he adds. "So our results show that depth perception is also a fundamental opposition."

Thus, the basic idea is that ability to judge the size of objects is embedded in the primary visual center as a code of opposed interaction of "nearness" and "farness" cells. Therefore, the neurons are pooled for depth perception; lab work with monkeys earning rewards for correct depth identification bears this out.

In addition to Allman, the authors are Jozsef Fiser of the University of Southern California; and Allan C. Dobbins and Richard M. Jeo of the Caltech Division of Biology.

Written by Robert Tindol