For distance, eyes see like ears hear.

We take for granted our ability to judge the depth and distance of objects. To do that, the brain needs both eyes, so it can compute these measurements based on slight differences in how each eye perceives the object. A simple test, holding up a finger and looking at it first with one eye closed and then with the other closed, reveals that this angular difference, or disparity, does indeed exist, as the finger will seem to shift depending on the viewing eye.

Neurobiologists studying visual processing in the barn owl (Tyto alba) now report that the owl's brain perceives depth using the same computations it uses to determine the location of sounds.

To locate a sound, the brain assesses the difference in the time the sound takes to reach each ear. The brain pairs a signal from one ear with that from the other, and specific nerve cells respond depending on the length of the delay between the two signals. Consequently, each of these so-called characteristic delay cells winds up firing most vigorously when that sound comes from a particular place relative to the head. These cells create an auditory spatial map that helps the brain pinpoint sounds, says Hermann Wagner of the Max Planck Institute for Biological Cybernetics in Tubingen, Germany.

Though a hearing researcher, Wagner wanted to test whether what held true for ears also worked for eyes. So he and Barrie Frost of Queen's University in Kingston, Ontario, created visual signals that paralleled the audio signals -- tones and noise -- used to demonstrate how auditory processing occurs. These signals consisted of regular or irregular black-and-white stripe patterns. Wagner and Frost projected moving patterns in front of the owl and placed a prism over one of its eyes to create the illusion of depth or distance. At the same time, the researchers monitored the electrical impulses from 58 sites in the part of the owl's brain analogous to the visual cortex in other animals.

Different nerve cells there do respond to specific angular differences between two visual signals, Wagner and Frost report in the Aug. 26 NATURE. The owl brain contains visual cells with "characteristic disparity" just as auditory nerve cells have characteristic delays, Wagner says. Thus holding the finger 30 inches away would cause one of these nerve cells to fire, while a finger 10 inches away would set off a different one, thereby creating a three-dimensional view of the world.

The experiments help resolve a question that vision researchers have grappled with for years, says Curtis L. Baker Jr. of McGill University in Montreal. "The interdisciplinary aspect of making an analogy to the auditory system in order to get an idea for a new visual experiment shows the value of studying more than one subspeciality," he adds.

In addition, "We speculate that the brain uses similar algorithms to solve similar problems," says Wagner, citing hints that the brain may use this approach in memory, for matching.