When you jaywalk, your ability to keep track of that oncoming truck despite your constantly changing position can be a lifesaver. But scientists do not understand how such constant updating of depth and distance takes place, suspecting that the brain receives information not just from the eye but also from the motion-detecting vestibular system in the middle ear.

In studies with monkeys reported in the October 6, 2005, issue of Neuron, Nuo Li and Dora Angelaki of Washington University School of Medicine in St. Louis have demonstrated how such depth motion is updated and strongly implicated the vestibular system in that process.

In their experiments, the researchers trained the monkeys to perform memory-guided eye movements. The animals were first shown a light a fixed distance away from their head. Then the researchers flashed one of eight other, closer "world-fixed" target lights. Next, with the room lights turned off, the monkeys were moved either forward or backward and the fixed-distance light flashed, signaling the monkeys that they should look at where they remembered the world-fixed light had flashed. Finally, the room lights and target light were turned on, so the monkey could make any corrective eye movement to the re-lit target. For comparison, the researchers also conducted experiments in which the monkeys were not moved.

Such an experimental design using passive motion enabled the researchers to study depth-tracking in the absence of any clues the monkeys might have gleaned from their own motor movements--leaving vestibular system as the most likely source of information.

Finally, the researchers eliminated the vestibular systems in two of the monkeys and performed the same eye-movement experiments.

They found that the eye motion of monkeys in the first experiments indicated that they were clearly able to update their perception of the depth of the target, even in the absence of information from their own motor movements. By contrast, the monkeys that lacked vestibular systems showed compromised ability in the task.

"These results demonstrate not only that monkeys can update retinal disparity information but also that intact vestibular motion cues are critical in reconstructing three-dimensional visual space during motion in depth," concluded Li and Angelaki.

The researchers include Nuo Li and Dora E. Angelaki of Washington University School of Medicine in St. Louis. The work was supported by NIH grants.