New Research Shows How the Eyes Help the Body Navigate
PASADENA— Neuroscientists have new results on how our brains and eyes work together in getting our bodies from point A to point B without mishap. The research appears in today's issue of the journal Science.
According to Dr. Richard Andersen, James G. Boswell Professor of Neuroscience at the California Institute of Technology, the research shows how different parts of the brain work together to allow for navigation. When a driver is speeding down the interstate and looking at exit signs, for example, he can competently remain traveling in a straight line because of the way his brain is wired. The study in Science reveals more about the precise nature of this wiring.
Andersen's studies of neural impulses in the brain show that humans and animals see things literally in a straightforward way if they are moving and looking directly ahead. When the movement is simple, a fingernail-size area located within the cortex of each hemisphere of the brain an inch or two above each ear interprets the data from the eyes and allows the body to navigate forward. This region is known as the dorso-medial superior temporal area (MSTd), and is sufficient to compute the direction of self-motion as long as the eyes and body are both pointed directly ahead.
But when it becomes necessary or convenient to look around while the body is traveling forward, things get a bit more complicated in the brain. Andersen's research shows that the MSTd area still likes to interpret motion with the same visual neurons at work, but that additional information must come from another part of the brain to help process the more complicated information.
"When the eyes move, the images on the eyes shift as a result of the eye movements," says Andersen. "The brain knows that it's moving its eyes, so a signal about the eye movement is sent to MSTd."
The eye-movement information is then combined with the incoming visual signals to allow for reliable navigation to take place. The process is automatic, and presumably evolved so that humans and animals could walk forward while glancing to the side, Andersen adds.
"We know that the brain has no problem in doing this, just from the fact that people don't drive off the road when they look around," says Andersen.
Though the Science article reveals much about how the brain combines images and motor signals to allow for navigation, Andersen says that additional research should reveal more about the precise mechanism.
For example, the researchers are not yet sure whether motor areas of the brain that are responsible for moving the eyes are indeed projecting the signals back into this sensory area. Another possibility is that sensors in the eye muscles themselves tell the brain what the eyes are doing when they glance about. Also, the researchers would like to find out precisely how the mechanism works when the head rotates but the eyes do not move at all. Again, from everyday experience, humans are able to glance about either by moving the eyes or moving the entire head while moving in another direction, and one would suspect that head movements are also accounted for during navigation. Nonetheless, there are situations in which the eyes can play tricks on the brain, Andersen says. And this research could have several applications in the future for helping humans orient themselves. For example, the knowledge gained could help in the design of better flight instruments for pilots in cases where visual information is misleading.
Also, the research could lead to more realistic flight and driving simulations. And at a more basic level, the research is already providing new insights into the nature of vision and the brain. "The experiments will help us to understand how we perceive and act within the world around us," says Andersen.
The research was funded by the National Eye Institute, the Sloan Center for Theoretical Neurobiology at Caltech, the Office of Naval Research, and the Human Frontier's Scientific Program. Other authors of the paper were David C. Bradley, Marsha Maxwell, and Krishna V. Shenoy, all of Caltech; and Martin S. Banks of the University of California at Berkeley.