Pinpointing the body's global positioning system

A tiny section in the back of the brain is critical for people to understand where they are in the world and how they got there, a sort of mental map that keeps location in context, scientists have found.

Researchers at the University of Rochester (N.Y.) Medical Center report Friday in the journal Science that the region, called the medial superior temporal cortex, functions not only to constantly update our body's position in our surroundings, it also allows us to navigate where we want to go.

The findings, based on recordings of activity in the brains of monkeys as they rode on a motorized sled, show that the section "encodes heading, path and place information" to identify where in the world you are, serving as a biological global positioning system.

"There's a continuous interaction between where you've been, where you're going and where you are," said Dr. Charles Duffy, a neurologist who's been studying the brain region for more than a decade. "What we've done is peeked into that process."

The findings help explain why people with Alzheimer's disease have such a difficult time finding their way and so often exhibit confused, wandering behavior.

Doctors know that brain cells in the medial superior temporal cortex are destroyed in large numbers in those with the disease. Duffy has previously described a condition known as "motion blindness" that explains why Alzheimer's patients lose the ability to keep track of their movements.

"We believe this new discovery will help us develop new ways to treat people with Alzheimer's disease who lose the ability to understand where they are or where they're going," said Duffy, who did the latest experiments with graduate student Michael Froehler.

"For these patients, it's truly a tragedy when the disease reaches a point where they can't find their way through their home town, their neighborhood or even their own home. Often, this is the first step toward the loss of independence," he added.

In treating Alzheimer's patients, Duffy has seen how many struggle to compensate for the loss of critical brain functions.

"One former patient would drive from her home to visit her husband in a nursing home every day. She'd drive a couple of miles, then lose track of where she was, so she'd pull over, get out and walk around looking for a building or some other landmark to remind her of where she was.

"Then she'd get back in the car, drive a couple more miles, lose track of where she was and do the same thing, over and over again, until she reached her destination. She did this day after day."

The woman had lost her ability for dead reckoning, which scientists call path integration. The experiments confirmed that the function is based in that tiny portion of the brain.

"Path integration is crucial to our ability to navigate the world," Duffy said. "It's the difference between turning right and heading down the road, or turning right and ending up in a ditch. In both cases, you're turning right, but the significance of your moment-by-moment heading depends on the context in which in occurs.'

Web posted Tuesday, April 2, 2002

By Lee Bowman
Scripps Howard News Service