A tiny section of the brain that is ravaged by Alzheimer's disease is more important for our ability to orient ourselves than scientists have long thought, helping to explain why people with the disease become lost so easily.
The findings by neuroscientists at the University of Rochester Medical Center are reported in today's issue of Science.
Neurologist Charles Duffy, M.D., Ph.D., previously discovered that a small section of brain tissue slightly above and behind the ear -- known as the medial superior temporal area (MST) -- acts much like a compass, instantly updating your mental image of your body's movements through space.
(Editor's Note: See Alzheimer's Patients Get Lost From Motion Blindness.)
In new research, Duffy and graduate student Michael Froehler show that the MST acts not only as a compass but also as a sort of biological global positioning system, providing a mental map to help us understand exactly where we are in the world and how we got there.
The findings help explain why people with Alzheimer's disease have such a difficult time finding their way in the world, Duffy says. Doctors already know that brain cells in the MST die in great numbers in patients with the disease, and four years ago Duffy described a condition known as "motion blindness" that explains why Alzheimer's patients lose the ability to keep track of their own movements.
Now he and Froehler have found that the same region is even more crucial than previously thought for keeping us oriented.
"We believe this 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," says Duffy. "For these patients it's truly a tragedy when the disease reaches a point where they can't find their way through their town, their neighborhood, or even their own home; often this is the first step toward a huge loss of independence. Understanding the damage done by Alzheimer's disease is our best hope toward finding our way to treatments and prevention."
In common with all doctors who treat patients with the disease, Duffy is full of stories about patients who scramble to find routines to make their way through the world despite the void left by the ongoing death of their brain cells.
"One former patient would drive from her home to visit her husband in a nursing home every day. She'd get in the car, 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."
This patient had lost her ability for dead reckoning, or what scientists call path integration -- the ability to figure out where you are based on the knowledge of where you started and how you moved. If your house is two blocks west of a grocery store and you leave your house and walk two blocks west, it's the sense of dead reckoning that tells you to expect a grocery store when you arrive.
It's this ability that Froehler and Duffy discovered resides in the MST. Scientists have known for more than a decade that the MST governs the ability to understand self-movement -- where you're headed at that very moment. The team showed that the same brain region provides some overall context to help identify where in the world you are, much like a global positioning system does. In the example above, the MST not only tells you you're heading west at the moment, but it provides the context and knowledge for you to realize that a grocery store should appear two blocks away.
"There's a continuous interaction between where you've been, where you're going, and where you are. What we've done is peeked into that process," says Duffy. "Path integration or dead reckoning is crucial to our ability to navigate the world. 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 it occurs."
The study was funded by the National Eye Institute, the National Institute on Aging and the Alzheimer's Association. - By Tom Rickey
[Contact: Charles Duffy, Tom Rickey]