‘Motion Blindness,’ Not Just Poor Memory, Causes Alzheimer's Patients to Lose Their Way
March 23, 1999
A previously unobserved condition in some Alzheimer’s patients that physicians are calling "motion blindness" is a big reason such patients become disoriented and lose their way, asserts a University of Rochester study in the March 23 issue of Neurology.
For years doctors generally have thought that Alzheimer’s patients become lost simply because they forget directions or where they’re going. But the Rochester team has found that while Alzheimer’s patients certainly do have memory problems, those are separate from the motion blindness that is due to brain damage in a highly sophisticated part of the brain that interprets motion.
"People with Alzheimer’s get lost not because they can’t remember where they’ve been, but because they can't see where they’re going," says lead author Charles Duffy, a neurologist at the University’s Strong Memorial Hospital who made the finding using computer patterns that look like snowflakes rushing toward the viewer. "Many of these patients are basically blind to the kinds of cues most of us absorb unconsciously every day. It’s almost like they’re walking around with their eyes closed," says Duffy, associate professor of neurology and ophthalmology and a member of the Center for Visual Science. "It’s a disorder of perception as well as memory."
The finding creates the possibility of pinpointing which patients will encounter serious difficulty driving or getting around their neighborhoods, or even their own homes. That would allow some patients to live independently longer than they otherwise might while alerting other families that they should be extra vigilant in keeping tabs on their relatives. It should also spur providers to encourage patients to refer to specific landmarks that are independent of motion when getting around.
The findings are based on experiments in Duffy’s Visual Orientation Laboratory. Patients sit in a chair and watch computer-generated moving patterns of dots of light on a screen six feet wide and eight feet high. The moving dots form patterns, like snowflakes rushing at the windshield as one drives through a storm, that our brains use to understand how we’re moving. In a car the way the flakes part helps most individuals realize they’re moving forward; Alzheimer’s patients would see the flakes move, but they would have a much harder time understanding what this tells them about their own movement. Interpreting such motion helps us understand while driving down a country road that we’re the ones moving at 60 miles per hour, not the cows or barns that appear to be rushing past.
Like most neurologists, Duffy sees Alzheimer’s patients who regularly complain about getting lost. "I hear the same story over and over from new patients and their families. The world becomes a strange place, and the street they’ve lived on for 30 years becomes unfamiliar." Duffy, who holds both an M.D. and a Ph.D., was part of the research team that several years ago first identified the part of the cerebral cortex that interprets self-movement, and he recognized that the symptoms his patients complained about would result from damage to this brain area.
So with funding from the National Institute on Aging and the National Eye Institute, Duffy decided to test patients’ abilities to interpret the "optic flow field," the scientific term for the motion one sees as a result of one’s own movement. Research subjects sit in front of computer-generated patterns of dots and press buttons to indicate which direction the dots seem to be moving. Together with researchers Sheldon Tetewsky and Hope O’Brien and physician Lisa Lebedovych, Duffy tested six healthy young adults, 12 healthy elderly adults, and 11 elderly Alzheimer’s patients. Scientists found that while the ability to interpret motion diminished slightly with increasing age, people with Alzheimer’s disease were much more likely to suffer from motion blindness. Alzheimer’s patients needed nearly twice as much visual information -more dots traveling in the same direction - to understand the patterns.
In another part of the experiment, research subjects were escorted from the front lobby of the University’s Medical Center to Duffy’s laboratory, then were asked questions about the route. Young people answered 88 percent of the questions correctly, healthy elderly people answered 73 percent correctly, and Alzheimer’s patients got only 32 percent right.
The ability to perceive motion wasn’t diminished in all the Alzheimer’s patients studied. Of the 11, only six did poorly analyzing the optic flow field, and they also had the most difficult time remembering the way to the laboratory. Duffy thinks the research may form the basis for a test that would allow doctors to monitor patients closely, perhaps even predict more accurately the impact of the disease on a patient’s ability to live alone or to drive.
"If these patients can’t find their way, the car keys are taken away and they’re basically locked in their house. This loss of independence can be devastating. Right now doctors don’t have a way of assessing who should be told to stop driving, and who might be safe to continue regardless of age. We’ve identified one ability that may help pinpoint that," says Duffy. He is now setting up another laboratory to use with patients at the University’s main Alzheimer’s treatment site at Monroe Community Hospital, to help monitor patients.
Because the veil of confusion that comes with Alzheimer’s is so evident, most people think that it’s mainly the hippocampus - our memory center - that is ravaged by the disease. But pathologists know that the disease also wipes out neurons in an adjacent part of the cortex where the temporal, occipital and parietal lobes come together. It’s this highly sophisticated processing hub that is home to our ability to interpret motion. While both areas are ultimately damaged, in the early stages the disease usually seems o affect one area more than the other. "Some patients will lose their way, and others will lose their memory," says Duffy. There are patients who continue drive years after diagnosis, though they can’t remember their son’s name or whether or not they’re employed. Others can’t even walk out their front door without getting lost, even though they have a firm grip on the circumstances of their lives.