New research has added to the evidence that patients with Alzheimer’s disease lose their way not simply because their memory is failing, but because they are subject to a unique form of brain damage that causes symptoms doctors call, “motion blindness.”

In a study published in the journal Cerebral Cortex, researchers from the University of Rochester Medical Center reported new driving test data from a small number of patients, where they have linked motion blindness to the loss of one specific driving skill: the ability to stay in one’s lane while driving.

“While it’s obvious that people with Alzheimer’s disease are losing their memory, that’s only part of the reason why they become lost,” says neurologist Charles Duffy, M.D., Ph.D., who leads the research team. “These patients also lose their ability to perceive their own motion. That’s ultimately what puts them at much greater risk than others of becoming lost.”

In the current study, the team examined 26 elderly patients with Alzheimer’s disease, 50 healthy elderly adults, and 32 healthy young adults. Scientists performed a variety of tests to measure participants’ vision and ability to perceive motion, and then put the Alzheimer’s patients who were still driving through a standard New York State driving test.

As in previous studies, older patients had more difficulty with memory than younger patients, and among older patients, Alzheimer’s patients had a much harder time detecting motion than their healthy counterparts.

In addition, the 11 Alzheimer’s patients who were still able to drive took the New York State driving test. Results showed they performed adequately on all aspects of the test except the portion that measures their knowledge of their location on the road. They tended to drift out of their lane, either across the middle or to the right, and they had difficulty knowing how close or far away they were from the car in front of them.

“These people weren’t just bad drivers. They were bad drivers in a particular way,” says Duffy. “They couldn’t judge where they were in their lane.”

Laura Cushman, Ph.D., a clinical neuropsychologist and associate professor of Physical Medicine and Rehabilitation, arranged the driving tests and analyzed the results. She has found that a combination of three tests, which measure concentration, ability to shift attention between tasks, and the ability to see and respond to useful information, pinpoints potentially dangerous drivers.

The American Academy of Neurology advises doctors to tell all patients with the disease not to drive, but Duffy and Cushman would like to see the recommendation modified to take into account an individual’s driving ability.

“Most doctors agree that in the early stages of the disease, patients can still drive safely, but the family needs to be aware that the disease is progressive, and that at some point, the person will no longer be able to drive, and he or she will need assistance to realize this,” says Cushman. “The only way to assess an individual’s driving ability is to do a real-life test on the road.”

As a result of the small number of drivers tested in the current study, Cushman believes the specific drift-from-lane result should be considered preliminary. Driving skills of both people with Alzheimer’s and healthy people should be studied more closely, she says, before the ability to stay in one’s lane becomes part of any test measuring Alzheimer’s disease.

Duffy first reported the phenomenon of motion blindness three years ago, when he found that many Alzheimer’s patients have much greater-than-normal difficulty detecting motion as they move in the world. While it’s tempting to think that patients get lost, or become disoriented simply because they forget information such as where they live, Duffy found that memory failure explains only a small part of why patients become lost. Instead he discovered that brain damage in a highly specialized part of the brain that interprets motion, leaves the patients unable to perceive motion in a way that most people unconsciously do all the time.

“Relying on your memory to get around is a strategy of the young and healthy,” says Duffy. “In people with Alzheimer’s, once they begin losing their memory, the nervous system adapts and patients instead rely on their perception of self-movement. But once their ability to detect motion goes, they’ve lost all the tools in their tool chest for finding their way around.