Monday, May 21, 2012

Motion Blindness in Alzheimer’s Patients

The standard consideration among general public is that Alzheimer's patients easily become lost simply because they forget directions or where they're going. But the University of 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.

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"Some 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."

This finding created the possibility of creating more elaborate prognosis of 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.

These 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.

In another part of the experiment, research subjects were escorted from the front lobby of the University's Medical Center to Duffy's laboratory, and 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 examined in the study, 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.

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 to 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.

So, 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. In the follow-up study, Duffy and Michael Froehler confirmed 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.

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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. "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."

Just recently, researchers at the Montreal Neurological Institute and Hospital and the University of Maryland have figured out the scientific background of the MST performance – the mathematical calculations that specific neurons employ in order to inform us of our distance from an object and the 3D velocities of moving objects and surfaces relative to ourselves. Highly specialized neurons located in the brain’s visual cortex, in an area known as MST, respond selectively to motion patterns such as expansion, rotation, and deformation. “Area MST is typical of high-level visual cortex, in that information about important aspects of vision can be seen in the firing patterns of single neurons. A classic example is a neuron that only fires when the subject is looking at the image of a particular face. This type of neuron has to gather information from other neurons that are selective to simpler features, like lines, colors, and textures, and combine these pieces of information in a fairly sophisticated way,” says Dr. Christopher Pack, neuroscientist at The Neuro and senior author. “Similarly, for motion detection, neurons have to combine input from many other neurons earlier in the visual pathway, in order to determine whether something is moving toward you or just drifting past.”

The brain’s visual pathway is made up of building blocks. For example, neurons in the retina respond to very simple stimuli, such as small spots of light. Further along the visual pathway, neurons respond to more complex stimulus such as straight lines, by combining inputs from neurons earlier on. Neurons further along respond to even more complex stimulus such as combinations of lines (angles), ultimately leading to neurons that can respond to, or recognize, faces and objects for example. The mathematical models successfully account for the stimulus selectivity of some of the brain’s complex motion neurons – which are vitally important in helping navigate us through the world.

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