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To be truthful, we are not totally certain how DBS works, but it does help if you understand a few basic facts about the brain. It is simplistic to think that right side of the brain only controls the left side of the body and vice versa. The brain is a complicated collection of neural "networks" that connect distant areas of the brain. Imagine that you are sitting in your air-conditioned office on a warm summer day. The lights, AC, computers and elevators all hum along without a glitch. You hear a light rain hitting the windows, but ten miles away a lightning bolt strikes the relay transformers on the power grid. Your computer blinks off, and suddenly you realize that all the power is gone. Nothing happened at your office that would have cut the power, but since you are on the same power grid as where the lightning struck, all your services shut down. The same thing can happen in your brain.

Think about moving your right arm. There are cells on the left side of the brain that make the actual movement happen, but through a series of networks and connections throughout the brain there are other cells and structures that ensure that your arm moves accurately and smoothly when it reaches for a glass of water and brings it to your lips. To treat the tremors that prevented our patient from drinking his glass of water, the stimulating wires were placed in a small almond sized structure called the thalamus. It acts as a hub and connects the many axons that travel throughout the brain. In this case, the stimulation may keep certain cells from firing (that is, inhibit the cells). But with other diseases, like Alzheimer's, we may want to stimulate specific groups of cells to make them more active. Stimulating different parts of the brain produces different results.

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The use of DBS for essential tremor and Parkinson's disease has become routine in the United States and Europe, improving the lives of over 100,000 people. In addition to these uses, like I mentioned before, new studies explore the use of DBS for depression, obsessive-compulsive disorder, obesity, chronic pain, and Alzheimer's disease (AD). Like so many things, the idea to use DBS for the treatment of AD was serendipitous. While studying the use of DBS in obesity, the researchers observed that it produced increased memory. This led first to safety trials and now to clinical trials.

In Parkinson's disease, DBS treats symptoms, but does not alter the progressive nature of the disease. The question with Alzheimer's disease is whether stimulation can both improve a person's memory and alter the course of the disease. With more than 115 million new cases of AD predicted worldwide in the next 40 years, this would be a huge accomplishment. In AD test studies, the stimulating electrodes are placed in the fornix, a major bundle of over one million nerve fibers that carry information concerned with memory. It connects the hippocampus, so named because it looks like a seahorse, with many other areas of the brain. The hippocampus is intimately involved with memory and is one of the first areas involved in Alzheimer's disease. Unlike Parkinson's disease, where the stimulation attempts to inhibit or suppress nerve cells, we want to use DBS for AD to increase neural activity in this memory circuit. Early in AD, specific areas of the brain related to memory show a decreased utilization of glucose, the brain's fuel. We can measure that decrease with a PET brain scan. The brain scans of the patients who received DBS for AD revealed an improved use of glucose in memory areas of the brain that were near the area of stimulation and also in areas that were remote from the area of stimulation.

Research for these sort of things starts slowly. A 2010 study of just 6 patients found that DBS not only caused a reversal of the brain's decreased utilization of glucose, the brain's fuel, but also may have slowed the rate of cognitive decline. There were a few interesting descriptions during the initial stimulations in two patients. One had "a sensation of being in a garden, tending to plants on a sunny day," while another reported "fishing on a boat on a wavy blue colored lake with his sons and catching a large green and white fish." Both episodes were real life occurrences, memories of which were elicited by stimulation of the fornix, deep inside the patients' brains.