Diffusion MRI Tracks Memories and May Detect Alzheimer’s at Early Stage

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Israeli scientists have developed a new way to monitor the effect of memory on brain structure.

Dr. Yaniv Assaf, from Tel Aviv University’s (Israel) department of neurobiology, developed the approach. “With a specific MRI [magnetic resonance imaging] methodology called diffusion imaging MRI, we can investigate the microstructure of the tissue without actually cutting into it,” he explained. “We can measure how much capacity our brain has to change structurally, what our memory reserve is, and where that happens.”

Dr. Assaf’s study, presented at the annual meeting of the Human Brain Mapping Organization in San Francisco, CA, USA, in August 2009, has been pivotal to the way scientists view the effect of memory on the brain. Scientists used to believe that the brain took days or weeks to change its microstructure. Dr. Assaf’s new observations demonstrate that the microstructure can change in just hours. “It gives us a quantifiable measure of the plasticity of each individual brain,” he said. “It’s possible that before a person experiences any memory loss, the plasticity is affected--that is, the ability of one's brain to adapt to change. A lack of ability for change in the brain could mean susceptibility to dementia. Now, we have the means to monitor this ability.”

To track changes in the brain, Dr. Assaf initiated a study that focused on spatial learning and memory. “Usually, scientists distinguish between functional and structural plasticity,” he stated. Functional plasticity refers to neuronal activity in the brain, while structural plasticity refers to the physical shape of the brain itself. “From animal studies we know that spatial memory tasks have consequences for both.”

First, study volunteers were scanned using diffusion MRI. Then, they were asked to play two hours of a racetrack video game, going over the same virtual racetrack 16 times. “This measured a special form of memory--spatial memory,” noted Dr. Assaf. “Each time they circled the track, the time they took to complete it decreased. At the end of the two hours, we put them back into the MRI to see the difference.”

Dr. Assaf and his team saw a noticeable change measured by diffusion MRI in the characteristics of brain microstructure. The memorization of the virtual racetrack affected the hippocampus, motor, and visual areas of the brain. “The most striking thing about this study is that it shows structural plasticity happening in only two hours,” he noted. “This changes what we think structural plasticity is. It shows that memory is rapidly changing the structure of the cells, and that may lead to a lasting effect on the brain.”

According to Dr. Assaf, most of the research on Alzheimer’s disease and dementia focuses on its aftereffects. Diffusion MRI, he believes, could be used for early detection of the disorder. “We can study the memory capacity of an individual at high risk for these disorders, and compare it to the morphological plasticity of people who are not at risk,” Dr. Assaf commented. “Such an approach may allow us to develop an intervention at an early stage, possibly in the form of drugs, one that may not be appropriate at a later stage.”

Another similar study, now being conducted in collaboration with Tel Aviv University’s Prof. Daniel M. Michaelson, involves working with MRI and animals with mutations of Alzheimer’s.