Alternative neural networks help compensate for Alzheimer's

While verbal memory decline associated with Alzheimer’s disease has been well documented, a similarly relevant area of research has gone relatively uncharted until now. Those affected by the neurodegenerative disease also suffer loss of nonverbal memory function and researchers are mapping the effects of Alzheimer’s disease on brain regions dedicated to this area of cognition and finding newly blazed trails of neural activity, according to a study published April 26 in Neurology.

Karen E. Anderson, MD, of the Taub Institute for Research on Alzheimer’s Disease and the Aging Brain, College of Physicians and Surgeons of Columbia University in New York City, and colleagues are using a series of non-verbal recognition tasks in conjunction with O-15 PET to observe recruitment of compensatory neural networks in the face of neurological damage to regions strongly associated with nonverbal memory.

“Relative fusiform and inferior frontal differences may reflect the Alzheimer disease (AD) patients’ compensatory engagement of alternate brain regions,” wrote the authors. “The strategy used by patients with AD is likely to be a general mechanism of compensation, rather than task-specific.”

The study included 11 Alzheimer’s patients with mild dementia and a mean age of 68 years and 17 cognitively healthy elderly subjects with a mean age of 71 years. Education levels were matched between the groups. All subjects underwent O-15 functional PET imaging during evaluations of nonverbal recognition for cycles of both information encoding and retrieval. 

The study presents a novel approach to cognitive activation, due to its control for level of performance and task difficulty, according to researchers. Participants underwent extensive initial neuropsychological testing. For the functional PET imaging studies, subjects were introduced to looped shapes that could not be identified by verbal memory, as tested in previous studies.

Here patients were given study items and test items to activate both an encoding phase and a recognition phase. Pictures appeared every five seconds in order of difficulty. The shape study list size (SLS) was titrated prior to imaging for normalization of recognition at 75 percent accuracy.

Average SLS was 4.55 shapes for Alzheimer’s patients and 7.53 shapes for healthy controls. Six activation imaging studies and one rest scan were performed for all enrollees and performance according to their SLS was then calculated and compared to functional O-15 PET scans.

Results of the study showed that both Alzheimer’s subjects and healthy elderly controls performed similarly, with Alzheimer’s patients getting a score of 72.17 percent and controls 72.25 percent accuracy. O-15 PET studies displayed higher mean differences in the left fusiform and inferior frontal regions for the titrated tasks and controls.

“The primary finding from the current study suggests that patients with AD engage compensatory brain regions when performing at a similar accuracy level to that seen in elderly controls,” wrote the researchers. “Patients with AD showed greater activation in the fusiform gyrus and inferior frontal gyrus, whereas controls engaged the middle frontal gyrus, suggesting that patients with AD utilized additional posterior neural resources.”

Investigators indicated that Alzheimer’s patients might be using alternate cognitive strategies as a functional coping mechanism, lighting up differential brain regions from healthy controls for the same actions.

“There are three means by which formerly viewed information can be recalled: comparison with items still available in working memory, pattern familiarity using implicit memory (priming), and conscious recognition using explicit memory,” the authors wrote. “Greater activation in frontal regions seen in the patients with AD might suggest that they utilized either short term explicit memory or working memory strategies to complete the task.”

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