Compensatory reallocation of brain resources supporting verbal episodic memory in Alzheimer's disease.

Conscious recall of past events that have specific temporal and spatial contexts, termed episodic memory, is mediated by a system of interrelated brain regions. In Alzheimer's disease (AD) this system breaks down, resulting in an inability to recall events from the immediate past. Using subtraction techniques with PET-acquired images of regional cerebral blood flow, we demonstrate that AD patients show a greater activation of regions of cerebral cortex normally involved in auditory-verbal memory, as well as activation of cortical areas not activated by normal elderly subjects. These results provide clear evidence of functional plasticity in the AD patient's brain even if those changes do not result in normal memory function, and provide insights into the mechanism by which the AD brain attempts to compensate for neurodegeneration.