Psychologist Examines Role of
Hippocampus in Memory, Aging
February 21, 2000
Impairment in the hippocampus, a portion of the brain critical to establishing or consolidating certain types of long-term memories, contributes to age-related learning and memory deficits in animals, according to Etan Markus, an assistant professor of psychology.
This type of impairment may underlie the memory deficits seen in the elderly and those with Alzheimer's disease.
In two articles published in a recent issue of the journal Neurobiology of Aging, Markus examines the functioning of the hippocampus in rats. Markus' study suggests that memory consolidation, a gradual process by which an experience is transformed into a long-term memory, deteriorates during old age. Although the ability of the hippocampus to process information is reduced as rats age, it continues to contribute to animals' learning and memory, Markus finds.
The current findings indicate that normal aging results in a reduced ability to shift between alternative memory strategies and an increased dependence on a dysfunctional hippocampus.
In the first study, detailed in the article "Hippocampal Dysfunction During Aging I: Deficits in Memory Consolidation," Markus and students Mattison Ward and Jonathan Oler examined memory for a mildly aversive experience in aged and middle-aged rats.
According to Markus' findings, middle-aged rats exhibited the temporally graded retrograde amnesia associated with memory consolidation, while aged rats with hippocampal lesions didn't. The lack of temporally graded retrograde amnesia in the aged rats with lesions suggests an impaired consolidation process.
This difference in hippocampal consolidation may underlie human memory deficits in normal aging and in Alzheimer's disease.
In the companion article, titled "Hippocampal Dysfunction During Aging II: Deficits on the Radial-Arm Maze," Markus and students Mattison Ward and Carl Stoetzel found aged animals continued to rely on a partially dysfunctional hippocampus.
Middle-aged and aged rats from the first study were tested on their ability to remember the location of food in an eight-arm maze. Although the animals quickly learned to avoid the arms without food and not to return to an arm already visited, they made some errors.
For all types of memory errors, removing the aged rats' hippocampus disrupted memory performance beyond the age-related decline.
These findings suggest that although its functioning has degraded, the hippocampus still plays a role in encoding memories in aged rats.
In the second part of the study, the examiners tried to determine whether environmental, visual or spatial cues were guiding the animals' behavior. The results indicated that middle aged rats compensated for a hippocampal lesion by attending to visual cues. Aged animals did not show this compensation.