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Abstract:
Background: There is controversial discussion regarding the influence of retrieval frequency or remoteness of an event on the memory performance and their dependence on the neurodegenerative processes in Alzheimer's disease (AD). Regarding this issue we conducted several studies with neuropsychological and imaging methods in terms of memory performance in autobiographical and historical events.
Methods: In healthy controls (HC) and AD patients, we examined the recall performance of historical events, depending on the remoteness to the event or its retrieval frequency. The ranking of the retrieval frequency was operationalized through a paired comparison of all events. Furthermore, we investigated in HC subjects and patients with AD the memory performance of autobiographical events depending on the remoteness of the event or its retrieval frequency. With functional magnetic resonance imaging, we then examined in HC subjects and patients with AD the influence of the remoteness to the event or its retrieval frequency on brain activity. In a fourth study, we correlated the dependence of the retrieval frequency for the memory of historical events with the hippocampal degeneration in HC subjects and patients with AD using voxel-based morphometry.
Results: The behavioral data showed that as well in the HC as in the AD groups the memory performance for historical and autobiographical events depended on the retrieval frequency. The memory was accompanied by bilateral activation in the hippocampus, medial temporal lobe, angular gyrus, precuneus and medial prefrontal regions. The activation in this network was reduced in patients with AD, whereas in HC and AD subjects activity in the hippocampus increased with the retrieval frequency. In contrast to rarely retrieved events the memory performance for frequently accessed events was independent of the degree of atrophy of the hippocampus.
Conclusions: In summary, our studies have shown that the memory performance for historical and autobiographical events depends on retrieval frequency and this is associated with an increase in brain activity in the hippocampus. Moreover, the memory performance of frequently retrieved events appears to be less affected by the atrophy of the hippocampus in AD, either by compensatory hyperactivity in this region of the brain, or by additional activity in other brain regions.
