Memory performance of past public events depends on retrieval frequency but not remoteness in Alzheimer's disease

Background Patients with Alzheimer's disease (AD) typically develop retrograde amnesia. But whereas some studies found a better preservation of older memories than more recent ones, others could not detect such a temporal gradient (TG). Therefore the views of involvement of the in AD early affected hippocampus in the storage and retrieval of declarative knowledge are ambiguous. In two previous studies we investigated autobiographical memory and remembrance of historic events in patients with AD. While we could show a clear TG in recall of autobiographical information, we did not find a TG for knowledge concerning public events in the same patients with AD. We concluded that recall of more frequently retrieved memories as older autobiographical incidents becomes more independent of the hippocampal formation than more seldomly retrieved remembrances. Consequently, we investigated memory retrieval of past public events in patients with AD in dependency of retrieval frequency with functional magnetic resonance imaging (fMRI). Methods In 28 healthy controls (HC) and 26 patients with AD we assessed the remembrance of historic events of the last 60 years divided in 4 time segments. Retrieval frequency was operationalized by a paired comparison analysis. The events were matched head-to-head with each other and the subjects were required to estimate which of the two events was more often remembered during lifetime. In the scanner an imagination task was chosen as control task. Results As well for the HC group as for the patients with AD we could show a better remembrance of frequently retrieved historic events. No influence of time segment was found. Neuronal network of memory of historic events comprised hippocampus, parts of the medial temporal cortex, precuneus, gyrus angularis and medial prefrontal cortex. Brain activity in this network was reduced in AD. In both groups activity in hippocampus and precuneus increased with retrieval frequency but did not differ with time period. Conclusions The results of the present study suggest a dependency of memory performance on retrieval frequency accompanied by enhancement of brain activity in hippocampus and precuneus in HC and AD. Thus, we conclude that cognitive activities can delay the onset of memory decline in persons who develop AD