The role of synchronized gamma activity in declarative memory formation

Fell, J.1, Elger, C. E.1, and Fernández, G.2
1Department of Epileptology, University of Bonn, Germany; 2F. C. Donders Center for Cognitive Neuroimaging, Nijmegen, The Netherlands

Human declarative memory formation crucially depends on processes within the medial temporal lobe (MTL). These processes can be monitored in real-time by recordings from depth electrodes implanted in the MTL of epilepsy patients, which undergo presurgical evaluation. In our studies, patients performed a word memorization task during depth EEG recording. Afterwards, the difference between event-related potentials corresponding to subsequently remembered versus forgotten words was analyzed. These kind of studies revealed that successful memory encoding is characterized by an early process generated by the rhinal cortex within 300 ms following stimulus onset preceding a hippocampal process, which starts about 200 ms later. Direct evidence for a memory related cooperation between both structures, has been found in a study analyzing induced gamma activity around 40 Hz. Here, successful as opposed to unsuccessful memory formation was accompanied by an initial enhancement of rhinal-hippocampal phase synchronization, which was followed by a later desynchronization. Present knowledge about the function of synchronized gamma activity suggests that this phase coupling and decoupling initiates and later terminates communication between both MTL structures. Furthermore, the memory related changes of gamma synchronization were found to be correlated with increases of rhinal-hippocampal coherence in the theta range. This correlation may suggest an interaction of both mechanisms during memory formation. During sleep a general reduction of rhinal-hippocampal EEG coherence compared to waking state was observed, which was most pronounced within the gamma band. The decrease of rhinal-hippocampal EEG coherence may yield an electrophysiological explanation for the sleep related deficiency of declarative memory.