Episodic encoding and retrieval and semantic memory retrieval involve multiple brain areas including prefrontal, and lateral temporal neocortex, as well as the hippocampal formation.
In the present study, 18 healthy rh subjects were investigated with MEG and fMRI using a rapid-presentation event related memory paradigm including trials for episodic encoding, episodic retrieval and semantic retrieval. The same paradigm with a different set of items was used in fMRI and 306-channel MEG. Cortical activation maps were averaged across all subjects and structural and functional MRI were combined with MEG to obtain spatiotemporal maps of cortical activity with high spatiotemporal resolution. Data were compared with results of multi-micro-laminar entorhinal cortex (EC) recordings in one patient with medically intractable temporal lobe epilepsy.
fMRI constrained MEG showed a greater involvement of prefrontal areas for pisodic retrieval vs episodic encoding. Semantic retrieval showed greater ativation of the mesial temporal lobe peaking at ~420-550 msec than episodic retrieval. All conditions show hippocampal involvement in fMRI. Multi-microelectrode recordings in the left entorhinal cortex showed strong current sinks indicating population EPSPs, to all conditions, most strongly in superficial layers to episodic retrieval, with sustained firing in deep layers. Recall evoked strong increases in theta power lasting >1s in deep EC layers and phasic wideband increases in superficial EC layers.
This study shows that the medial temporal lobe is strongly active in both semantic and episodic declarative memory tasks, and suggests that there is a sustained contribution of the entorhinal cortex during intentional retrieval processes, in interaction with prefrontal and lateral temporal sites.