Human entorhinal gamma and theta oscillations selective for remote autobiographical memory

Current source densities in different layers of the human entorhinal cortex (ER) were recorded using a linear array of 24 microelectrodes during three memory conditions: a remote autobiographical condition eliciting recollections of events that occurred 10 or more years ago in the participant's past, a semantic icon condition invoking the mental image of a well-known object, and a semantic knowledge condition asking about general information. Our data demonstrate theta, gamma, and delta oscillations in left ER particularly for remote autobiographical memory. Gamma is predominant in hippocampally projecting layers during presentation of the memory cue, whereas theta is prolonged and dominant in cortically projecting layers during memory retrieval. Gamma interactions between ER and hippocampal formation (HF) may elicit an HF contribution to recall under influences relayed from the association cortex (AC). This contribution may then be relayed back to AC during retrieval of the memory orchestrated by theta interactions with ER. Accordingly, theta oscillations were recorded simultaneously in frontal and temporal cortices.