Declarative Memory Formation for High- and Low Frequency Words Investigated by Event-related Potentials Recorded Directly from the Medial Temporal Lobe.

Abstract:
Our aim was to explore how subprocesses of declarative memory formation, performed by the rhinal cortex and the hippocampus, are influenced by item novelty and the amount of semantic content. We recorded event-related potentials (ERPs) from medial temporal depth electrodes while epilepsy patients (n=9) studied visually presented high frequency (less novel/ rich semantic content) and low frequency words (more novel /less semantic content). ERPs were sorted into subsequently recalled and unrecalled high- and low frequency words. As subsequent memory effects, rhinal ERPs were more negative between 400 and 800 ms (anterior medial temporal lobe N400: AMTL-N400) and hippocampal ERPs were more positive between 600 and 1600 ms for subsequently recalled than unrecalled words. As frequency effects, the AMTL-N400 and the hippocampal positivity occurred earlier for high- than low frequency words and the AMTL-N400 amplitude was smaller for low- than high frequency words. As an interaction between subsequent memory and word frequency, the hippocampal subsequent memory effect was less sustained for low- than high frequency words. Results indicate that the rhinal AMTL-N400 and its subsequent memory effect reflects rather mnemonic/semantic processing than novelty detection and the hippocampal subsequent memory effect may reflect specific mnemonic processes utilizing the semantic input provided by the rhinal cortex.