The role of theta and gamma oscillations in item memory, source memory, and memory confidence

Abstract

Theta and gamma oscillations have been linked to episodic memory processes in various studies. Both oscillations seem to be vital for processes guided by the medial temporal lobe, such as the retrieval of information from memory. While theta oscillations increase with successful memory, it is unclear what the unique contribution of theta is to various subcomponents of memory. On the other hand, memory-related gamma oscillations have been mainly reported in the hippocampus, leaving the role of neocortical gamma in memory underexplored. In the current study, we explored how unique variability in memory accuracy and memory confidence contributes to fluctuations in theta and gamma power. To this end, we recorded EEG from 54 participants while they performed a source memory task. From this task we obtained their item memory accuracy, source memory accuracy, item memory confidence, and source memory confidence. These behavioral measures were put in a trial-by-trial linear mixed effects model to uncover their unique contribution to the oscillatory power in frontal and parietal regions. Our results are in line with the involvement of theta oscillations in both memory accuracy and confidence, but seem to indicate a main role for theta oscillations in memory-related confidence. In addition, we found that gamma oscillations play various roles in memory-processing, dependent of brain region.

Figure 1.

Schematic overview of the memory task. In the encoding phase, participants either had to imagine a spatial scene (place task) or rate the pleasantness (pleasant task) of the presented word. They indicated how successful they were in completing this encoding task. In the retrieval phase, participants first made an old/new response. In the case of an ‘old’ response, participants were asked to indicate what encoding task was performed when first encountering that word.

Figure 2.

The relationship between theta power and item memory. (A) Predicted standardized power obtained from the linear mixed effect model for the memory accuracy conditions (hits, misses, correct rejections, false alarms), for frontal and parietal regions. The values shown reflect predicted theta while controlling for the other predictors in the model, like memory confidence (B) Predicted standardized power obtained from the linear mixed effect model for the memory confidence conditions (high-confidence, low-confidence), for frontal and parietal regions. The values shown reflect predicted theta while controlling for the other predictors in the model, like memory accuracy. (C) The difference in power between hits and correct rejections for the frontal EEG channels. (C) The difference in power between hits and correct rejections for the parietal EEG channels. (C) The difference in power between high- and low-confidence for the frontal EEG channels. (D) The difference in power between high- and low-confidence for the Parietal EEG channels. The frequency and time window used for the models is indicated in the lower plots by a rectangle.

Figure 3.

The relationship between theta power and source memory. (A) Predicted standardized power obtained from the linear mixed effect model for the memory accuracy conditions (source hits, source misses), for frontal and parietal regions. The values shown reflect predicted theta while controlling for the other predictors in the model, like memory confidence (B) Predicted standardized power obtained from the linear mixed effect model for the memory confidence conditions (high-confidence, low-confidence), for frontal and parietal regions. The values shown reflect predicted theta while controlling for the other predictors in the model, like memory accuracy. (C) The difference in power between source hits and source misses for the frontal EEG channels. (C) The difference in power between source hits and source misses for the parietal EEG channels. (C) The difference in power between high- and low-confidence for the frontal EEG channels. (D) The difference in power between high- and low-confidence for the Parietal EEG channels. The frequency and time window used for the models is indicated in the lower plots by a rectangle.

Figure 4.

The relationship between gamma power and item memory. (A) Predicted standardized power obtained from the linear mixed effect model for the memory accuracy conditions (hits, misses, correct rejections, false alarms), for frontal and parietal regions. The values shown reflect predicted gamma while controlling for the other predictors in the model, like memory confidence (B) Predicted standardized power obtained from the linear mixed effect model for the memory confidence conditions (high-confidence, low-confidence), for frontal and parietal regions. The values shown reflect predicted gamma while controlling for the other predictors in the model, like memory accuracy. (C) The difference in power between hits and correct rejections for the frontal EEG channels. (C) The difference in power between hits and correct rejections for the parietal EEG channels. (C) The difference in power between high- and low-confidence for the frontal EEG channels. (D) The difference in power between high- and low-confidence for the Parietal EEG channels. The frequency and time window used for the models is indicated in the lower plots by a rectangle.

Figure 5.

The relationship between gamma power and source memory. (A) Predicted standardized power obtained from the linear mixed effect model for the memory accuracy conditions (source hits, source misses), for frontal and parietal regions. The values shown reflect predicted gamma while controlling for the other predictors in the model, like memory confidence (B) Predicted standardized power obtained from the linear mixed effect model for the memory confidence conditions (high-confidence, low-confidence), for frontal and parietal regions. The values shown reflect predicted gamma while controlling for the other predictors in the model, like memory accuracy. (C) The difference in power between source hits and source misses for the frontal EEG channels. (C) The difference in power between source hits and source misses for the parietal EEG channels. (C) The difference in power between high- and low-confidence for the frontal EEG channels. (D) The difference in power between high- and low-confidence for the Parietal EEG channels. The frequency and time window used for the models is indicated in the lower plots by a rectangle.