Enhancement of Theta and Alpha Oscillations Prior to Crossmodal Memory Encoding

Background

Episodic memory is a crucial component of human memory, with one of its core mechanisms being the formation of associations through stimuli from different sensory channels. Current theories suggest that during crossmodal associative encoding, the phase and power of theta band (3-7 Hz) oscillations play functional roles. Furthermore, sustained oscillatory activity in the theta band (3-7 Hz), alpha band (8-12 Hz), and low beta band (13-20 Hz) prior to stimulus presentation regulates subsequent memory processes and cognitive processing.

Research Question

This study aims to test the hypothesis that low-frequency pre-stimulus activity characteristics are associated with the successful formation of crossmodal memory. The research design is specifically tailored to investigate associative memory independent of single item memory. Participants were required to remember audiovisual stimulus pairs and differentiate them from newly rearranged pairs in a subsequent recognition task.

Source of the Paper

The study was authored by Jan Ostrowski and Michael Rose and published in the journal “Scientific Reports” as article number 7895 in the 2024 volume. Both authors are from the Department of Systems Neuroscience, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.

Overview of the Research Process

Experimental Design

In the study, participants (N = 51) needed to remember a series of audiovisual stimulus pairs and distinguish them in a later task. The experiment was divided into encoding phases, an interference task, and subsequent recognition phases. Out of a total of 55 participants, data from 51 participants were included in the analysis due to data quality considerations.

Encoding Phase

In the encoding phase, participants were required to remember a series of simultaneously presented audiovisual pairs, followed by a recognition test. During encoding, partially related audiovisual pairs were presented together, and participants were instructed to judge whether these pairs were related to animals and to memorize the stimulus pairs with full effort.

Interference Task

The interference task required participants to count backward from numbers like 100, 115, and 125 at different intervals to prevent further thinking about the content of the encoding phase.

Recognition Phase

The recognition phase presented the pairs from the encoding phase along with new disturbed pairs, and participants were asked to judge whether the pairs in memory were the same as before.

Main Results

Behavioral Data Analysis

Participants had an accuracy of 92.48% in the encoding task. In the recognition task, the average hit rate was 52.98%, and the false positive rate was 14.27%. The d’ value, as a measure of sensitivity, indicated that memory performance was significantly above chance level.

EEG Data Analysis

Preprocessing

EEG data were collected using a 60-electrode setup and high-pass filtered to remove low-frequency fluctuations. Independent Component Analysis (ICA) was used to identify and remove components corresponding to eye blinks and lateral eye movements.

Power Analysis

Power analysis revealed significant differences between successful (rem) and unsuccessful (notrem) trials during the pre-stimulus period and early encoding phase (from -1s to 0.9s), with rem trials showing significantly higher theta (3-7 Hz), alpha (8-12 Hz), and low beta (13-18 Hz) power than notrem trials.

Phase Connectivity Analysis

Phase-based connectivity analysis among different electrodes showed no significant difference in functional connectivity between visual and auditory regions for remembered and forgotten trials during the pre-stimulus and post-encoding phases.

Conclusions

The study confirmed that increases in theta and alpha oscillation power during the pre-stimulus period benefit crossmodal memory formation. Moreover, the d’ value of memory performance positively correlated with differences in theta and alpha oscillation power before and after the stimulus, indicating a functional relationship between pre-stimulus features of theta and alpha waves and successful encoding. However, the lack of phase connectivity differences suggests that only theta power, not phase connectivity, plays a critical role in memory formation.

Significance and Value of the Research

This study emphasizes the role of theta and alpha waves in crossmodal associative memory and indicates that the brain state during the pre-stimulus period influences subsequent processing. By exploring oscillatory activities across different frequency bands, the research suggests their joint role in the memory encoding process. Further studies are needed to clarify the interactions of these oscillations with hippocampal-cortical EEG synthesis and to explore the synergistic actions of other relevant oscillatory bands and brain regions.

Research Highlights

1. Functional Relevance of Theta and Alpha Waves in Memory Formation: Increased theta and alpha oscillations during the pre-stimulus period significantly benefit crossmodal memory formation.
2. New Perspective on Crossmodal Memory Encoding: By analyzing differences in theta, alpha, and beta wave power and their relationship with memory performance, the study reveals the fine mechanisms involved in crossmodal associative memory formation.
3. Multiband Synergistic Action: Theta frequency might regulate crossmodal information binding, while alpha and beta frequencies might support task-specific inhibition and semantic processing, respectively.

Methodological Discussion

The study employed a multi-step data acquisition and processing process, including Independent Component Analysis, high-pass filtering, and time-frequency decomposition, ensuring data accuracy and reliability. Additionally, by combining the ICC algorithm and non-parametric permutation test methods, the scientific authenticity of the results was further enhanced.