Study on Theta Oscillations in Hippocampus-Prefrontal Interaction Supporting Sequential Working Memory

Academic Background

The dorsolateral prefrontal cortex (DLPFC) and the hippocampus play crucial roles in sequential working memory, but the specific interaction mechanisms are not yet clear. Previous studies have shown that these two brain regions interact through theta oscillations in episodic memory and spatial navigation, but their specific roles in working memory need further exploration. Some studies have found that hippocampal and prefrontal cortex theta coherence is related to the learning of spatial and object contexts, and lesions or dysfunctions can affect such memory capabilities. The purpose of this study is to explore the interaction mechanisms between the DLPFC and the hippocampus in real-time sequential performance, aiming to reveal their specific roles.

Paper Source

This paper was published in “Neurosci. Bull.”, and its main authors are Minghong Su, Kejia Hu, Wei Liu, Yunhao Wu, etc. The authors are from institutions such as the Institute of Psychology, Chinese Academy of Sciences, Ruijin Hospital affiliated with Shanghai Jiao Tong University, among others. The article was published online on October 17, 2023.

Study Procedure and Experimental Details

Subjects and Methods

The subjects were 20 epilepsy patients (8 females, aged 27.6±8.2 years) who underwent stereotactic electroencephalography (SEEG) recordings at the Functional Neurosurgery Center of Ruijin Hospital. These recordings covered the DLPFC and the hippocampus. The study used a line sorting task where patients were required to arrange lines in a clockwise order and remember their direction after a short delay.

Experimental Procedure

The experiment specifically included the following steps:

1. Line Sorting Task: Participants needed to complete 4 to 6 experimental blocks after a 3-minute practice phase, with each block lasting 9 minutes. Each block included 32 sorting trials and 32 random trials interleaved.

2. Data Collection and Preprocessing: SEEG data were collected from the patients, focusing on the DLPFC and the hippocampus, with data from 56 hippocampal sites and 43 DLPFC sites being collected.

3. Time-Frequency Analysis (TFRs): Morlet wavelet transform analysis was used to obtain power spectra at different task stages.

4. Phase Coherence Analysis: The imaginary coherence method was used to analyze theta phase coherence data between different brain regions.

5. Granger Causality Analysis: Directional influence between the DLPFC and the hippocampus was identified using time-domain causality analysis methods.

Experimental Results

Task Performance

Task performance results showed that the thinking time in random trials was significantly longer than in sorting trials, and the recall error rate was also higher. This suggests that random sorting tasks have higher demands on working memory.

Relationship Between Theta Power and Task Performance

The study found that the instantaneous theta power increase in the hippocampus during the encoding phase was closely related to task performance. In contrast, the theta power increase in the DLPFC was more sustained during the encoding and delay phases. For trials with better task performance, the low-frequency theta (2.5–5 Hz) power in the hippocampus was particularly crucial. Additionally, the study found that theta phase coherence between the hippocampus and the DLPFC significantly increased during random trials, especially for trials with higher memory accuracy.

Directional Influence in Theta Band

Granger causality analysis showed that in random tasks, the influence of the DLPFC on the hippocampus was more significant than the influence of the hippocampus on the DLPFC. This corresponds to the previously mentioned coherence enhancement results, indicating that the DLPFC has a stronger controlling effect on the hippocampus during the processing of sequential information.

Conclusion and Significance

This study found that theta oscillations support the interaction between the DLPFC and the hippocampus in sequential working memory. This finding underscores the importance of the prefrontal cortex and the hippocampus in encoding and manipulating sequential information and suggests that this interaction may be achieved through a competitive queuing mechanism. This is significant for understanding the neural mechanisms of working memory and its applications. Further studies can explore the specific neural activity and anatomical foundations of this mechanism in more depth.

Research Highlights

1. Important Discovery: The study for the first time clearly identifies that the DLPFC and the hippocampus interact through theta oscillations, highlighting their importance in real-time sequential working memory.
2. Relationship Between Task Performance and EEG Activity: Reveals the dynamic changes in theta power in the hippocampus and DLPFC at different task stages and their relationship with memory accuracy, enhancing the understanding of task performance.
3. Directional Influence: Granger causality analysis shows for the first time that the DLPFC has a significant influence on the hippocampus, demonstrating the directionality of brain region interactions.

Other Valuable Information

This study not only provides new insights into understanding the role of brain regions in working memory but also may offer new ideas for the diagnosis and treatment of brain diseases, such as improving memory function by modulating theta oscillations. The current study’s limitation lies in not distinguishing the different roles of the anterior and posterior parts of the hippocampus. Future research may further refine this issue, laying a foundation for a comprehensive understanding of the hippocampus’s role in memory.

Summary

This study innovatively reveals the important role of interaction between the DLPFC and the hippocampus through theta oscillations in sequential working memory. This finding provides new evidence for the study of the neural mechanisms of working memory and offers new directions for future clinical applications and basic research.