Analysis of Time-varying Brain Network Activity Using Functional Connectivity and Graph Theory during Memory Retrieval

In many memory impairment cases, memory failure is due to impaired retrieval and not loss of memory information. Studies on memory retrieval using electroencephalography have mainly focused on changes in power and connectivity in the gamma band, which is a high-frequency region (> 30 Hz). However, previous research has not focused in detail on network activity during memory retrieval. To clarify this, we quantitatively compared retrieval and non-retrieval conditions using network analysis for time-varying functional connectivity. This study analyzed memory retrieval using a paired associative learning task. Consequently, this research found gamma band responses similar to those observed in previous time-frequency analysis in high gamma band. Furthermore, the high-gamma characteristic path length in the target condition was significantly higher than that in the distractor condition. The network becomes more efficient in the non-retrieval condition in the high-gamma band (50-80 Hz). We considered that this was due to the high workload, resulting in distracted memory retrieval. In non-retrieval conditions, participants must focus only on the next stimuli, which may increase network efficiency. We believe that this study shows the potential of a time-varying network analysis for revealing complex brain network activity.