Temporal Dynamics of Memory Trace Formation: a MEG Study

In the current study we investigated temporal dynamics of memory trace formation using subsequent memory paradigm (SM) in MEG. SM procedure implies that neural activity in the encoding predicts success in the retrieval. We analyzed time window which included online (during the stimulus presentation) and offline (before and after the stimulus presentation) intervals. Negative and neutral visual stimuli were selected from the International Affective Picture System (IAPS). Before the MEG experiment two preparatory studies were conducted: online ranking study was conducted to select the set of visual simuli for Russian population; behavioral memory study was conducted to determine the optimal duration of the time separating encoding and retrieval. In the main study we compared correctly retrieved items, or hits (total, negative and neutral), with forgotten items, or misses (total, negative and neutral). The purposes of the MEG study were to localize brain regions involved in the formation of episodic memories and to track temporal dynamics of this activity; to analyze memory-related activity in various frequency bands (theta, alpha, beta, gamma and high gamma). We hypothesized that online and offline activity during the encoding that would predict subsequent memory formation; both desynchronization and synchronization in various frequency bands served memorization; memory formation was a result of neural activations in different frequency bands and various regions. Neurophysiological data of 19 participants were analyzed using the following methods: event-related fields (ERFs) analysis was performed as a sanity check for alertness and involvement; time-frequency (TF) analysis in sensor space allowed to estimate general temporal and spectral patterns of memory-related activity; finally, permutation statistics applied to the results of source reconstruction (SR) of spectral power allowed to check for synchronization/ desynchronization patterns in frequency bands of interest. Our results are in line with previous research. SM approach revealed neural activity in the encoding that is significant for the subsequent retrieval. Analysis of temporal dynamics showed that both online and offline neural activity contribute to successful memorization. Synchronization (frontal lobes)/ desynchronization (parieto-temporal regions) interplay in theta, as well as alpha and beta desynchronization are essential features of subsequent memorization. Synchronization in beta however was not described in the literature and we speculate that it might be the pattern specific for the formation of emotional memories with negative modality. All in all, our data support previous findings shows similar patterns of memory formation in three different modalities: hits and misses without consideration of emotional component, negative hits misses, neutral hits and misses. We expect that our findings will provide insights on episodic memory formation and the role of emotional component in the memory story.