On April 29 th 2019, a research paper entitled “ Spaced Learning Enhances Episodic Memory by Increasing Neural Pattern Similarity Across Repetitions” from Gui Xue's group at the State Key Laboratory of Cognitive Neuroscience and Learning, IDG/McGovern Brain Research Institute at Beijing Normal University was published online on the Journal of Neuroscience. This work provides the first empirical data to show that spaced learning enhances memory by improving the spatiotemporal similarity, which further supports the “neural pattern reinstatement” hypothesis for effective learning (Xue et al, 2010; 2018).

   One of the most robust and fundamental phenomena in learning and memory is the spacing effect. First demonstrated by Ebbinghaus in the late 1800s. Compared with restudying the material in immediate succession (i.e., massed learning), interleaving repetitions (i.e., spaced learning) benefits memory more. Although many cognitive theories and computational models have been proposed to account for the spacing effect, whether and how the neural representations contribute to it remain unknown. Previous studies in Gui Xue's lab have demonstrated that spacing could enhance memory performance and reduce neural repetition suppression, which suggests that spacing improves memory by enhancing the processing strength (Xue et al., 2011; Zhao et al., 2015). However, it remains unclear whether and how this increased processing strength modulates the neural representation and pattern reinstatement to affect memory.

Figure 1. Experimental paradigm and behavioral results

By leveraging the neural representational analysis on scalp EEG data, this study replicated previous finding that greater spatiotemporal similarity that occurs at a late time window is associated with better subsequent memory performance (Lu et al., 2015). More importantly, this memory-related STPS is larger under the spaced learning condition than the massed learning condition and it partially mediates the spacing effect on memory. How could spaced learning generate stronger pattern similarity than massed learning? Consistent with previous findings, this study finds that spacing reduces repetition priming effect and N400 repetition suppression, leading to stronger retrieval processing (more positive-going LPC). This result suggests that spaced learning improves long-term memory by increasing retrieval effort and enhancing the pattern reinstatement of prior neural representations, which might be achieved by reducing the momentary retrieval strength as the extended repetition lags might help to eliminate the residual representation in working memory. This study advances our understanding of the mechanisms of effective learning and the spacing effect.

Figure 2. Spaced learning was associated with greater item-specific STPS, which predicted better subsequent memory performance