Switching on the behavioral and neural rhythmicity to retrieve memories when the number of retained items exceeds four

Does our nervous system rely on the same neural mechanism for memory retrieval across all contexts? Here, we report an intriguing phenomenon where the nervous system "switches on" behavioral and neural rhythms under specific conditions, such as when the number of items being learned concurrently exceeds four. In our experiments, participants repeatedly learned and retrieved 2, 3, 4, or 5 color-letter pairs. Analyses of hundreds of reaction times revealed a significant tendency for memory recall to occur at discrete timings corresponding to theta-alpha (4-13 Hz) cycles in the 5-pair condition, while no such behavioral rhythmicity was observed in the 2, 3, and 4-pair conditions. The behavioral rhythmicity showed a clear increasing trend as the trials progressed. This rhythmicity was more prominent when recalling a different item from the immediate previous trial. Analyses of electroencephalographic (EEG) data also revealed an increased cue-locked neural theta-alpha rhythm in the 5-pair condition. Interestingly, the behavioral and EEG rhythms exhibited distinct characteristics, suggesting they each visualize different aspects of the underlying neural rhythm. Our results indicate that our nervous system adaptively engages a neural rhythm, as if switching it on, to achieve memory retrieval in specific contexts.