Spatiotemporal patterns of transcranial electrical stimulation can strengthen the metamemory of individual episodic memories in humans
Targeted memory reactivation (TMR) during slow-wave oscillations (SWOs) in non-rapid eye movement (NREM) sleep has been demonstrated with sensory cues to achieve about 5-12% improvement in post-nap memory performance on simple laboratory tasks. But prior work has neither addressed the one-shot aspect of episodic memory acquisition, nor dealt with the presence of interference from ambient environmental cues in real-world settings for the sensory cues. Moreover, TMR with sensory cues may not be scalable to the multitude of experiences over ones lifetime. We designed a novel non-invasive paradigm that tags one-shot experiences of minute-long naturalistic episodes within immersive virtual reality (VR) with unique spatiotemporal amplitude-modulated patterns (STAMPs) of transcranial electrical stimulation (tES) and cues them during SWOs. In particular, we demonstrate that these STAMPs can be re-applied as brief pulses to temporally coincide with UP states of SWOs (0.4167 - 1 s) on two consecutive nights to achieve about 20% improvement in the metamemory of targeted episodes at 48 hours after the one-shot viewing, compared to the control episodes. Post-sleep metamemory of the targeted episodes was driven by an interaction between their pre-sleep metamemory and the number of STAMP applications for those episodes during sleep. Overnight metamemory improvements were mediated by spectral power increases from 6.18 to 6.7 s following the offset of STAMPs in the slow-spindle band (9-12 Hz) for left temporal areas in the scalp electroencephalography (EEG) during sleep. These results prescribe an optimal strategy to leverage STAMPs for boosting metamemory and suggest that real-world episodic memories can be modulated in a targeted manner even with coarser, non-invasive spatiotemporal stimulation.