Enhancing memory capacity by experimentally slowing theta frequency oscillations using combined EEG-tACS

The coupling of gamma oscillation (~ 40+ Hz) amplitude to the phase of ongoing theta (~ 6 Hz) oscillations has been proposed to be directly relevant for memory performance. Current theories suggest that memory capacity scales with number of gamma cycles that can be fitted into the preferred phase of a theta cycle. Following this logic, transcranial alternating current stimulation (tACS) may be used to adjust theta cycles (increasing/decreasing theta frequency) to decrease or increase memory performance during stimulation.?

In this study, individualized EEG-informed theta?tACS?was used to (1) experimentally "slow down" individual theta frequency (ITF), (2) evaluate cognitive after effects on a battery of memory and learning tasks, and (3) link the cognitive performance changes to?tACS-induced effects on theta-band oscillations as measured by post EEG.?

Observed frequency- and task-specific?tACS?after effects demonstrate a specific enhancement in memory capacity. This?tACS-induced cognitive enhancement was specific to the visual memory task performed immediately after?tACS?offset, and specific to the ITF-1 Hz (slowing) stimulation condition and thus following a protocol specifically designed to slow down theta frequency to enhance memory capacity. Follow-up correlation analyses in this group linked the enhanced memory performance to increased left frontal-parietal theta-band connectivity. Interestingly, resting-state theta power immediately after?tACS?offset revealed a theta power increase not for the ITF-1 Hz group, but only for the ITF group where the?tACS?frequency was 'optimal' for entrainment.?

These results suggest that while individually calibrated?tACSat peak frequency maximally modulates resting-state oscillatory power,?tACS?stimulation slightly below this optimal peak theta frequency is better suited to enhance memory capacity performance. Importantly, the results further suggest that such cognitive enhancement effects can last beyond the period of stimulation and are linked to increased network connectivity, opening the door towards more clinical and applied relevance of using?tACS?in cognitive rehabilitation and/or neurocognitive enhancement.

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