Program of Electroneurophysiology, Vocational School, Istanbul Medipol University, Istanbul, Turkey.
Section Brain Stimulation and Cognition, Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands.
Sci Rep. 2022 Aug 20;12(1):14199. doi: 10.1038/s41598-022-18665-z.
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. Here, we used individualized EEG-informed theta tACS 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. We found frequency- and task-specific tACS after effects demonstrating 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 tACS at 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, our 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.
伽马振荡（40+ Hz）幅度与持续的θ振荡（6 Hz）相位的耦合被认为与记忆性能直接相关。目前的理论表明，记忆容量与可以拟合到θ周期首选相位的伽马周期数成正比。根据这一逻辑，经颅交流电刺激（tACS）可用于调整θ周期（增加/减少θ频率），以在刺激期间降低或提高记忆性能。在这里，我们使用个体化 EEG 指导的θ tACS 来：（1）实验性地“减慢”个体θ频率（ITF）；（2）评估一系列记忆和学习任务的认知后效；（3）将认知表现的变化与 tACS 诱导的θ带振荡效应联系起来，通过 EEG 后测量。我们发现频率和任务特异性 tACS 后效，表现出记忆容量的特异性增强。这种 tACS 诱导的认知增强是特定于 tACS 关闭后立即进行的视觉记忆任务，并且特定于 ITF-1 Hz（减慢）刺激条件，因此遵循专门设计用于减慢θ频率以增强记忆容量的方案。在该组中进行的后续相关性分析将增强的记忆表现与左额顶叶θ带连通性的增加联系起来。有趣的是，tACS 关闭后立即的静息状态θ功率并没有增加 ITF-1 Hz 组，而是仅增加了 tACS 频率对同步最“最佳”的 ITF 组。这些结果表明，虽然针对个体峰值频率进行校准的 tACS 可以最大程度地调节静息状态振荡功率，但略低于最佳峰值θ频率的 tACS 刺激更适合增强记忆容量性能。重要的是，我们的结果进一步表明，这种认知增强效果可以持续到刺激期之外，并与增加的网络连通性相关联，为使用 tACS 在认知康复和/或神经认知增强中具有更广泛的临床和应用相关性打开了大门。
