80 Hz but not 40 Hz, transcranial alternating current stimulation of 80 Hz over right intraparietal sulcus increases visuospatial working memory capacity

Abstract

Working memory (WM) is a complex cognitive function involved in the temporary storage and manipulation of information, which has been one of the target cognitive functions to be restored in neurorehabilitation. WM capacity is known to be proportional to the number of gamma cycles nested in a single theta cycle. Therefore, gamma-band transcranial alternating current stimulation (tACS) should be dependent of the stimulation frequency; however, the results of previous studies that employed 40 Hz tACS have not been consistent. The optimal locations and injection currents of multiple scalp electrodes were determined based on numerical simulations of electric field. Experiments were conducted with 20 healthy participants. The order of three stimulation conditions (40 Hz tACS, 80 Hz tACS, and sham stimulation) were randomized but counterbalanced. Visual hemifield-specific visual WM capacity was assessed using a delayed visual match to the sample task. High gamma tACS significantly increased WM capacity, while low gamma tACS had no significant effect. Notably, 80 Hz tACS increased WM capacity on both the left and right visual hemifields, while previous tACS studies only reported the effects of tACS on contralateral hemifields. This is the first study to investigate the frequency-dependent effect of gamma-band tACS on WM capacity. Our findings also suggest that high gamma tACS might influence not only WM capacity but also communication between interhemispheric cortical regions. It is expected that high gamma tACS could be a promising neurorehabilitation method to enhance higher-order cognitive functions with similar mechanisms.

Causal functional maps of brain rhythms in working memory

“…The available data features a total of 39 different stimulation electrode positions ( 21 – 23 , 29 – 53 ), each resulting in a unique combination of anatomical target and off-target brain areas. The frontal cortex was targeted most frequently, as was indicated by the largest average induced electric field across studies ( Fig.…”

Attention improvement to transcranial alternating current stimulation at gamma frequency over the right frontoparietal network: a preliminary report

“…In addition to offering a research gateway to modulate brain rhythms and cognitive processes, studies have begun to support tACS as a treatment option for neuropsychiatric conditions (Lee et al, 2023, Manippa et al, 2024b). Previous research has investigated the potential of tACS at the gamma range to ameliorate various cognitive dysfunctions, predominantly episodic memory and working memory (Park et al, 2022, Varastegan et al, 2023, Manippa et al, 2024a, Wischnewski et al, 2024). Relatively few studies have delved into its impact on attention function (Hopfinger et al, 2017, Wostmann et al, 2018).…”

Transcranial alternating current stimulation over multiple brain areas with non-zero phase delays other than 180 degrees modulates visuospatial working memory performance

“…In this study, we attempted to induce a systematic phase delay of the gamma band (80 Hz) in the bilateral parietal areas and to explore how such a phase delay modulates VWM performance. The stimulation frequency (80 Hz) was selected over 40 Hz, a conventional gamma tACS frequency, because 80 Hz affected VWM capacity significantly while 40 Hz did not 35 . To deliver focal msmp-tACS, we introduced a new method to simulate and optimize tACS protocols using complex values.…”