Abstract
A single blind sham-controlled study was conducted to explore the effects of theta and gamma transcranial alternating current stimulation (tACS) on offline performance on working memory tasks. In order to systematically investigate how specific parameters of tACS affect working memory, we manipulated the frequency of stimulation (theta frequency vs. gamma frequency), the type of task (n-back vs. change detection task) and the content of the tasks (verbal vs. figural stimuli). A repeated measures design was used that consisted of three sessions: theta tACS, gamma tACS and sham tACS. In total, four experiments were conducted which differed only with respect to placement of tACS electrodes (bilateral frontal, bilateral parietal, left fronto-parietal and right-fronto parietal). Healthy female students (N = 72) were randomly assigned to one of these groups, hence we were able to assess the efficacy of theta and gamma tACS applied over different brain areas, contrasted against sham stimulation. The pre-post/sham resting electroencephalogram (EEG) analysis showed that theta tACS significantly affected theta amplitude, whereas gamma tACS had no significant effect on EEG amplitude in any of the frequency bands of interest. Gamma tACS did not significantly affect working memory performance compared to sham, and theta tACS led to inconsistent changes in performance on the n-back tasks. Active theta tACS significantly affected P3 amplitude and latency during performance on the n-back tasks in the bilateral parietal and right-fronto parietal protocols.
Highlights
Functional imaging of the human brain has shown that maintaining information in working memory is associated with activity in the prefrontal cortex (Courtney et al, 1998; D’Esposito et al, 1999; Haxby et al, 2000; Pessoa et al, 2002) and in the posterior parietal cortex (Honey et al, 2000; Pessoa et al, 2002; Todd and Marois, 2004; Mitchell and Cusack, 2008)
The effects of transcranial alternating current stimulation (tACS) on resting EEG activity were examined in delta, theta, alpha, beta, low gamma and high gamma frequency bands
Theta amplitude increased after tACS, whereas during the theta tACS session, theta amplitude decreased after stimulation; these effects depended on the location of the recorded EEG data and on the site of stimulation
Summary
Functional imaging of the human brain has shown that maintaining information in working memory is associated with activity in the prefrontal cortex (Courtney et al, 1998; D’Esposito et al, 1999; Haxby et al, 2000; Pessoa et al, 2002) and in the posterior parietal cortex (Honey et al, 2000; Pessoa et al, 2002; Todd and Marois, 2004; Mitchell and Cusack, 2008). The authors propose that crossfrequency coupling between low (theta, alpha) and high (beta, gamma) frequencies enable processing of distinct working memory information. The latter was operationalized in the theta-gamma coding theory proposed by Lisman and Jensen (2013), which assumes that the maximal number of stored items is limited by the number of gamma cycles that fit into a theta cycle
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