Changes of WM network activity following HD-tACS revealed by simultaneous measurement of fMRI

The working memory (WM) is one of our most essential cognitive functions. Besides, WM deficits are observed in neuropsychiatric diseases such as schizophrenia. Thus, trying to enhance the WM with non-invasive brain stimulation methods like transcranial alternating current stimulation (tACS) seems attractive. In a previous study, we could show that HD-tACS with a frequency of 5 Hz over the left dorsolateral prefrontal cortex (DLPFC) is able to enhance working memory performance. In this work, we directly investigate effects driven by such a tACS by the means of functional magnetic resonance imaging during a WM task.
Healthy subjects are stimulated with high density transcranial alternating current stimulation with a frequency of 5 Hz (theta frequency range) and a stimulation current of 1.5 mA targeting the left DLPFC. Behavioural measures and BOLD response changes compared to a sham condition following a delayed-match-to-sample (DMTS) task comprising the presentation of abstract, non-natural visual stimuli are measured.
We find higher BOLD responses under a tACS compared to a sham stimulation during the encoding phase in frontal areas of the WM network. However, activation patterns following a stimulation differ across subjects.
To our knowledge, this is the first study applying HD-tACS simultaneously with fMRI during a working memory task. Transcranial alternating current stimulation combined with simultaneous fMRI seems feasible also with high-density setups. This method provides a promising tool to investigate changes of neural activity during more focal HD-tACS. Different activation patterns suggest that future studies might benefit from more advanced protocols with individual stimulation models.