Investigating individualised theta transcranial alternating current stimulation (tACS) effects in healthy brain (tACS-theta 1)

Transcranial alternating current stimulation (tACS) is a form of non-invasive brain stimulation that delivers a weak electrical current that alternates at a specified frequency back and forth between electrodes. Theta-tACS involves applying low-frequency alternating current to the scalp to modulate theta brainwave activity. Specifically, the study investigates whether theta-tACS produces greater effects when applied at individualised theta frequency (matched-ITF) or at 1 Hz below ITF (offset-ITF) and comparing both conditions to sham tACS. Administration of tACS will occur via a custom-built tACS-EEG device called the Rio Transceiver, manufactured by eemagine Medical Imaging Solutions GmbH. The device consists of a battery-driven unit that allows for the combined recording of electroencephalography (EEG) signals and administration of tACS. The device delivers a voltage controlled current across stimulation electrodes made of conductive rubber encased in commercially supplied saline soaked sponges and are held in place with a fitted cap. A within-subjects design will be utilised with all participants completing each stimulation condition. Participants will attend 3 sessions where they will complete working memory tasks while receiving one of the 3 stimulation conditions at each visit: 1. individualised theta frequency (aka matched-ITF), 2. offset at 1Hz below the ITF (aka offset-ITF), and 3. sham tACS (aka sham-tACS) The sessions will be held at the Clinical Suites of the John Curtin School of Medical Research, ANU. All procedures are conducted in person and performed under the direction/supervision of a study researcher. Participants will initially be sent an online tACS safety form to complete prior to their first visit. This will determine if they can safely receive tACS and participate in the study. Upon arrival to the initial visit, participants completed tACS safety will be reviewed by the study researcher, consent for participation obtained, and participants will complete a demographic information survey and the state-trait anxiety inventory (STAI). At all visits, participants will also complete 3 visual analogue scales that assess mood, anxiety, and quality of sleep. Initially, electroencephalography (EEG) will be used to record brain activity during a working memory (WM) task in the absence of tACS to determine the fronto-midline ITF for each participant prior to the stimulation period. Participants will undergo the WM task a second time while exposed to a tACS condition. The iTF will be recalibrated during breaks offered throughout the working memory task to account for any drift from the initial ITF determined. The applied tACS frequency will be updated accordingly for subsequent stimulating periods while completing the task. The WM task will be completed a third time in the absence of tACS with EEG, and the same visual analogue scale to assess mood and anxiety will be completed for a second time. Resting EEG will be recorded before and after the completion of each WM task. The WM tasks will be presented in 4 blocks of 5-minute stimulation with a 1-minute rest period between blocks. The WM task used is called n-back task, where letters are systematically presented on the screen. Participants are required to identify target letters which is a current letter that matches a previous letter that was 'n' back. For example, in a 3-back task, the target will be a current letter that matches the letter presented 3 letters prior. For the two active tACS conditions, a peak-to-peak intensity of 1.5mA will be administered for a maximum of 20 minutes, with a 10s ramp-up and 10s ramp-down every time stimulation starts and stops. In the sham condition, the current will be delivered at an intensity of 1.5mA in the first and last 30 seconds at the ITF (including 10s ramp up and down) to emulate tACS associated sensations without producing enduring physiological effects. A minimum of 72 hours and up to 3 weeks is required between sessions.