The Combination of Transcranial Alternating Current Stimulation and Electroencephalogram

Summary

This protocol establishes a practical approach for combining HD-tACS with EEG to examine stimulation effects on neural oscillations during cognitive tasks, using theta-frequency stimulation in working memory as an exemplar.

Abstract

Neural oscillations are integral to various cognitive processes, including working memory, attention, and perception. Transcranial alternating current stimulation (tACS) has emerged as a noninvasive tool for investigating causal relationships between brain oscillations and cognitive functions. By targeting specific frequencies, tACS can modulate oscillatory activity, providing insights into the role of neural rhythms in cognitive performance. To understand the impact of tACS on oscillatory dynamics and cognitive processes, it is essential to combine physiological measurements such as EEG with behavioral data. High-definition tACS (HD-tACS) improves the spatial precision of stimulation, enabling more localized targeting of specific cortical areas. This protocol outlines a method for combining HD-tACS with EEG to assess changes in theta-frequency (4 Hz) oscillations in the left parietal cortex. EEG recordings are taken before and after stimulation during a 2-back working memory task to examine theta activity. This approach offers a powerful tool for exploring the neural mechanisms that underlie cognitive modulation via rhythmic brain stimulation.

Introduction

Transcranial alternating current stimulation (tACS) is a noninvasive neuromodulation method that delivers weak sinusoidal electrical currents to the brain at specific frequencies, or combinations of frequencies^1,2. As part of the broader category of transcranial electrical stimulation (tES), which also includes transcranial direct current stimulation (tDCS), transcranial random noise stimulation (tRNS), and transcranial pulsed current stimulation (tPCS)³, tACS is particularly notable for its capacity to modulate endogenous brain rhythms. Unlike other tES modalities, tACS can synchronize with ongoing neural oscillations, thereby enhancing or restoring cognitive functions by entraining neural oscillations at targeted frequencies^1,4,5,6. Furthermore, tACS is recognized as a potential therapeutic option for the treatment of a wide range of psychiatric disorders, such as improving memory performance in patients with Alzheimer's disease⁷ and alleviating depressive symptoms⁸.

While traditional pad-based tACS has demonstrated its potential, its relatively broad current distribution can limit precise targeting⁹. High-definition tACS (HD-tACS) addresses this issue with a 4×1 ring montage, where a central electrode is surrounded by four return electrodes, thereby increasing focality by concentrating current flow beneath the central electrode^10,11,12. This montage can achieve greater intensity and longer durations compared to traditional pad-based tACS^13,14.

Brain oscillations reflect the synchronized activity of neuronal ensembles and are fundamental to various cognitive processes¹⁵, including working memory^10,16. However, it remains an open question whether these oscillations play a causal functional role or merely correlate with cognition. To further reveal a causal relationship, researchers can apply brain stimulation protocols like tACS to modulate oscillatory activity and observe corresponding behavioral changes¹. A comprehensive understanding of how external stimulation influences cognition requires examining not only behavioral performance but also physiological measures such as EEG. EEG allows researchers to monitor whether and how tACS-induced entrainment alters neural activity^1,17.

The massive artifacts generated by tACS made it difficult to reliably analyze EEG signals recorded during stimulation^18,19. Although there are many researchers who attempt to separate tACS-induced artifacts from brain activity, such as PCA²⁰, temporal filtering²¹, and beamforming²², the validity of these attempts is still in doubt¹⁸. After stimulation ceases, the neural population gradually returns to its baseline state, giving rise to an entrainment echo. These entrainment echoes provide a valuable opportunity to assess neural entrainment using EEG, as they occur after stimulation and are thus free from tACS-induced artifacts. However, the short duration of entrainment echoes reduces the likelihood of observing reliable neural entrainment in the EEG. Therefore, a practical approach to recording EEG immediately after HD-tACS stimulation is urgently needed.

The aim of the current protocol is to demonstrate how to combine HD-tACS with high-quality EEG to evaluate the effects of stimulation. By recording EEG before and after a 10 min HD-tACS stimulation (4Hz, theta oscillations) targeted at the left parietal region within a working memory 2-back task, this protocol enables researchers to assess neurophysiological effects of stimulation without tACS-induced artifacts. The 2-back task is a widely used paradigm for assessing working memory, which involves both manipulation and storage of information^23,24. Working memory processes, including those engaged during the 2-back task, are closely linked to theta oscillations in frontal-parietal networks. This provides a strong theoretical rationale for targeting theta-frequency stimulation in this paradigm^16,25,26. To ensure the EEG data quality, we recommend maintaining EEG electrode impedance below 5 kΩ and recording the data in a standard electrically shielded environment. The methods outlined can be adapted for other stimulation targets (e.g., dorsolateral prefrontal cortex, DLPFC), other frequency bands (e.g., alpha oscillations), and other cognitive paradigms (e.g., attention). Stimulation parameters such as intensity and duration, which are critical for the effectiveness of stimulation, can be determined based on the specific research goals. We hope this protocol will facilitate broader applications of tACS-EEG integration and promote more rigorous exploration of neural entrainment dynamics.

Protocol

The protocol was approved by the Institutional Review Board of Southwest University and conducted in accordance with institutional guidelines. This protocol demonstration presents data from one healthy adult participant (age = 25, female) to illustrate the protocol procedures. The participant provided written informed consent prior to the experiment. The equipment used are listed in the Table of Materials.

1. Determination of stimulation parameters

Based on prior research investigating neural oscillations underlying working memory^16,25,26, we selected a 4 Hz stimulation frequency, 10-min stimulation duration, and the left parietal region as the target site for this protocol.

2. Pre-stimulation EEG recording

Baseline EEG data were recorded while participants completed a working memory 2-back task. Prior to the experiment, the participant's scalp was cleaned. All EEG electrodes were filled with conductive gel using a plastic syringe. Specifically, the syringe tip was used to part the hair through the electrode openings, and the gel was injected to ensure direct contact between the electrode and the scalp. The scalp was gently rubbed with the syringe tip until the impedance of each electrode was reduced below 5 kΩ. The EEG braincap used in this protocol contained 64 electrodes arranged according to the 10-20 system. Electrode impedance was monitored by clicking on the Impedance option in the software interface (Figure 1). To ensure high quality of the EEG recording, the impedance display range was set to 0-5 kΩ. Electrodes with impedance values displayed in orange or green were interpreted as below 5 kΩ. Once impedance across all electrodes was confirmed to be below 5 kΩ, EEG recording was initiated by pressing Start in the software as the participant performed the 2-back task (Figure 2). The EEG signals were also visualized during the task (Figure 2). The procedure of the 2-back task in our protocol is shown in Figure 3.

3. Preparation for stimulation

Participants were first assessed to confirm that they had no history of neurological or psychiatric disorders for tES. The participant's scalp was cleaned again prior to stimulation. All necessary materials were prepared in advance (Figure 4). The equipment was assembled as follows²⁷: Batteries were installed and confirmed to be fully charged. The input cable was used to connect the 2-channel tES stimulator to the 4×1 multi-channel stimulation interface. The output cable was then connected to the 4×1 interface, and five Ag/AgCl electrodes were attached to the output cable (Figure 5).

After all device connections were completed, the 2-channel tES stimulator and the 4×1 interface system were powered on. Five plastic high-definition (HD) casings were embedded into specific electrode sites (P3, CP3, P1, PO3, P5) in the EEG braincap, and the cap was then placed on the participant's head (Figure 6).

An electrically conductive gel was applied to the scalp surface through the opening of the HD casing. Using the tip of a syringe, the hair was parted to expose the scalp, and conductive gel was applied directly to the exposed area. Five electrodes were then inserted into HD casings, with electrode No.5 positioned at the center of the ring-shaped configuration (i.e., P3). The remaining four electrodes were positioned around the central electrode at the adjacent sites (i.e., P1, PO3, P5, and CP3; Figure 6). Similarly, all EEG electrodes were filled with conductive gel, and their impedances were confirmed to be below 5 kΩ (Figure 1).

4. Stimulation

Before initiating stimulation, the default mode on the stimulator was confirmed to be set to "SCAN". In this mode, the system displayed the impedance of each electrode individually (Figure 7). The impedance value of each electrode was viewed by pressing the corresponding numbered buttons on the 4×1 interface. Impedance values below 1.5 indicated acceptable quality^27,28. If any impedance value exceeded this threshold, the corresponding plastic casing was adjusted by opening the cap and scrubbing the scalp with the tip of a syringe to obtain the desired impedance value.

Stimulation parameters were set on the 2-channel tES stimulator, including current waveform, duration, and intensity (Figure 7). Four knobs were used to set the respective parameters. A joystick was used to toggle the sham condition on or off. In this protocol, stimulation was delivered in an alternating current waveform, 2 mA and 4 Hz for 10 min. Next, the "RELAX" level was switched to full current. The stimulator mode was then changed from SCAN to PASS using the MODE SELECT button. The stimulation polarity (center-anode or center-cathode) was selected by pressing the POLARITY button.

Stimulation was initiated by pressing the START button on the 2-channel tES stimulator. The current intensity gradually ramped up until the target intensity was reached. The timer showed the remaining stimulation time. If the participant experienced discomfort during the current ramp-up, the intensity could be slightly reduced using the "RELAX" lever. Once the participant felt comfortable again, the "RELAX" lever was gradually pushed forward to resume full current intensity. If necessary, stimulation could be terminated at any moment using the "ABORT" button.

5. Post-stimulation EEG recording

Following stimulation, the plastic HD casings and stimulation electrodes were removed from the EEG braincap. The EEG electrodes previously removed from positions P3, CP3, P1, PO3, and P5 were reinserted into their original locations. Each electrode was gently inserted through its designated hole in the cap, following the 10-20 system layout, and secured by aligning it with the underlying scalp. The tip of a plastic syringe was used to part the hair through the opening to ensure direct contact with the scalp, and conductive gel was applied to each electrode using the same syringe. The impedance of each EEG electrode was then checked and confirmed to be below 5 kΩ before initiating the post-stimulation EEG recording.

Results

EEG data can be analyzed in the time domain (e.g., event-related potentials), frequency domain (e.g., spectral analysis), or time-frequency domain (e.g., time-frequency analysis). In this case, time-frequency analysis was used to examine changes in oscillatory activity associated with stimulation. EEG data acquired pre- and post-stimulation from a representative subject were pre-processed using the EEGLAB toolbox in MATLAB. The pre-processing steps included the following: The continuous EEG was filtered with 40 Hz low-pa...

Discussion

Transcranial alternating current stimulation (tACS) offers a noninvasive approach to modulate specific brain rhythms. Due to its excellent resolution, EEG is well-suited for examining both spontaneous and externally entrained neural oscillations. Combining tACS with EEG provides a multimodal approach to investigate the function and structure of the brain, and to further explore the underlying neural mechanisms of tACS effects. Thus, this protocol presents a practical method for integrating HD-tACS with EEG.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
EEG Braincap	BRAIN PRODUCTS	BC-64-X42-UAMW-56	64 channels based on 10-20 system
4´1 HD-tES	Soterix medical	4´1-C3A	The Soterix 4×1 HD-tDCS / HD-tES adaptor converts any 2-channel Soterix Medical tDCS stimulator into an HD-tDCS / tES device. The 4×1 HD-tES adaptor is not a stand-alone stimulator, and during stimulation does not generate any current. Rather, the 4×1 HD-tES adaptor intelligently divides and guides current generated by a 2-channel device, converting it to HD-tES (HD-tDCS, HD-tACS, HD-tRNS, HD-tPCS, HD-tODCS).