The overall goal of this protocol is to demonstrate how to successfully set up concurrent transcranial alternating current stimulation and EEG, with important technical considerations in order to acquire a successful recording. Transcranial alternating current stimulation, or tACS, is a method to experimentally control rhythmic activities in the brain. And to study the cordal association of these rhythmic activity with behavior.
EEG offers high temporal resolution data for studying endogenous and entrained authority annular activity. Simultaneous EEG recording during the tACS can help answer key questions concerning the immediate neurophysiological effect of tACS. This, in turn, can help to further understanding of the rhythmic dynamics of the brain, as well as understanding the mechanisms and optimizing parameters of tACS, for clinical neuromodulatory applications.
This experimental protocol can also be adapted for other montages and cortical targets. This video will demonstrate a montage with tACS electrodes placed on the shoulder and the scalp. Begin by preparing the skin of the shoulder for the tACS electrode by using a gauze pad and an abrasive skin gel.
Apply electroencephalography, or EEG, gel with appropriate viscoscity on the tACS electrode and place the electrode on the shoulder. Secure the electrode on the shoulder with adhesive tape. Next, put the EEG cap on the participant.
Adjust the position of the cap according to the International 10-20 system for electrode positioning and fasten the chin strap. Use a water-based red pen to mark a spot indicating where to position the tACS electrode on the scalp. If the pen does not reach the scalp, paint the tip of a wood stick and use this tip to mark the scalp.
Remove the EEG cap and check if the marking was successful. Fill in the marking if needed so that it can be easily spotted later. Next, for participants with long hair, place the tACS electrode on the head with its center marked by the red spot on the scalp.
Thread out all the hair inside the inner ring of the tACS electrode, and bind the threaded-out hair with cable binders. Ensure that the hair located around the tACS electrode does not get bound up by the cable binders. After the hair has been bound, remove the tACS electrode.
Next, connect the scalp and shoulder tACS electrodes to the stimulator, but do not turn it on. Apply a thin layer of EEG gel onto the tACS electrode and carefully place it back on the head. If the participant has longer hair, thread the bound hair back through the inner hole of the tACS electrode without letting the hair touch the EEG gel.
Ensure that the red mark on the scalp is in the middle of the tACS electrode, as the position may no longer be changed once placed. Then, remove the cable binders from the hair. Now, turn on the stimulator and monitor the impedance.
While carefully putting some pressure on the tACS electrode, pay very close attention that the red marking spot is always kept in the middle of the tACS electrode. Carefully lift the edges of the tACS electrode and apply some more EEG gel beneath the hair. Continue putting pressure on the tACS electrode until impedance is stably below 10 kilohms.
Pay attention to any gel escaping from the tACS electrode, and remove excess EEG gel with a cotton swab. Once impedance is below the threshold of 10 kilohms, gently mount the EEG cap again, and fasten the strap. Apply EEG gel with appropriate viscosity to the EEG electrodes to create contact between the scalp and EEG electrodes.
Begin with the ground and reference EEG electrodes, and then proceed to the electrodes located in the middle and near the tACS electrode. Then continue with the remaining electrodes. For EEG electrodes surrounding the tACS electrode, inject gel with the needle tip pointing in a direction away from the tACS electrode.
Gently push down the EEG electrodes while applying gel, so that gel does not escape from beneath the electrodes. To increase the contact between the EEG electrodes and the scalp, in particular EEG electrodes located in the vicinity of the tACS electrode, push down the gel with a wooden stick towards the scalp. Keep the angle of the stick orthogonal to the scalp while gently rubbing the scalp in a rotating motion as sideways movements of the stick will spread out the gel under the electrode.
Once optimal impedance has been achieved, apply some more gel to stabilize the contact between the EEG electrode and the scalp. Next, test for bridging between the tACS electrode and surrounding EEG electrodes due to leaking gel by applying a brief stimulation, and checking whether any channel of the EEG amplifier is saturated. Finally, check impedances of the EEG electrodes once more and then begin recording.
This protocol explains how to successfully set up a concurrent tACS and EEG experiment and avoid bridging via leaking of the EEG gel, as well as large tACS artifacts. If bridging between the tACS and EEG electrodes occurs, the recorded EEG signal will become saturated as seen here. As opposed to a successfully recorded signal.
Concurrent tACS EEG recording is a technically challenging method. After watching this video, you should now have a better understandings of how to successfully set up this type of experiment to study neurophysiological mechanism of tACS.
