Non-invasive electrical brain stimulation can modulate cortical function and behavior, both for research and clinical purposes. This protocol describes different brain stimulation approaches for modulation of the human motor system.
The overall goal of this procedure is to demonstrate the localization of the primary motor cortex, and correct placement of electrical brain stimulation electrodes, to modulate the motor system. This method can help answer key questions in the field of neuroplasticity, through the targeted modulation of brain networks that improve motor functions, or counteract motor disabilities. The main advantage of non-evasive electrical brain stimulation is that it is easy to administer even as a bedside technique, without inducing pain or discomfort.
This method of brain stimulation can provide insight into the function of the motor system. It can also be applied to other regions of interest, such as the pre-frontal cortex, or the visual cortex. Visual demonstration of the localization of target cortical areas and electrode placement are crucial for the success of these experiments.
Otherwise, electrical current flow may not reach the intended brain areas, resulting in a lack of effect, or unexpected findings. Begin by asking the participant to sit comfortably in the chair. Clean the participant's skin by rubbing preparation paste in the regions of the hand where electrodes will be placed.
Remove excess with a clean gauze pad. Attach the electromyography, or EMG, surface electrodes in a belly-tendon montage, on the hand muscle of interest, and connect a ground electrode. Then, turn on the magnetic stimulator and charge the capacitor by pressing the corresponding charge button.
Place a figure of eight TMS coil on the participant's scalp on the motor cortex region. Hold the TMS coil at a 45 to 50 degree angle, referenced to the inter-hemispheric fissure, with the handle oriented backwards. When the magnetic stimulator is charged, discharge the stimulator by either pressing the trigger button, by stepping on the foot switch, or automatically with a software program.
Next, start with a low stimulation intensity, and watch for motor evoked potentials, or MEP's, visible on the EMG amplifier. If no MEP is visible, increase the stimulation intensity, until an MEP is clearly present. Repeat stimulation by pressing the trigger button.
Move the coil radially, in one centimeter steps, around the initially stimulated site, to find the spot with the largest MEP response, following the application of single TMS pulses. From there, start moving the coil again to secure the hot spot. Finally, mark the hot spot position, and coil orientation with a non-permanent skin marker.
Connect cables to rubber electrodes, and place the electrodes inside the sponge bags. Ensure that the electrode size and sponge bag size match. Soak sponge bags on both sides with isotonic sodium chloride solution, but avoid excessive soaking to prevent salt bridges, or dripping onto the volunteer.
Next, find the head markings indicating the motor cortical hot spot, and separate the hair around the area. To improve conductance, clean the skin before electrode placement with a swab soaked with 40 to 50 percent alcohol, or skin preparation paste. Remove excess with a swab, and clean the area again with isotonic sodium chloride solution.
Dry the area afterwards. Place one electrode on the head marking for the motor cortex area of interest, and keep the sponge in direct contact with the skin as much as possible. For unilateral stimulation, place the second electrode over the contralateral supra-orbital area.
Alternatively, for bilateral stimulation, place the second electrode on the opposite motor cortex, following the head marking ipsilateral to the limb used in the study. Place all of the electrode cables towards the participants back, to avoid disturbance during stimulation, and to ease connection to the non-invasive electrical brain stimulation, or any BS device. Then, cover the head twice, with an elastic bandage, circularly in the mediolateral direction, to stabilize the M1 electrode.
Use the remaining bandage to cover the head circularly in the anterior posterior direction, to stabilize both electrodes. Use adhesive tape to fix the end of the bandage, and secure the cables with adhesive tape on the participant's neck, or shirt. Finally, connect the electrode cables to any BS device.
Begin by switching on any BS device. Adjust the any BS device settings, regarding stimulation type, intensity, duration, ramping up and down, and frequency spectrum for transcranial random noise stimulation. Next, inform the participant about potential side effects associated with any BS.Then start the stimulation, and check for continuity of stimulation, during ramping up, and throughout the session.
If any BS is co-applied with the execution of a motor task, start the testing after stimulation is ramped up, and after the participant is feeling comfortable with stimulation. Finally, after the stimulation session is complete, disinfect the electrodes and sponges with 40 to 50 percent alcohol. Then thoroughly rinse in water, and let the materials dry before storing.
This protocol describes multiple approaches for non-invasive electrical brain stimulation, for modulation of the human motor system. Here the time course of cortical excitability is shown over the course of 90 minutes after stimulation of the primary motor cortex. Note that TRNS exerts similar effects on cortical excitability compared to TDCS.
Within three days of motor training, motor skill increases significantly over time, in the Sham control group, and is augmented further by each any BS strategy. After watching this video, you should have a good understanding of how to locate the primary motor cortex by TMS, and place electrodes for the different any BS montages. Once mastered, this technique can be completed in 10 to 15 minutes, if it is performed properly.
Don't forget that working with magnetic and electrical brain stimulation can be harmful if used improperly, and safety screening of participants should always be done before performing this procedure.
