The overall goal of this experimental protocol is to measure changes in blood oxygen level dependent activity induced during transcranial electrical stimulation. Combining transcranial alternating current stimulation with functional magnetic resonance imagine has a potential to answer key questions in cognitive neuroscience. A main advantage of combining these techniques is the possibility to measure brain function while manipulating neuro oscillations in a frequency-specific manner.
The implications of this technique extend towards diagnosis and therapy of neurological disorders associated with abnormal synchronization. Though this technique can provide insight into human visual cognition, it can also be applied to other systems, such as motor, behavior, and memory. Generally, individuals new to this method might struggle with the complexity of the set up that is required to ensure noise-free and trigger synchronized experiments.
We became interested in TCS because we saw it as a way of studying the cause of all of oscillations to conscious perceptions. Our MRI technician is Ilona Pfahlert, with Paule Wunsch as our subject volunteer. Pre-screen all subjects for any contraindications to MR scanning as well as to transcranial alternating current stimulation.
When the subject arrives, describe the details of the experiment and obtain informed consent. Begin by using a tape measure to determine the distance on the subject's head from the nasion to the inion. And then from ear to ear over the top of the head.
Mark the intersection of these lengths as the C-Z position, according to the 10-20 EEG system. Now, place the EEG cap without electrodes on the subject's head with C-Z aligned to the marked location. Then, determine and mark the desired locations for the electrodes using the same placement for all subjects to ensure experimental consistency.
Next, using alcohol and cotton pads, clean the hair and skin on and around the marked spots on the scalp to remove oils and hair products. Then, spread gel on the rubber electrodes and press each electrode firmly on these locations, ensuring full contact from electrode, to conductive gel, to the scalp with minimal impedance. Use a shielded LAN cable and the MR safe cables to connect the filter boxes to the stimulator and to the rubber electrodes.
Then, turn on the stimulator and test the impedance. If it is not below 20 kilo ohms, press the electrodes onto the scalp. Or, add electrode gel as necessary.
Be careful to limit electrode gel coverage to approximately the same surface area as the electrodes, and remove any excess gel from the area. Next, allow the stimulator to output current for a few seconds to familiarize the subject with the sensory experience. Ask them about sensory perception during this test, including whether they can feel, and withstand, any tingling sensation, and the extent and location of any phosphines during stimulation.
At this point, leave the cable plugged into the rubber electrodes on the subject, but disconnect the stimulator, the spare LAN cable, and the outer and inner filter boxes so the subject can move to the scanner. Now, connect the outer filter box to the LAN cable that runs through the wave guide, leaving as little cable outside of the wave guide as possible. Then, connect the stimulator cable.
Also double check that the stimulator is connected to the presentation computer trigger output. After ensuring that the subject is free of magnetic materials and ready for the MRI experiment, lead them into the scanner room. Provide ear plugs for hearing protection, and then have the subject lie on the scanner bed.
Lay the electrode cable flat in a position that will comfortable for the subject to lie on for the duration of the experiment, and then place pillows around and under their head. Also, place a pillow under the subject's legs for comfort, and to reduce movement. Provide and alarm ball as well as an MRI safe response button box positioned such that minimal movement is required to push a button.
Now, secure the head coil with a mirror attached such that the subject will see the projection screen in the correct orientation. Then, temporarily secure the free end of the electrode cable from the rubber electrodes to the head coil such that it does not catch when the bed is moving. Move the scanner bed into position.
Then, from the back end of the scanner bore, connect the electrode cable from the rubber electrodes to the inner filter box that connects to the LAN cable. To prevent excess motion during the scans, secure the cables and filter box along the scanner bed railing to the right of the bore with tape and sand bags. Then, place the projector screen into the rear end of the scanner bore.
Finally, again test the impedance on the stimulator to ensure that all cable connections are properly made before starting the experiment. Begin by testing that the presentation computer registers when the subject pushes the response buttons. Then, acquire a high resolution T1 weighted anatomical image with a one millimeter isotropic resolution.
Once acquired, adjust the contrast and windowing levels on this image to low and high extremes to visually detect noise that may result from the stimulator set up. Continue this monitoring during the functional imaging acquisition. Now, start the experiment on the presentation computer so that it will begin with the scanner trigger, and start the stimulator to wait for this computer's output trigger.
At this point, start the FMRI acquisition. Leave the stimulator on and connected throughout the FMRI experiment to avoid differences in the temporal signal to noise ratio between the stimulator on and off conditions. Also, monitor the stimulator display to guarantee that current is sent at desired times throughout the experimental runs.
Once scanning is complete, unplug the inner filter box from the cable connected to the rubber electrodes. Once the subject is out of the scanner, remove the electrodes, leaving the subject free to wash their hair. This figure shows results of testing the effects of current strength of 16 hertz CZOZTACS on the bold signal during central cross fixation.
Event related averages of the signal for statistically significant clusters are shown, with increasing effect on the signal with increased current strength. Here, we see current strength specific T score maps illustrating regional specificity of effects as well as increasing effects on bold activity with increased current strength. This figure shows representative results testing the frequency dependence of TACS effects during a visual perception task.
This schematic illustrates the experiment timing with visual presentation and TACS periods between blocks of central cross fixation. TACS condition and frequency effect interaction maps, and cluster post-hoc tests show frequency specific effects in the parietal cortex, with 10 hertz TACS decreasing, and 60 hertz increasing signal. These T score maps show specific effects of 60 hertz TACS extending beyond the parietal cortex to include some occipital and frontal regions.
Once mastered, this experiment can be done in as little as two hours, depending on the length of the experiment design. While attempting this procedure, it is especially important to make sure that the stimulator is properly synchronized with the MRI acquisition time. After watching this video, you should have a good understanding of how to perform combined TACS FMRI experiments.
