The overall goal of this procedure is to extract reliable ERPs from EEG data recorded. During FMRI guided transcranial magnetic stimulation. This is accomplished by first performing a high resolution functional MRI scan to define the cortical areas.
To stimulate, the later step is to design an EEG experiment that allows extraction of a TMS artifact template that will later be subtracted from the recorded EEG data to obtain clean ERP. Next, the T-M-S-E-E-G experiment setup is carefully prepared using a stereotactic navigation system. To locate the FMRI defined target on the participant's head, the final step is to record EEG data concurrently with TMS application, ultimately, subtraction of the TMS artifact template is used to obtain reliable ERP.
The main advantage of this technique over existing methods like simultaneously G-F-M-R-I, is that it allows us to make an inference about a brain area that is defined with FMRI and an electrophysiological signal that is measured with EEG. The main challenge in E-E-G-T-M-S simultaneous combination is to remove the artifacts generated by the TMS. This is accomplished by first cutting the EEG data collected during the magnetic pulse, and then correcting for any other residual artifact related to the TMS Demonstrating this procedure will be Ziv Perman, who is trained on this procedure by Boaz Ade.
Run a functional MRI task using a high resolution echo planner imaging sequence to determine the desired regions of activation to be targeted with transcranial magnetic stimulation known as TMS. Then run a T one weighted structural scan to obtain neuro anatomical data. Make sure the entire subject's face is included in the field of view.
External markers such as the tip of the nose will be used later on to coregister the subject's head with its scan. After the scan. Use the Mars bar toolbox for SPM to define the brain regions of interest based on contrasts between experimental conditions.
For example, define the occipital face area by subtracting activations to objects from activations to faces, and exclude any body selective voxels to define the extra striate body area. Subtract activations to objects from activations to bodies and exclude any face selective voxels. Design an experimental paradigm where all stimulus conditions are presented.
Randomly use at least 50 trials per condition with time. Jittered interra stimulus intervals. Use a blank screen condition where A TMS pulse is administered, but no visual stimulus appears.
This is important for generating the TMS artifact template. Then set the latency at which the TMS will be triggered after the start of the trial. In this example, double pulse TMS will be administered at 60 and 100 milliseconds.
After stimulus onset, feed the structural T one scans into the neuro navigation system software, then overlay the functional MRI contrasts onto the structural images. Mark the desired target brain areas using the neuronavigation software. Also find the external markers that will be used to coregister the head's location such as the tip of the nose, the nasion, and the tragus of each ear.
When the subject arrives, mount the EEG cap on their head and connect the electrodes. Use as little gel as needed to maintain impedance below five kilo ohms. Make sure the electrode wires do not cross each other or form loops and are oriented away from the coils location.
Also, place the reference in ground electrodes as far from the coil as possible while measuring the EEG signal. Use a high sampling rate of at least one kilohertz for a better representation of the TMS artifact. Next, mount the infrared detector on the subject's head coregister the head location with the navigation system.
Using the previously defined markers, select the target for TMS on the navigation software holding the neuro navigation's pointer tool perpendicular to the surface of the head. Navigate to the target location and mark on the electrode cap up seat. The subject comfortably in front of the screen with their chin on the chin.
Rest the chin rest will help the subject to refrain from any movement, which is crucial for measuring a clean and reliable TMS artifact. Guide the coil's center to the target brain area and fix it in place. Use a holder to stabilize the coil so it does not move during the session.
Set the TMS to the desired intensity and administer a test pulse to the subject's approval before beginning the session. Administer TMS to each cortical target area on a different block. Additionally, run a no TMS block where all stimuli, including the blank screen condition are presented during the no TMS condition.
Use a sham coil or place the coil against the head and tilt it perpendicularly. Repeat the identification of navigated targets in between each block. Unless you have a sample in hold system, you need to remove the pulse artifact.
This is done by cutting out a time window around the pulse or pulses in case of repetitive simulation. In most cases, a window of a few milliseconds will be enough. In the example shown here, two consecutive pulses were administered and are removed together as one piece.
There are two ways to connect the two ends created. After removing the pulse, you can either directly join them or you can interpolate a line between them. The brief pulse artifact is sometimes followed by a long residual artifact that may last a few hundreds of milliseconds.
Use the subtraction technique to remove this long lasting artifact without losing data. For each experimental block, calculate an averaged ERP template of the blank screen trials by time locking them to the start of the trial. In other words, you calculate an ERP in the absence of an experimental stimulus, thus computing a template of the TMS noise alone Within each block.
Subtract this template from each trial of all other stimulus conditions. A clean ERP results from the subtraction of the TMS template from the raw ERP repeat the subtraction for each block individually. EEG recordings were made while subjects were presented with either face or body stimuli.
While viewing stimuli, double pulse TMS was administered to either the occipital face area or the extra striate body area at 60 and 100 milliseconds after stimulus onset. By subtracting the recording made during TMS alone from recordings of stimulus presentation with TMS an ERP can be obtained. That contains the N one component, a prominent visually evoked potential that is larger when viewing faces and bodies compared to other stimuli.
When viewing faces. TMS to the OFA increased the amplitude of the N one component as compared to the EBA and the no TMS conditions. When viewing bodies, TMS to the EBA increased the amplitude of the N one component as compared to the NO TMS or to the OFA conditions.
These results indicate a dissociation between the neural networks underlying face selective and body selective ERPs. A particular advantage of the artifact removal technique showed in this video is that it allows the elimination of any artifact related to the use of TMS, whether it is of a mechanical, muscular, or cortical origin, even in electrodes, lying in proximity to the TMS coil so that no electrode needs to be excluded from analysis. After Watching this video, you should have a good understanding on how to design, execute, and analyze a simultaneously GTMS experiment that will allow you to obtain reliable event related potentials.
