Motor threshold estimation
To perform safe and effective transcranial magnetic stimulation (TMS), the resting motor threshold (rMT) of any subject must be determined. On this page you will find the necessary software, setup, and steps to do so.
As shown in the following diagram, an electric field is induced perpendicularly to the magnetic field. Flow of current in a magnetic coil and induced current in brain tissue are shown in this diagram. An electric field is induced perpendicular to the magnetic field in the coil. An electrode is placed over the cortex of the M1 muscle, and a magnetic coil is placed on the target muscle, the abductor pollicis brevis (APB) and first dorsal interosseus (FDI). For the FDI and APB muscles, MEP responses are detected with channels Ch1 and Ch2.
The demonstration will show how to get the resting motor threshold on FDI muscles.
- create a new project on BrainSights
To view the stimulation effects in real time, we need to create a new BrainSights project. Create a new empty project and load the freesurfer reconstructed brain images of the individual to define the stimulation site for the individual T1 images. In the absence of anatomical images for the subject, create a new MNI head project and load BrainSights' default images.
Create a new online session and select IOBox. Now you'll see two traces of EMG responses. As we usually only record from one paired channels, uncheck the one that won't record.
-
Download TMS Motor Threshold Assessment Tool (MTAT 2.0). It provides a procedure using the Maximum-Likelihood strategy for estimating motor thresholds.
-
Connect BrainSight EMG recording sets to the analog input of the IOBox
-
As an abrasive, use Brain Products Conductive Gel on the skin surface of targeted muscles and bones. Follow up by asking the subject to wash their hands. Clean the skin surface with alcohol cotton before attaching surface electrodes and connecting BrainSights short electrodes.
-
Two short electrodes should be placed on 1) the FDI muscle belly (signal +); 2) the insertion point of the FDI at the knuckle of the index finger (a minimum of 3.5 cm away from the FDI electrode; signal -). The reference lead demonstrated in BrainSights devices, commonly referred to as the ground electrode, should be placed on the ulna head.
- To determine whether there is significant fluctuation in the EMG trace, ask the subject to tap his or her right thumb and index finger. If there is not, adjust the surface electrodes of signal +.
- Head Measurement
Put a TMS cap to aid the measurement taking. Three sizes are available for TMS caps. Make sure the subject’s ears are visible. Measure from nasion to inion and mark the halfway point on each side of the ruler. Then, measure from the right tragus to the left trigs and mark the halfway point on each side of the ruler. This allows you to find the vertex of the head.
- Locate C3
This tool https://clinicalresearcher.org/eeg/ allows to locate C3 by entering the dimensions of tragus-to-tragus, nasion-to-inion, and circumference and submit it. After calculation, click at C3. It will show the distance from Cz (Vertex) to C3. In the example here, C3 is eight cm from the vertex to the left tragus. Make a mark X mark there.
- Make a grid centering the X
Draw a 3 x 3 grid around the X. Move one cm anteriorly, posteriorly, leftward and rightward.
- Set up the stimulator
Now you are ready to begin setting up the stimulator, press the main menu. On the setup menu scroll down until you find the letter D, underneath you’ll see a button that says Recall, make sure the setup menu on your left changes to the letter that you’ve chosen and then press Timing. Enable the coil to your left and raise the amplitude to about 50%.
- Determination of the stimulation site based on the highest motor output
Hold the TMS coil at a 45º angle to the midsagittal line over the X. Initially, stimulate the X you have created, then move forward to the anterior point, then move clockwise toward all points until you are able to find the most motor activity (twitch of the right index finger).
Launch MTAT 2.0 which you downloaded in advance. Select the condition to be applied, and then click Start. As an example, I have selected the condition "threshold estimation without a priori information." You will see a value at the left bottom corner. You should set the amplitude as suggested on the stimulator. The program will adjust the amplitude in response to the participant's motor output. If you see the participant's finger twitch or an EMG response greater than 50 microvolts, press the "Y" key. Press "N" if you do not observe any movement/significant EMG response. The program turns red and rings when the correct dose has been determined.
