Measuring the Contralateral Silent Period Induced by Transcranial Magnetic Stimulation

Take a human patient's hand. Place a negative electrode on the first dorsal interosseous, or FDI, muscle to record its activity. Place a positive electrode on the distal joint and a reference electrode on the wrist to establish baseline signal.

Position a force-measuring device between the thumb and the index finger.

Maintain a standardized force on the device and record the muscle activity.

Locate the primary motor cortex, or M1, on the contralateral side of the brain that regulates FDI activity.

Position a magnetic coil on M1 and identify the lowest-intensity threshold stimulus to generate action potentials in cortical neurons that lead to FDI contraction, known as motor-evoked potential or MEP.

Deliver a suprathreshold pulse to elicit a high-amplitude MEP.

The cortical interneurons release inhibitory neurotransmitters that block cortical neurons, preventing excessive muscle activity.

Record the contralateral silent period, or cSP, a period of reduced muscle activity before voluntary activity returns.

To select the muscle for positioning the electrodes, ask the subject to put their hand over the table in a prone position. Then, to identify the first dorsal interosseous or FDI muscle, ask the subject to abduct the index finger against the resistance, keeping the rest of the hand still and laying on the table, while palpitating the area. Clean the area with alcohol pads to remove skin oils and other factors that can increase impedance.

Place the negative electrode on the belly of the FDI muscle and the positive on the distal interphalangeal joint with an inter-electrode distance of at least 1.5 centimeters. Place the reference or neutral electrode on the wrist over the ulnar styloid process.

Next, determine the required muscle contraction force. Use a digital pinch dynamometer and quadrangular pyramid support to minimize mechanical distortions and elevate the sensitivity for minimal contractions. Place the dynamometer between the first and second fingers with the help of the pyramidal support. Ensure that the first and second fingers generate the forces of the pinching movement and capture the electromyography or EMG.

With the fixed position, ask the participant to press the dynamometer with the first finger and the side of the pyramid with the index finger, squeezing the dynamometer pyramid system with their maximum force and creating a strong contraction of the FDI muscle. Using this value as reference, determine the 20% of maximum force. The participant must practice maintaining the target at 20% of sustained contraction.

Next, to identify the initial location for hotspot searching, put a swim cap on the subject's head. All the reference points will be marked on it. Measure the sagittal circumference of the head from the nasion to the inion. Divide the value by two and mark the middle spot on the head.

Mark the location of the patient's nasion, inion, the helix of both right and left ears, and right and left supraorbital ridge. Measure the tragus-to-tragus distance and add a mark halfway. Mark the intersection between them — a point identified as the vertex.

From the vertex, move 5 centimeters laterally in parallel to the midsagittal line on the contralateral side to the selected muscle. This mark identifies the M1 on the same coronal level as the hand motor cortex. Use this as the first spot to initiate the search for the hotspot.

The hotspot is the area of the motor cortex where the lowest motor threshold is detectable. Set up a low intensity, and initiate the search by delivering multiple pulses to the first spot. Pursue with small intensity increments until identifying the lowest stimulus that detects electromyography or EMG-indexed response.

To deliver the stimuli, angle the figure of eight coil at 45 degrees to the midsagittal line with the handle pointed toward the posterior of the patient. To ensure that the best spot is identified, move around the first spot and test the subsequent three motor-evoked potential, or MEPs, at 1 centimeter each anterior, lateral, and posterior to it.

Once the hotspot is identified, mark the spot in the patient's head. Use this location during the experiment and the potential follow-up visits. Use both hands to support the coil on the subject's head.

Next, to determine the resting motor threshold, or rMT, apply 10 consecutive stimuli at the hotspot and select the lowest intensity that produced an MEP with a peak-to-peak amplitude of at least 50 microvolts on the target muscle in 50% of the trials. For the contralateral silent period, or cSP, deliver the 10 suprathreshold stimuli with the stimulation intensity of 120% of the rMT with 10-second period in between them to elicit MEPs during tonic voluntary contraction of the target muscle

During stimulation, ask the patient to maintain 20% of the maximum motor contraction of the target muscle. To capture the whole silent period, or SP, confirm that the EMG time window is long enough to capture up to 400 milliseconds of activity.