Using TMS to Measure Motor Excitability During Action Observation

1. Recruit 20 participants.

1. Participants should be right-handed and have no history of neurological or psychological disorders.
2. Participants should have normal or corrected-to-normal vision to ensure that they will be able to see the visual stimuli properly.

2. Pre-experiment procedures

1. Obtain written consent from the participant and explain what is involved in the experiment.
2. Explain that the participant will watch a series of short videos while TMS pulses are delivered to their brain. The subject may experience a light tap on the head from the TMS coil, but there should be no great discomfort associated with participation.

3. Prepare the subject for TMS.

1. Seat the subject in a comfortable chair in front of a computer screen. Their elbow should be bent at a 90º angle, and the hand should lie comfortably prone.
2. Use a chinrest to fix the movement of the head, ensuring that the subject's eyes are about 50 cm from the computer screen.
3. Prepare the skin of the hand for EMG electrode placement by cleaning with alcohol.
4. Place two EMG electrodes on the first-dorsal interosseous (FDI) muscle of the right hand.
   1. Locate the peak location of muscular tension by asking the participant to flex their FDI muscle and place the first electrode on that location.
   2. Place a second reference electrode on the nearby bone of the hand.
5. Connect the EMG electrodes to a computer that digitizes, amplifies, filters, and displays the signal.

4. Localize and calibrate TMS.

1. Instruct participants to relax their hand, so that there is no muscle tension.
2. Use a figure-8 TMS coil to locate the primary motor cortex.
   1. Place coil against the contralateral (left) surface of the head on the anterior portion of the scalp.
   2. Deliver a series of individual TMS pulses, systematically moving the location of the coil until twitches are visible in the FDI muscle and the recorded MEPs are stable and reliable. Stimulation intensity may be adjusted as needed during this phase to assist in locating the hotspot.
3. Determine the subject's motor threshold.
   1. Find the minimum stimulator output strength that produces an MEP of greater than 50 µV on 5 out of 10 stimulations. Record this value.
4. Measure the resting MEP amplitude.
   1. Deliver a series of 10 TMS pulses, each separated by 15 s, in the absence of any stimulus to measure the baseline MEP amplitude.

5. Experimental task

1. Play a series of 5-s video stimuli, one at a time.
   1. There are three kinds of videos: hand movement observation, arm movement observation, and control.
      1. In the control videos, a cup is presented on a table, and a right hand rests nearby. No movement is made.
      2. In the hand action videos, the right hand reaches and grasps the cup. This action involves contraction of the FDI muscle.
      3. In the arm action videos, the right arm reaches, is lifted, and moved around the area. There is no grasping action performed, and so the FDI muscle is not involved.
   2. Play each video 10 times, with 15 s of rest after each one, for a total of 30 videos. Play the videos in random order.
2. During the video, deliver a TMS pulse at 120% of motor threshold.
   1. Time the pulse to coincide with the action in the video. To achieve this, the pulse should occur 2 s after the start of the video.

6. Analyze the data.

1. For each MEP, calculate the peak-to-peak amplitude.
2. Discard MEPs that occur either before TMS stimulation, or more than 100 ms after stimulation to remove spurious spikes.
3. For each subject, calculate the average MEP amplitude for the baseline, action observation, and control conditions.
4. Perform analysis of variance (ANOVA) on the group data to test the hypothesis that MEP amplitude is affected by action observation.