This study investigates the effects of transcranial magnetic stimulation (TMS) on the excitability of cortical neurons and muscle activation. By utilizing EMG to measure muscle response, the research aims to understand the relationship between cortical stimulation and motor control.
Begin with a participant fitted with EMG electrodes on the right-hand FDI muscle, which controls index finger movement.
Attach reflective markers to the forehead to track and stabilize the TMS coil position.
Place the coil over the left primary motor cortex and deliver magnetic pulses to excite pyramidal neurons.
This cortical activation elicits contraction of the FDI muscle, producing an EMG response.
Identify the cortical site that produces maximal muscle contraction.
Then, lower the stimulation intensity to sufficiently activate motor neurons and produce a minimal contraction.
Finally, reduce the intensity further to induce cortical excitation without triggering muscle contraction.
Next, instruct the participant to press the finger against a weight to maintain neuronal activation and muscle engagement.
In the brain, the reduced stimulation intensity activates GABAergic interneurons, triggering GABA release and inhibiting pyramidal neurons.
This prevents unnecessary muscle activation and suppresses EMG activity, improving muscle movement control.
Begin by placing the reflective markers on the participant's forehead with double-sided adhesive tape.
Use a figure-of-eight coil attached to a TMS stimulator to deliver stimuli to the contralateral motor area. Then, find the optimal position, or hot spot, of the coil relative to the skull for eliciting motor evoked potentials, MEPs, in the FDI muscle by performing a classical mapping procedure. To do this, place the coil approximately 0.5 centimeters anterior to the vertex and over the midline with the coil handle pointing at 45 degrees towards the contralateral forehead.
Get the participant accustomed to the TMS stimuli by starting the intensities below 25% of maximum stimulator output, or MSO. Then, start to increase the stimulation intensity and move the coil in the medio-lateral and rostro-frontal direction to discover the hot spot. Once the hot spot is found, record the optimal position with the neuro-navigation system.
Determine the active motor threshold, or AMT, by adjusting the intensity of the stimulator output. Next, fill a bottle of water with the weight that represents 10% of Fmax. Ensure that the participant remains in a relaxed, comfortable position.
Then, find the optimal intensity for eliciting subTMS-induced EMG suppression by successfully diminishing in steps of 2% MSO from the AMT determined previously. Then, have the participants perform a few practice trials in order to match the target force. Have the participant perform four separate isometric index finger abductions at 10% of Fmax and record the EMG signal of the FDI.
Record 40 trials with, and 40 trials without TMS with randomized ISIs for each condition in a counterbalanced order. Use the same previously determined optimal stimulation intensity. The onset of subTMS EMG suppression is defined as the moment when the difference between the trials with and those without TMS is negative for at least four milliseconds in a time window from 20 to 50 milliseconds after the TMS.
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