The overall goal of these two transcranial magnetic stimulation protocols is to investigate the activity of intracortical inhibitory circuits within the primary motor cortex or M1 while performing motor tasks under different attentional foci. Although many studies have established the advantage of an external compared to an internal focus of attention, little is known about the underlying neural mechanisms. Despite enhanced motor performance, an external focus of attention is usually accompanied by a decreased muscular activity, therefore we speculated that inhibitory mechanisms might be involved.
The applied TMS methods, namely subTMS and SICI allow to better understand the impact of attentional foci on the activity of the inhibitory circuits within the primary motor cortex. The main advantage of the subthreshold transcranial magnetic stimulation technique is that there is no movement artifact due a low stimulation intensity. In contrast, the conventional paired-pulse protocol uses higher stimulation intensities and therefore leads to muscle contractions.
However, this method is well-established and it's impact on the GABAeric sinuses is thoroughly investigated. Begin by having the participant sit in the testing chair and place a monitor one meter in front of them. Next, place the left arm in a comfortable and relaxed position under the table resting on the left leg.
Place the right arm of the subject in a custom-belt splint in pronated position. Align the finger joint with the access of rotation of the custom-made device. Once the optimal position is found, take a picture of the anteroposterior and medial lateral positions of the splint to use comparable positions for each session in the experiment.
Begin by aligning the axis of rotation of the goniometer and the metacarpophalangeal joint. Place the force transducer in a way that allows for maximum voluntary contractions. Then, ask the participant to perform three maximal isometric abductions of the index finger with a 30-second break between each contraction and determine the Fmax.
Note the participant must push the lever against the force transducer without any instructions about the focus of attention. After maximal contractions, remove the force transducer allowing the index finger to move freely in the transverse plane. Then, fill a bottle of water to amount representing the 30%of Fmax and attach the weight of Fmax to the rope of the device.
Have the participant hold the finger in the target position by counteracting the weight, performing an abduction of the index finger. Have them perform the task until task failure. Press the record button on the recording software to start recording the goniometer signal and wait until task failure.
Once task failure is reached, press the stop recording button on the recording software to stop the recording and save the goniometer signal on the computer. Finally, remove the participant's hand from the orthopedic splint. Begin by placing the silver-silver chloride bipolar surface electrodes in a belly-tendon montage on the FDI muscle with one centimeter inter-electrode distance.
Connect the EMG cable or FDI muscle and the goniometer cable to an EMG amplifier and to an AD converter. Finally, use the surface electrodes to record and measure muscular activity in electrophysiological responses elicited by brain stimulation from the FDI muscle. 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 medial 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%MOS 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.
Finally, to conduct the paired pulse TMS, move the coil over to the hot spot at M1.Set the intensity on the stimulator, the ISI, at 2.5 milliseconds in the interval between paired and single-pulse TMS at 0.25 hertz. Finally, ask the participant to perform four separate isometric index finger abductions at 10%of Fmax and record the EMG signal of the FDI. During the isometric contraction, record 20 TMS stimuli for each condition in a counterbalanced order.
The amount of short-interval intracortical inhibition is defined as the ratio between the control MEP single-pulse and the conditioned MEP paired-pulse. The TTF was prolonged when the participants adopted an external rather than an internal focus of attention. As subthreshold intensities are used, no muscle twitches induced by stimulation which may disturb motor performance.
Results indicated that subTMS-induced EMG suppression was instantly enhanced when using an EF compared to an IF.Further, when adopting an EF, participants showed more short-interval intracortical inhibition. This is well in line with the subTMS results and suggests that GABAergic neurons constituting the intracortical inhibitory circuits are modulated differently within the M1 according to the type of attentional focus. Once mastered, this protocol can be done in approximately 90 minutes if it is performed properly.
While attempting the subthreshold TMS procedure, it is important to set the right stimulation intensity to get the subTMS-induced EMG suppression in the FDI muscle. For this purpose, it is important to average at least 40 trials with stimulation and 40 trials without stimulation. In addition to subTMS and paired-pulse TMS protocols measuring intracortical inhibition within M1, other methods like the measurement of surround inhibition can be performed in order to reveal additional mechanisms that might be involved to increase task efficiency with an external compared to internal focus of attention.
After watching this video, you should have a good understanding of how to apply subTMS and paired-pulse TMS over the primary motor cortex. These methods can be used in order to investigate the impact of different attentional foci on the activity of the primary motor cortex.
