Stimulating the Lip Motor Cortex with Transcranial Magnetic Stimulation

The overall goal of this procedure is to stimulate the representation of the lips in the left primary motor cortex by transcranial magnetic stimulation or TMS. This is accomplished by first attaching electrodes to the lip muscles to record electromyography. Next, the lip motor cortex is localized by recording motor evoked potentials from the lip muscles.

Then the optimal intensities of the pulses for single pulse and repetitive TMS experiments are defined. Ultimately, transcranial magnetic stimulation is used to show that the motor evoked potentials elicited by single TMS pulses are increased during listening to speech showing that the excitability of the lip motor cortex is enhanced and that repetitive TMS induced disruption in the lip motor cortex impairs participant's ability to discriminate speech sounds. This method can help to answer key questions in cognitive neuroscience, substance weather.

The areas inter motor cortex that contribute to speech production also contribute to speech perception. Generally, individuals new to this method might struggle because the lip representation is not easy to find in all participants. To begin, ask the participant to fill out a safety screening form.

Participants who have contraindications for TMS such as lack of sleep or on medications such as antidepressants or have a family history of epilepsy should not be stimulated. Explain the TMS procedure and the experimental details to the participant and obtain in informed consent. Use alcohol to clean the skin above the belly of the first dorsal inters or FDI muscle of the right hand and of the reference site such as the tendon of the FDI muscle.

Then attach electrodes to these sites. Clean the skin on the right side of the Orbicularis Aus or O muscle, and attach one electrode to the right corner of the upper lip and a second to the right corner of the lower lip. Then clean the skin around the site for the ground electrode and attach the electrode.

Next, using software such as Spike two, instruct the participant to contract and relax their hand and lip muscles and check the EMG signals recorded from them. If the signals look noisy, when the participant relaxes their lip and hand muscles, twist the electrode cables, reclean the skin at the electrode site, and or ask the participant to uncross their legs, remove their shoes, and have their feet on the floor for better grounding. To protect the participant's hearing.

Have them insert earplugs. Then in order to mark the position of the TMS coil, place the cap on the participant's head using a pen or a small sticker. Mark the vertex on the cap and measure the distance from the vertex to the left preauricular point.

Move 33%of this distance from the vertex towards the left preauricular point and mark this spot. Place the center of the figure eight TMS coil on this spot and orient to the handle of the coil 45 degrees from the midline. Press a foot pedal to deliver the first TMS pulse using a low intensity if the participant's motor threshold is not known.

If no motor evoked potential or MEP such as a muscle twitch is visible in the hand, move the coil slightly or increase the intensity. When an MEP is elicited. Move the coil in five millimeter steps around the stimulated area and wait at least five seconds in between TMS pulses to find an appropriate hotspot where the site and coil orientation elicits the maximal meps at a certain intensity.

When a hotspot is identified, mark it and mark the orientation of the coil on the cap. To localize the motor lip representation, measure two to three centimeters from the FDI spot along a straight line towards the corner of the left eye and place a coil in this spot. Explain to the participant that the TMS pulses may feel more intense in this location than in the previous location, and that they may experience facial twitches or involuntary eye blinks.

Instruct them to inform the experimenter at any time if the stimulation becomes unpleasant or painful, or if they wish to stop. Next, deliver the first pulses moving the coil slightly and or increasing the intensity. If an MEP is not elicited, if it is not, ask the participant to purse their lip muscles since this lowers the motor threshold and therefore makes it easier to find the motor lip representation.

After identifying the hotspot for the O muscle market and the orientation of the coil on the cap, if MEPS will be recorded from contracted lip muscles, train the participant to maintain a constant level of contraction by using the software to provide visual feedback about the power of the EMG signal. Continue the training until the participant can stably maintain the level of contraction for at least one minute. Next, place the coil on the hotspot for the OO muscle.

Then deliver 10 pulses with fixed intensity, keeping at least a five second break between the pulses. Estimate the sizes of MEPS by visually inspecting them. If the meps are very small or they were not consistent with every trial, increase the intensity and deliver another 10 pulses.

Continue to increase the intensity until a robust MEP is elicited on every trial. This intensity will be used for the single pulse experiment. Ask the participant to contract their lip muscles as hard as they can and visually inspect the EMG signal to determine the amplitude of this maximal contraction.

Instruct the participant to reduce the lip contraction, guiding them to reach a level that is roughly 20%of the maximum. Then ask the participant to hold this level for one minute. After the training is completed and the participant can hold the contraction for one minute, instruct the participant to contract and hold their lip to 20%maximum before delivering 10 pulses over the hotspot for the O muscle.

If less than five meps were elicited, increase the intensity. If more than five meps were recorded, lower the intensity. Repeat the experiment until a minimum intensity level is found that elicits meps on at least 50%of the trials to carry out low frequency repetitive TMS or RTMS.

Use the mag stim by STEM system that consists of two stimulators. Create a monophasic pulse sequence of point 66 hertz by delivering TMS pulses by alternating the triggering of the stimulators for 15 minutes. Monitor the recordings from the lip and the hand muscles during the RTMS to ensure nops are elicited.

That would indicate an increase in excitability or spreading effects to the neighboring representation. Also monitor the participant for signs of discomfort or changes in their level of alertness. This figure shows meps recorded from a relaxed and contracted lip muscle.

The intensity of the TMS pulses were kept constant across three levels of contraction. The motor excitability increases when the muscle is contracted, and consequently the mapps get larger. Shown here are lipps recorded while listening to speech and nonverbal noise and while watching I and speech movements.

The meps were enhanced during listening to and watching speech showing that the excitability of the lip motor cortex was enhanced during speech perception. Based on a recent study looking at changes in excitability in the lip motor cortex during observation of visual mouth movements, lipps were recorded during visual perception of known speech, unknown speech, non-PE mouth movements and a still face lipps were larger during observation of known speech than unknown speech or non-PE mouth movements. These findings suggest that the lip motor cortex participates in processing of visual signals during speech communication.

This figure shows that 15 minutes of repetitive TMS disrupted functioning of the lip motor cortex for about 15 minutes. Finally, TMS induced disruption of the lip motor cortex impaired discrimination of speech sounds that are produced using the lips. This suggests that the lip motor cortex contributes to speech perception in an articulator specific manner.

Following this procedure, TMS can be combined with other methods like E-G-M-E-G and FMI in order to answer additional questions like whether the lip motor cortex interacts with other brain areas during speech communication. After watching this video, you should have a good understanding of how to record motor oke potentials from the lip muscles and how to use single pulls and repetitive TMS.