Standardized Induction and Assessment of Long-term Potentiation-like Cortical Plasticity Using Transcranial Magnetic Stimulation

We study corticoplasticity to understand and measure synaptic changes in health and neurological disorders. Variability in stimulation, coil positioning, and the measurement timing makes reproducible iTBS plasticity studies challenging. To begin, explain the aim of the assessment, the main experimental procedures, and any potential risk factors associated with the study.

Request an acknowledgement of the consent process and obtain a signature on the informed consent form. Use the neuronavigation system to identify the functional motor hotspot in the primary motor cortex corresponding to the contralateral target muscle. Now, generate a three-dimensional head model.

Using the individual MRI scans, select and register anatomical landmarks across the axial, sagittal, and coronal planes. Perform scalp surface segmentation to complete the generation of the three-dimensional head model. After completing head model registration, perform coil calibration to ensure precise real-time tracking within the neuronavigation system.

In the software, navigate to Settings, click on TMS Coil, and select Calibrate Tool Faces. Calibrate fiducial points on the coil face, as well as the coils control point marker. Place the TMS coil with reflective markers on the designated calibration block.

After initial calibration, follow the system prompt to place the coil over the calibration block and confirm its position for accuracy. Then, click on Neuronavigation to validate single coil. Accept only those calibrations with a spatial error less than three millimeters.

Otherwise, repeat the calibration process. Ask the participant to sit in a comfortable chair with both back and arm support to minimize head and body movement. Instruct the participant to keep both hands completely relaxed and still throughout the procedure.

Attach infrared reflective markers to the participant's forehead to enable real-time head tracking during the session, and secure the markers using adhesive patches. Using a tract pointer, mark three anatomical landmarks on the participant's scalp:the nasion, the left supratragic notch, and the right supratragic notch. Confirm that the markers are recognized by the neuronavigation system, indicated by their green appearance on screen.

Using the same pointer, collect approximately 200 to 300 additional points on the scalp to improve registration accuracy. Allow the neuronavigation software to automatically generate an individualized head shape model from these points to optimize alignment between the MRI scans and the actual coordinate system. Next, place surface electrodes two centimeters apart on the abductor pollicis brevis muscle in a belly-tendon montage.

Position the reference electrode distally, near the interphalangeal joint of the thumb. Next, administer single-pulse TMS approximately five centimeters lateral and zero to one centimeters anterior to the vertex, contralateral to the target muscle. Use this to determine the primary motor cortex coordinates that elicit the maximal motor-evoked potential amplitude for motor hotspot localization.

Instruct the participant to keep their hand completely relaxed to prevent voluntary muscle contractions that may affect measurements. Orient the handle of the coil 45 degrees posterior to the midline to direct the electromagnetic current perpendicularly to the central sulcus. Systematically move the coil in one-centimeter steps around the motor cortex region in all directions, anterior, posterior, medial, and lateral, at five-second intervals.

Identify the motor hotspot as the site that produces the largest motor-evoked potential amplitude and the shortest latency in the relaxed abductor pollicis brevis muscle. Allow the system to automatically record key spatial parameters, including the distance between the coil and the target point, the tilt deviation, and the rotation deviation. Use these values to ensure accurate and reproducible coil placement during stimulation.

Using the same surface electromyography setup and parameters, measure the peak-to-peak amplitude of the motor-evoked potential. Define the resting motor threshold, or RMT, as the minimum TMS stimulus intensity that produces motor-evoked potentials over 50 microvolts in at least 5 of 10 single-pulse trials. Record 20 consecutive motor-evoked potentials at five-second intervals from the motor hotspot as a baseline measurement.

Set the stimulation intensity to 120%of the RMT to evoke approximately one-millivolt motor-evoked potentials. Deliver stimulation using a TMS device. Set the stimulation intensity to 80%of the RMT.

Apply bursts of three pulses at 50 hertz, repeated at five hertz. At 5, 10, 15, and 30 minutes after the iTBS, record 20 motor-evoked potentials using the same stimulation intensity to assess plasticity. Calculate the mean peak-to-peak amplitude of the 20 motor-evoked potentials recorded at baseline and at each post-stimulation time Point to quantify cortical excitability.

Express the post-stimulation amplitudes as normalized ratios relative to baseline to standardized measurements across sessions and individuals. Following iTBS stimulation, motor-evoked potential amplitudes were recorded at multiple time points to assess cortical excitability over time. Mean raw motor-evoked potential amplitude increased after stimulation, peaking at 15 minutes and declining by 30 minutes.

Normalized motor-evoked potential amplitude showed a similar trend, increasing above baseline and gradually declining by 30 minutes. The mean normalized motor-evoked potential amplitude across all post-stimulation time points exceeded 1.1, classifying the participant as facilitated. We offer standardized neuronavigation-guided iTBS protocol for reliable and reproducible plasticity assessment.

Our protocol uses neuronavigation, ensuring precise, reliable iTBS delivery and robust LTP-like plasticity with better consistency. They enable reproducible studies of synaptic plasticity, supporting therapeutic development and neurorehabilitation strategies.