Distance-adjusted motor threshold for transcranial magnetic stimulation.

Area of Science:

• Neuroscience
• Neurophysiology
• Biophysics

Background:

• Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation technique.
• Accurate calibration of TMS intensity is crucial for effective and safe cortical stimulation.
• Resting motor threshold (MT) is commonly used to index cortical excitability.

Purpose of the Study:

• To investigate the relationship between coil-cortex distance and TMS efficacy.
• To compare the impact of different figure-eight coil sizes (50 mm vs. 70 mm) on this relationship.
• To determine if the coil-cortex distance effect differs between the left and right motor cortex.

Main Methods:

• Coil-cortex distance was systematically altered using 5 mm and 10 mm acrylic separators.
• Resting motor threshold (MT) was measured to quantify the effect of cortical stimulation.
• Experiments were conducted on both left and right motor cortices.

Main Results:

• A linear increase in MT was observed with increasing coil-cortex distance.
• Each millimeter increase in distance required approximately 2.8% more stimulator output (0.062 T) to reach MT.
• No significant differences in the distance effect were found between hemispheres or between 50 mm and 70 mm coils.

Conclusions:

• Coil-cortex distance is a critical factor influencing TMS intensity requirements.
• A linear relationship exists between TMS efficacy and coil-cortex distance.
• Distance-adjusted MT provides a more reliable and accurate measure of cortical excitability compared to scalp-surface MT.