Simple Metric For Scaling Motor Threshold Based on Scalp-Cortex Distance: Application to Studies Using Transcranial Magnetic Stimulation

TLDR

Transcranial magnetic stimulation (TMS) is a focal brain‑stimulation technique whose intensity is usually set by motor threshold, yet most studies ignore how scalp‑cortex distance affects stimulation. The study proposes a simple adjustment to motor threshold that accounts for scalp‑cortex distance to improve TMS calibration. The authors adjust motor threshold by scaling it proportionally to scalp‑cortex distance, adding 3% per millimeter. They found that each millimeter of scalp‑cortex distance requires a 3% increase in TMS output, a gradient that can lead to under‑ or overstimulation if not accounted for.

Abstract

Transcranial magnetic stimulation (TMS) is a unique method in neuroscience used to stimulate focal regions of the human brain. As TMS gains popularity in experimental and clinical domains, techniques for controlling the extent of brain stimulation are becoming increasingly important. At present, TMS intensity is typically calibrated to the excitability of the human motor cortex, a measure referred to as motor threshold (MT). Although TMS is commonly applied to nonmotor regions, most applications do not consider the effect of changes in distance between the stimulating device and underlying neural tissue. Here we show that for every millimeter from the stimulating coil, an additional 3% of TMS output is required to induce an equivalent level of brain stimulation at the motor cortex. This abrupt spatial gradient will have crucial consequences when TMS is applied to nonmotor regions because of substantial variance in scalp-cortex distances over different regions of the head. Stimulation protocols that do not account for cortical distance therefore risk substantial under- or overstimulation. We describe a simple method for adjusting MT to account for variations in cortical distance, thus providing a more accurate calibration than unadjusted MT for the safe and effective application of TMS in clinical and experimental neuroscience.