Pulse width dependence of motor threshold and input–output curve characterized with controllable pulse parameter transcranial magnetic stimulation

ObjectiveTo demonstrate the use of a novel controllable pulse parameter TMS (cTMS) device to characterize human corticospinal tract physiology.MethodsMotor threshold and input–output (IO) curve of right first dorsal interosseus were determined in 26 and 12 healthy volunteers, respectively, at pulse widths of 30, 60, and 120 μs using a custom-built cTMS device. Strength–duration curve rheobase and time constant were estimated from the motor thresholds. IO slope was estimated from sigmoid functions fitted to the IO data.ResultsAll procedures were well tolerated with no seizures or other serious adverse events. Increasing pulse width decreased the motor threshold and increased the pulse energy and IO slope. The average strength–duration curve time constant is estimated to be 196 μs, 95% CI [181 μs, 210 μs]. IO slope is inversely correlated with motor threshold both across and within pulse width. A simple quantitative model explains these dependencies.ConclusionsOur strength–duration time constant estimate compares well to published values and may be more accurate given increased sample size and enhanced methodology. Multiplying the IO slope by the motor threshold may provide a sensitive measure of individual differences in corticospinal tract physiology.SignificancePulse parameter control offered by cTMS provides enhanced flexibility that can contribute novel insights in TMS studies.

► Controllable pulse parameter TMS (cTMS) is used to characterize the pulse width dependence of motor threshold and input–output (IO) slope in this first-in-human cTMS study.
► Average strength–duration curve time constant for motor cortex is estimated to be 196 μs, 95% CI [181 μs, 210 μs].
► IO slope increases with pulse width and is inversely correlated with the motor threshold both across and within pulse width, consistent with the predictions of a novel quantitative model.