Cortical excitability and response variability of transcranial magnetic stimulation.

The magnetic evoked potential (MEP) following transcranial magnetic stimulation is subject to several sources of variability. In this study, relationships between stimulus intensity and MEP latency, amplitude, duration, and area of the hypothenar muscles were assessed in 12 right- and 14 left-handed subjects. Effects of handedness, coil orientation, intensity of stimulation, and intersubject variability on these relationships were analyzed. The intraindividual variability was analyzed as the standard deviation (SD) and the coefficient of variation (CV) of four trials. The mean response threshold was significantly lower (p < 0.0001) for preferential stimulation (32%) than for nonpreferential stimulation (45%). With increasing stimulus intensities, MEP amplitudes still increased at 100% intensity in some subjects while in others the stimulus response-relations saturated. MEP amplitudes at an intensity of 20% above threshold ranged between 6 and 100% of MEP amplitude at maximum intensity. Differences between dominant and non-dominant hands were not seen, regardless of handedness. The SD of latency, amplitude, duration, or area depended on stimulus intensity. The CV however, was influenced by the intensity of stimulation: The CV for amplitude decreased from 46% at threshold to 10% at higher intensities. The variability of the MEP amplitude is only related to stimulus intensity when it is expressed in relation to it but not when it is expressed in absolute terms. The stimulus-response relation offers a more extensive measure of cortical excitability than the use of thresholds alone for the measurement of cortical excitability. The question can be raised whether high intensities should be used for clinical testing rather than threshold-related intensities.