A comparison of relative-frequency and threshold-hunting methods to determine stimulus intensity in transcranial magnetic stimulation

Objective: Stimulation intensity (SI) in transcranial magnetic stimulation is commonly set in relation to motor threshold (MT), or to achieve a motor-evoked potential (MEP) of predefined amplitude (usually 1mV). Recently, IFCN recommended adaptive threshold-hunting over the previously endorsed relative-frequency method. We compared the Rossini-Rothwell (R-R) relative-frequency method to an adaptive threshold-hunting method based on parameter estimation by sequential testing (PEST) for determining MT and the SI to target a MEP amplitude of 1mV (I1mV). Methods: In 10 healthy controls we determined MT and I1mV with R-R and PEST using a blinded crossover design, and performed within-session serial PEST measurements of MT. Results: There was no significant difference between methods for MT (52.6±2.6% vs. 53.7±3.1%; p=0.302; % maximum stimulator output; R-R vs. PEST, respectively) or I1mV (66.7±3.0% vs. 68.8±3.8%; p=0.146). There was strong correlation between R-R and PEST estimates for both MT and I1mV. R-R required significantly more stimuli than PEST. Serial measurements of MT with PEST were reproducible. Conclusions: PEST has the advantage of speed without sacrificing precision when compared to the R-R method, and is adaptable to other SI targets. Significance: Our results in healthy controls add to increasing evidence in favour of adaptive threshold-hunting methods for determining SI. © 2012 International Federation of Clinical Neurophysiology.