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

ObjectiveStimulation 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 1 mV). 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 1 mV (I1 mV).MethodsIn 10 healthy controls we determined MT and I1 mV with R–R and PEST using a blinded crossover design, and performed within-session serial PEST measurements of MT.ResultsThere 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 I1 mV (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 I1 mV. R–R required significantly more stimuli than PEST. Serial measurements of MT with PEST were reproducible.ConclusionsPEST has the advantage of speed without sacrificing precision when compared to the R–R method, and is adaptable to other SI targets.SignificanceOur results in healthy controls add to increasing evidence in favour of adaptive threshold-hunting methods for determining SI.

► Evaluated the latest International Federation of Clinical Neurophysiology recommendation for determining motor threshold.
► Adaptive threshold-hunting (PEST) determined threshold with fewer stimuli and with comparable results to the Rossini–Rothwell relative-frequency method.
► Equivalent results are obtained when targeting a supra-threshold MEP amplitude (1 mV).