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A comparison of relative-frequency and threshold-hunting methods to determine stimulus intensity in transcranial
B I Silbert1, H I Patterson, D D Pevcic
1Australian Neuro-Muscular Research Institute and Centre for Neuromuscular and Neurological Disorders, M518, University of Western Australia, Nedlands, WA 6009, Australia.
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 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 (I(1) mV).
Methods:
In 10 healthy controls we determined MT and I(1) mV 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 I(1) 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 I(1) mV. 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.
