Fast estimation of transcranial magnetic stimulation motor threshold.

Neuroscience, University of Southern California, Los Angeles, California 90089, USA.
Brain Stimulation (Impact Factor: 5.43). 01/2011; 4(1):50-7. DOI: 10.1016/j.brs.2010.06.002
Source: PubMed

ABSTRACT In Transcranial Magnetic Stimulation (TMS), the Motor Threshold (MT) is the minimum intensity required to evoke a liminal response in the target muscle. Because the MT reflects cortical excitability, the TMS intensity needs to be adjusted according to the subject's MT at the beginning of every TMS session.
Shorten the MT estimation process compared to existing methods without compromising accuracy.
We propose a Bayesian adaptive method for MT determination that incorporates prior MT knowledge and uses a stopping criterion based on estimation of MT precision. We compared the number of TMS pulses required with this new method with existing MT determination methods.
The proposed method achieved the accuracy of existing methods with as few as seven TMS pulses on average when using a common prior and three TMS pulses on average when using subject-specific priors.
Our adaptive Bayesian method is effective in reducing the number of pulses to estimate the MT.

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    ABSTRACT: BACKGROUND: High-strength static magnetic field stimulation (SMS) results in a period of reduced corticomotor excitability that may be mediated through a decrease in membrane excitability. OBJECTIVE: As resting motor threshold (RMT) is thought to reflect membrane excitability, we hypothesized that SMS may increase RMT and that there would be an inverse relationship between RMT and motor-evoked potential (MEP) amplitude. METHODS: Ten healthy subjects (aged 20-29; 4 females) participated in a double-blinded crossover design comparing MEP amplitude and RMT before and after a 15-min period of SMS or sham stimulation over primary motor cortex (M1). RESULTS: MEP amplitude was initially significantly reduced post-SMS (∼20%), and returned to baseline by 6 min post-intervention. MEP amplitude and RMT were inversely correlated (r(2) = 0.924; P = 0.001). Sham stimulation had no effect on MEP amplitude (P = 0.969) or RMT (P = 0.549). CONCLUSION: After SMS, corticomotor excitability is transiently reduced in association with a correlated modulation of RMT. SMS after effects may be mediated in part by a reduction in membrane excitability, suggesting a possible role for non-synaptic (intrinsic) plasticity mechanisms.
    Brain Stimulation 04/2013; · 5.43 Impact Factor
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    ABSTRACT: Objective While the standard has been to define motor threshold (MT) using EMG to measure motor cortex response to transcranial magnetic stimulation (TMS), another method of determining MT using visual observation of muscle twitch (OM-MT) has emerged in clinical and research use. We compared these two methods for determining MT.Methods Left motor cortex MTs were found in 20 healthy subjects. Employing the commonly-used relative frequency procedure and beginning from a clearly suprathreshold intensity, two raters used motor evoked potentials and finger movements respectively to determine EMG-MT and OM-MT.ResultsOM-MT was 11.3% higher than EMG-MT (p < 0.001), ranging from 0% to 27.8%. In eight subjects, OM-MT was more than 10% higher than EMG-MT, with two greater than 25%.Conclusions These findings suggest using OM yields significantly higher MTs than EMG, and may lead to unsafe TMS in some individuals. In more than half of the subjects in the present study, use of their OM-MT for typical rTMS treatment of depression would have resulted in stimulation beyond safety limits.SignificanceFor applications that involve stimulation near established safety limits and in the presence of factors that could elevate risk such as concomitant medications, EMG–MT is advisable, given that safety guidelines for TMS parameters were based on EMG-MT.
    Clinical neurophysiology: official journal of the International Federation of Clinical Neurophysiology 01/2014; 125(1):142–147. · 3.12 Impact Factor
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    ABSTRACT: 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 (I(1mV)). METHODS: In 10 healthy controls we determined MT and I(1mV) 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(1mV) (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(1mV). 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.
    Clinical neurophysiology: official journal of the International Federation of Clinical Neurophysiology 10/2012; · 3.12 Impact Factor


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