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Qi F, Wu AD, Schweighofer N. Fast estimation of transcranial magnetic stimulation motor threshold. Brain Stimul 4: 50-57

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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    • "Some TMS devices already have built-in EMG options, similar to modern ECT devices. A number of methods for estimating MT presently exist beyond the commonly used relative frequency method (Rossini et al., 1994; Rothwell et al., 1999), including adaptive methods such as maximum-likelihood threshold-tracking algorithms (Awiszus, 2003; Mishory et al., 2004) and Bayesian adaptive methods (Qi et al., 2011), the two-threshold method in which lower and upper thresholds are found and averaged (Mills and Nithi, 1997), and supervised parametric estimation, in which MT is estimated from the input/output curve across TMS intensities (Tranulis et al., 2006). The relative frequency method itself can be improved upon, for instance by beginning at a level below the MT of a subject, and increasing in 5% increments of maximum device output until MEPs greater than 50 uV are consistently evoked, and then decrementing in 1% steps until less than five out of ten positive responses are found, as suggested by Groppa et al. (2012). "
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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. DOI:10.1016/j.clinph.2013.06.187 · 2.98 Impact Factor
  • Brain Stimulation 01/2011; 4(1):58-9; discussion 60-3. DOI:10.1016/j.brs.2010.09.004 · 5.43 Impact Factor
  • Brain Stimulation 01/2011; 4(1):60-1. DOI:10.1016/j.brs.2010.09.005 · 5.43 Impact Factor
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