Abstract
Most single-pulse transcranial magnetic stimulation (TMS) parameters (e.g., motor threshold, stimulus-response function, cortical silent period) are used to examine corticospinal excitability. Paired-pulse TMS paradigms (e.g., short- and long-interval intracortical inhibition (SICI/LICI), short-interval intracortical facilitation (SICF), and short- and long-latency afferent inhibition (SAI/LAI)) provide information about intracortical inhibitory and facilitatory networks. This has long been done by the conventional TMS method of measuring changes in the size of the motor-evoked potentials (MEPs) in response to stimuli of constant intensity. An alternative threshold-tracking approach has recently been introduced whereby the stimulus intensity for a target amplitude is tracked. The diagnostic utility of threshold-tracking SICI in amyotrophic lateral sclerosis (ALS) has been shown in previous studies. However, threshold-tracking TMS has only been used in a few centers, in part due to the lack of readily available software but also perhaps due to uncertainty over its relationship to conventional single- and paired-pulse TMS measurements. A menu-driven suite of semi-automatic programs has been developed to facilitate the broader use of threshold-tracking TMS techniques and to enable direct comparisons with conventional amplitude measurements. These have been designed to control three types of magnetic stimulators and allow recording by a single operator of the common single- and paired-pulse TMS protocols. This paper shows how to record a number of single- and paired-pulse TMS protocols on healthy subjects and analyze the recordings. These TMS protocols are fast and easy to perform and can provide useful biomarkers in different neurological disorders, particularly neurodegenerative diseases such as ALS.
Highlights
Transcranial magnetic stimulation (TMS) of the motor cortex is a non-invasive method for examining cortical physiology and the pathophysiology of many neurological conditions, including neurodegenerative diseases[1 ]
In the first version of this approach applied to SICI, namely 'serial' threshold-tracking (T-SICIs), a similar tracking method was used to the one employed in nerve excitability testing: the 'threshold' was first estimated at one interstimulus interval (ISI) and tracked serially at successive interstimulus intervals (ISIs)
resting motor threshold (RMT) for a 200 μV (RMT200) or a 1000 μV (RMT1000) peakto-peak response were detected by a '4→2→1' tracking rule and logarithmic regression as previously described[18]
Summary
Transcranial magnetic stimulation (TMS) of the motor cortex is a non-invasive method for examining cortical physiology and the pathophysiology of many neurological conditions, including neurodegenerative diseases[1 ]. An alternative approach is threshold-tracking TMS, first described over 20 years ago12 , 13 In this case, the intensity of successive stimuli is varied to achieve a fixed target amplitude response. The intensity of successive stimuli is varied to achieve a fixed target amplitude response Both conventional and thresholdtracking techniques can be used with different ISIs. In the first version of this approach applied to SICI, namely 'serial' threshold-tracking (T-SICIs), a similar tracking method was used to the one employed in nerve excitability testing: the 'threshold' was first estimated at one interstimulus interval (ISI) and tracked serially at successive ISIs. In the first version of this approach applied to SICI, namely 'serial' threshold-tracking (T-SICIs), a similar tracking method was used to the one employed in nerve excitability testing: the 'threshold' was first estimated at one interstimulus interval (ISI) and tracked serially at successive ISIs This method has been widely used by one group and advocated as a potential biomarker for ALS because of its high diagnostic utility14 , 15 , 16 , 17. The TMS method involves three stages: 1) preparation of the subject, 2) recording the TMS, and 3) analyzing the results
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