Table 1.
Common silent period outcome measures
| Outcome Measure | Description | Interpretation |
|---|---|---|
| Resting Motor Threshold (RMT) | ||
| RMT | Minimal stimulator intensity needed to reliably induce a MEP when a TMS pulse is applied to the motor hotspot. See Table 2 for common methods used to determine the RMT | Measure of cortical (i.e., corticospinal neuron) excitability; lower RMTs are interpreted as greater cortical excitability |
| Cortical Silent Period (cSP) | ||
| Duration | Time elapsed between the onset and offset of a cSPa | Duration of suppression of the contralateral EMG signal. Greater cSP duration is interpreted as greater intrahemispheric inhibition |
| MEP Amplitude | Spikes in the muscle activity resulting from the activation of corticospinal neurons. During cSP trials, a MEP (typically) precedes the silent period in the target (ON) muscle | Measure of cortical (i.e., corticospinal neuron) excitability. Greater MEP amplitude is interpreted as greater cortical excitability. Larger MEPs may predict longer cSPs (Orth & Rothwell, 2004). For this reason investigators should consider including the MEP : cSP ratio as an extra outcome variable |
| MEP : cSP Ratio | Ratio of MEP amplitude to the duration of the corresponding cSP | Intrahemispheric inhibition, controlling for cortical excitability (Orth & Rothwell, 2004). Provides a measure of the net excitability of the corticospinal tract (i.e., the balance between inhibition and excitability). If the cSP duration is greater in Group A than Group B and there is no difference in the corresponding MEP amplitudes, then Group A is exhibiting increased intrahemispheric inhibition compared to Group B. |
| Ipsilateral Silent Period (iSP) | ||
| Duration | Time elapsed between the onset and offset of an iSPa | Duration of suppression of the ipsilateral EMG signal. Greater duration is interpreted as greater interhemispheric inhibition |
| Depthb | Average or maximum EMG signal during the iSP, normalized to the pre-stimulus EMG level | Average or maximal amount of suppression of the ipsilateral EMG, accounting for the effect of the pre-stimulus muscle contraction level. Greater depth is interpreted as greater interhemispheric inhibition |
| Areab | Integral of the rectified EMG trace during the iSP (i.e., between the iSP onset and offset)a | Amount of suppression of the ipsilateral EMG. Laraer area is interpreted as greater interhemispheric inhibition |
| Normalized Areab | Area of the rectified EMG trace between the onset and offset of the iSP, normalized to the pre-stimulus EMG level | Amount of suppression of the ipsilateral EMG, accounting for the effect of the pre-stimulus muscle contraction level. Greater normalized iSP is interpreted as greater interhemispheric inhibition |
| Onset Latency | Time elapsed between the TMS pulse and the iSP onset | Speed of inter-hemispheric signal transmission. Shorter iSP onset latency is interpreted as faster interhemispheric signal transmission |
| Transcallosal conduction time (TCT) | Time elapsed between the contralateral MEP onset and the iSP onset | Speed of inter-hemispheric signal transmission. Shorter TCT is interpreted as faster interhemispheric signal transmission |
Table 1 Note. MEP = motor-evoked potential; TMS = transcranial magnetic stimulation; EMG = electromyography
a
See Section 5.2 and Table 2 for a discussion of common methods for determining silent period onsets and offsets
b
These metrics (depth, area, and normalized area) may also be calculated for cSPs and could provide useful additional outcome metrics for cSP studies. However, these metrics are less frequently reported for cSPs compared to iSPs