Variability of motor potentials evoked by transcranial magnetic stimulation depends on muscle activation

Abstract

The purpose of this research was to determine whether motor cortex excitability assessed using transcranial magnetic stimulation (TMS) is less variable when subjects maintain a visually controlled low-level contraction of the muscle of interest. We also examined the dependence of single motor evoked potential (MEP) amplitude on stimulation intensity and pre-stimulus muscle activation level using linear and non-linear multiple regression analysis. Eight healthy adult subjects received single pulse TMS over the left motor cortex at a point where minimal stimulation intensity was required to produce MEPs in extensor digitorum communis (EDC). Voluntary activation of the muscle was controlled by visual display of a target force (indicated by a stable line on an oscilloscope) and the isometric force produced as the subject attempted to extend the fingers (indicated by a line on the oscilloscope representing the finger extension force) while subjects were instructed to: exert zero extension force (0%) and produce forces equal to 5 and 10% of maximum voluntary finger extension under separate conditions. Relative variability (coefficient of variation) of single MEPs at a constant stimulus intensity and of pre-stimulus muscle EMG was lower during maintained 5 and 10% contractions than at 0% contraction levels. Therefore, maintaining a stable low intensity contraction helps stabilize cortical and spinal excitability. Multiple regression analyses showed that a linear dependence of single MEPs on stimulation intensity and pre-stimulus muscle activation level produced similar fits to those for a non-linear dependence on stimulus intensity and a linear dependence on pre-stimulus EMG. Thus, a simple linear method can be used to assess dependence of single MEP amplitudes on both stimulus intensity (to characterize slope of the recruitment curve) and low intensity background muscle activation level.

Fig 1

TMS-induced MEPs recorded from EDC at resting motor threshold stimulus intensity (1.0RMT) and at 1.2RMT and 1.5RMT in two subjects (S4, S5). Each panel shows five superimposed MEPs at a single stimulus intensity at one activation level in the same subject. The stimulus was delivered at 50 ms in all records. Note stimulus artifact and onset of MEP response (dashed vertical lines at 0% contraction level)

Fig 2

Peak-to-peak MEP amplitude versus stimulus intensity (relative to resting motor threshold) for three different levels of finger extensor force (0, 5, 10% of voluntary maximum). Each point is the average MEP amplitude for eight subjects at one stimulus intensity. The three plotted lines are best-fit non-linear (Boltzmann equation) regression lines for 0, 5 and 10% contraction levels. The error bars represent the mean of the standard deviations of MEP amplitude for eight subjects

Fig 3

a Mean coefficient of variation (CV) of peak-to-peak MEP amplitude versus stimulation intensity at different muscle contraction levels. b Mean absolute variability (SD) of peak-to-peak MEP amplitudes (% of max MEP) versus stimulus intensity at different activation levels. c Mean coefficient of variation of pre-stimulus EMG levels versus stimulus intensity at three different finger extensor contraction levels (0, 5 and 10% of maximum voluntary force). Each plotted point is the mean of within-subject coefficients of variation or standard deviations from eight subjects at a single stimulus intensity and activation level

Fig 4

Motor cortex recruitment curves (peak-to-peak MEP amplitude versus stimulus intensity) at different pre-stimulus activation levels for three subjects (a, b, c). Each plotted point is the mean MEP amplitude for five consecutive stimuli at one stimulus intensity and contraction level (0, 5, 10% of maximum voluntary finger extensor force) in a single subject. The three plotted lines are best-fit non-linear (Boltzmann equation) regression lines for 0, 5 and 10% contraction levels

Fig 5

Scattergraphs showing actual MEP amplitude versus MEP amplitude predicted from linear multiple regression with stimulus intensity and average pre-stimulus EMG amplitude as independent variables in two subjects. Each plotted point represents the peak-to-peak amplitude of a single MEP in one subject