Motor Evoked Potentials Size Research Articles

Variability of motor potentials evoked by transcranial magnetic stimulation

We studied the effect of stimulus intensity, coil size, mental alertness and prestimulus muscle contraction on the variability of motor evoked potentials (MEPs) produced by magnetic cortical stimulation (MCS). In 5 healthy subjects we delivered MCS either with a circular coil centered at the vertex or a figure-8 coil centered over the motor cortex hand area, recording from first dorsal interosseous. With the subject at rest or exerting 5% maximum voluntary contraction, 30 consecutive stimuli were given at 4 stimulus intensities (SIs) in 10% increments above resting motor threshold. Concurrent mental arithmetic constituted mental alertness. Spectral analysis was performed on data from 300 consecutive stimuli. The variability of MEP response size was inversely related to stimulus intensity, prestimulus voluntary muscle contraction, the recruitment of motoneurons and the size of the field generated by the magnetic coil. The MEP variability was larger than and not correlated with the variability of the H-reflex. Fast Fourier transformation and cross-correlation analysis did not identify a consistent dominant frequency, suggesting that the variability in MEP size is essentially random. We suggest that the variability in MEP response is caused by constant, rapid, spontaneous fluctuations in corticospinal and segmental motoneuron excitability levels. Any maneuver that raises this level or increases the probability of motoneuron firing will decrease MEP variability.

Motor potentials evoked by paired cortical stimuli

We recorded the motor evoked potentials (MEPs) from the abductor pollicis brevis muscle, after supramaximal electrical transcranial stimulation, and studied the effect of paired transcranial shocks with varying interstimulus time intervals, in 10 normal subjects, 4 patients with median nerve neuropathy and 2 patients with motoneurone disease. In relaxed muscles the amplitude of the MEP evoked by a single shock averaged 30% of the M wave. With intervals from 1 to 2.5 msec 2 shocks evoked one MEP far larger in size than the control MEP (70% of the M wave). With intervals of 10 msec and longer, the 2 shocks evoked 2 independent MEPs; the size of the MEP following the second shock (test) was inversely correlated with the size of the control MEP: the more the control MEP approached the size of the M wave, the smaller the test MEP. Single motor unit records showed that, in the normal subjects and patients with peripheral neuropathy, the same motor unit was activated either by the first or the second shock, whereas in the patients with motoneurone disease it fired twice. In active muscles, the control MEP averaged 70% of the M wave. With intervals of 10 msec and longer the test MEP was markedly suppressed; with 100 msec intervals it fully recovered. In relaxed muscles, by delivering a double shock at a 1.5 msec interval, thus evoking a large MEP, followed by a second double-shock, the test MEP was completely suppressed for a period of 20 msec; it began to recover at 50 msec intervals and fully recovered after 150 msec. These results indicate that: (1) high-threshold spinal motoneurones can profit from temporal summation if double-shocks are delivered at short time intervals; (2) the synchronous excitation of the motoneuronal pool produced by transcranial stimulation is followed by a 20 msec period of absolute inhibition, possibly through a massive activation of the Renshaw system; (3) during voluntary contraction, only a portion of the motoneuronal pool remains refractory, possibly because of the enhanced spinal excitability.

Amplitude asymmetry of hemi-field pattern reversal VEPs

TMS techniques have provided controversial information on motor cortical function in Huntington's disease (HD). We investigated the excitability of motor cortex in patients with HD using repetitive transcranial magnetic stimulation (rTMS).Eleven patients with HD, and 11 age-matched healthy subjects participated in the study. The clinical features of patients with HD were evaluated with the United Huntington's Disease Rating Scale (UHDRS). rTMS was delivered with a Magstim Repetitive Magnetic Stimulator through a figure-of-8 coil placed over the motor area of the first dorsal interosseus (FDI) muscle. Trains of 10 stimuli were delivered at 5 Hz frequency and suprathreshold intensity (120% resting motor threshold) with the subjects at rest and during voluntary contraction of the target muscle.In healthy subjects at rest, rTMS produced motor evoked potentials (MEPs) that increased in amplitude over the course of the trains. Conversely in patients, rTMS left the MEP size almost unchanged. In both groups, during voluntary contraction rTMS increased the silent period (SP) duration.Because rTMS modulates motor cortical excitability by activating cortical excitatory and inhibitory interneurons these findings suggest that in patients with HD the excitability of facilitatory intracortical interneurones is decreased.We suggest that depressed excitability of the motor cortex in patients with HD reflects a disease-related weakening of cortical facilitatory mechanisms.