VARIABILITY OF MOTOR POTENTIALS-EVOKED BY TRANSCRANIAL MAGNETIC STIMULATION

We studied the effect of stimulus intensity, coil size, mental alertne ss and prestimulus muscle contraction on the variability of motor evok ed potentials (MEPs) produced by magnetic cortical stimulation (MCS). In 5 healthy subjects we delivered MCS either with a circular coil cen tered 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 s timuli were given at 4 stimulus intensities (SIs) in 10% increments ab ove resting motor threshold. Concurrent mental arithmetic constituted mental alertness. Spectral analysis was performed on data from 300 con secutive stimuli. The variability of MEP response size was inversely r elated to stimulus intensity, prestimulus voluntary muscle contraction , the recruitment of motoneurons and the size of the field generated b y the magnetic coil. The MEP variability was larger than and not corre lated with the variability of the H-reflex. Fast Fourier transformatio n and cross-correlation analysis did not identify a consistent dominan t frequency, suggesting that the variability in MEP size is essentiall y random. We suggest that the variability in MEP response is caused by constant, rapid, spontaneous fluctuations in corticospinal and segmen tal motoneuron excitability levels. Any maneuver that raises this leve l or increases the probability of motoneuron firing will decrease MEP variability.