Trial-to-trial size variability of motor-evoked potentials. A study using the triple stimulation technique.
Motor-evoked potentials (MEPs) vary in size from one stimulus to the next. The objective of this study was to determine the cause and source of trial-to-trial MEP size variability. In two experiments involving 10 and 14 subjects, the variability of MEPs to cortical stimulation (cortical-MEPs) in abductor digiti minimi (ADM) and abductor hallucis (AH) was compared to those responses obtained using the triple stimulation technique (cortical-TST). The TST eliminates the effects of motor neuron (MN) response desynchronization and of repetitive MN discharges. Submaximal stimuli were used in both techniques. In six subjects, cortical-MEP variability was compared to that of brainstem-MEP and brainstem-TST. Variability was greater for MEPs than that for TST responses, by approximately one-third. The variability was the same for cortical- and brainstem-MEPs and was similar in ADM and AH. Variability concerned at least 10-15% of the MN pool innervating the target muscle. With the stimulation parameters used, repetitive MN discharges did not influence variability. For submaximal stimuli, approximately two-third of the observed MEP size variability is caused by the variable number of recruited alpha-MNs and approximately one-third by changing synchronization of MN discharges. The source of variability is most likely localized at the spinal segmental level.
Rösler
Kai M
KM
Department of Neurology, Inselspital, Bern University Hospital and University of Bern, 3010 Bern, Switzerland. kai-michael.roesler@insel.ch
Roth
Denise M
DM
Magistris
Michel R
MR
D000200:Action Potentials / Q000502:physiology; D000328:Adult; D003344:Cortical Synchronization; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D005528:Foot / Q000294:innervation / Q000502:physiology; D006225:Hand / Q000294:innervation / Q000502:physiology; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D009431:Neural Conduction / Q000502:physiology; D015588:Observer Variation; D011712:Pyramidal Tracts / Q000502:physiology; D011930:Reaction Time / Q000502:physiology; D015203:Reproducibility of Results; D013116:Spinal Cord / Q000502:physiology*; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation