TRIAL-TO-TRIAL VARIABILITY OF CORTICOSPINAL VOLLEYS IN HUMAN-SUBJECTS

The trial-to-trial variability of the different components of corticos pinal volleys evoked by transcranial electrical stimulation of the mot or cortex using a constant stimulus intensity was measured from epidur al recordings during surgery to correct scoliosis. The recordings were made when there was no operative interference, and blood pressure, te mperature, ventilation and anaesthetic regimen were stable. A simple D wave with a single negative peak of 10-30 mu V amplitude was recorded in 4 patients. It varied little in amplitude (S.D.s <8% for 100 conse cutive single responses). In 4 patients the stimulus was adjusted to p roduce a complex D wave with 3 components, the earliest 2 of which ari se from subcortical/brain-stem sites. The variability of amplitude of these components was high (S.D.s of 13-50%), but the variability of la tency was low (S.D.s of 2-3%). Eighteen I waves were recorded in 6 of the subjects. Their variability from trial to trial was similar to tha t of the components of the complex D wave. It is argued that there wou ld be greater trial-to-trial variability of the corticospinal volley i n the awake state, particularly when the stimulus was magnetic rather than electrical. Explanations for changes in the compound muscle actio n potential produced by transcranial stimulation, electrical or magnet ic, must take into account that a constant stimulus does not evoke an identical descending volley.