Central motor tract propagation in man: studies with non-invasive, unifocal, scalp stimulation

Motor-evoked potentials (MEPs) to unifocal, anodal scalp stimulation have been recorded in 45 healthy volunteers from proximal and distal upper limb muscles. Optimal responses were obtained through a pericranial cathode consisting of 6 or more regularly spaced, interconnected plaques whose impedance was carefully balanced with that of a 0.8-cm2 stimulating anode on the scalp. Individual rectangular pulses with threshold intensity (70-86 mA) 100-200 microseconds in duration, with rise-decay times shorter than 50 microseconds resulted more efficient in eliciting individual MEPs in the target muscle. The foci of maximal response for hand and shoulder muscles were localized. The scalp-to-cervical cord conduction time along the motor tracts governing the hand muscles was 5.21 +/- 0.42 ms. This index was highly correlated with the subject's height and stable in time when repeatedly tested. Collision between orthodromically and antidromically propagated motor impulses was obtained by simultaneous stimulation of scalp and median nerve at wrist. Response facilitation was achieved by means of prestimulus voluntary contraction of the target muscle, continuous vibration of its tendon or scalp stimulation with paired shocks. Facilitation of MEPs was obtained by prestimulating the ipsilateral motor cortex 8-24 ms before the stimulation of the one contralateral to the target muscle. This was considered at least in part mediated by transcallosal connections. An efferent volley secondary to scalp stimulation was recorded for the nerve trunk with the near-nerve technique. Segmental and suprasegmental mechanisms underlying MEP facilitation provoked by phasic and tonic contractions of the target muscle have been investigated.