DIRECT COMPARISON OF CORTICOSPINAL VOLLEYS IN HUMAN-SUBJECTS TO TRANSCRANIAL MAGNETIC AND ELECTRICAL-STIMULATION

1. The effects of graded transcranial magnetic and anodal electrical s timulation of the human motor cortex were compared in human subjects u ndergoing orthopaedic operations on the spine, before and after withdr awal of volatile anaesthesia. Corticospinal volleys were recorded from the spinal cord in the low-cervical and low-thoracic regions (six sub jects) or the mid-thoracic region (two subjects) using bipolar electro des inserted into the epidural space. 2. Electrical stimuli were deliv ered using anode at the vertex and cathode 7 cm laterally. The cortico spinal volley at threshold consisted of a single deflection with a mea n latency to peak of 4.17 ms at the rostral recording site. With furth er increases in stimulus strength the latency of this D wave shortened in two steps, first by 0.89 ms (seven subjects) and then by a further 0-8 ms (two subjects), indicating that the site of activation of some corticospinal neurones had shifted to deep subcortical sites. 3. When volatile anaesthetics were given, a corticospinal volley could not be defined in three subjects with magnetic stimuli of 70, 80 and 100 % m aximal stimulator output with the coil at the vertex (Novametrix Magst im 200, round coil, external diameter 14 cm). In the remaining five su bjects, the component of lowest threshold was a D wave recorded at the rostral site at 4.0 ms when stimulus intensity was, on average, 70 %. With stimuli of 90-1 00 % a total of five small I waves could be defi ned in the five subjects (i.e. on average one I wave per subject). 4. After cessation of volatile anaesthetics in seven subjects, the thresh olds for D and I waves were lower and their amplitudes were greater. T he D wave remained the component of lowest threshold in all subjects, appearing at the low-cervical level with magnetic stimuli of 50 %. How ever, in three subjects I waves also appeared at D wave threshold, and the D wave was smaller than with electrical stimulation at I wave thr eshold. There was no consistent change in latency of the magnetic D wa ve as stimulus intensity was increased to 1 00 % . 5. These findings s uggest that the previously reported difference in latency of the EMG p otentials produced in upper-limb muscles by anodal stimulation and mag netic stimulation of the human motor cortex is not because the cortico spinal volley induced by magnetic stimulation lacks a D wave. While it is unlikely that the volleys evoked by magnetic stimulation in the pr esent study arose exclusively from upper-limb motor cortex, the result s provide no support for the 'D and I wave hypothesis', as originally formulated.