Corticospinal volleys evoked by anodal and cathodal stimulation of the human motor cortex.

Abstract

SUMMARY1. In fifteen neurologically normal subjects, corticospinal volleys evoked by transcranial stimulation of the motor cortex were recorded from the spinal cord using epidural electrodes in the high-thoracic and low-thoracic regions during surgery to correct scoliosis.2. Anodal stimulation at the vertex produced complex corticospinal volleys that could be recorded at both sites, with multiple waves analogous to the D and I waves documented in animal experiments. These volleys were of higher amplitude when the cathode was 7 cm lateral to the vertex rather than 7 cm anterior. There were no differences in conduction time between the two recording sites for D and I waves, when these waves could be identified at the low-thoracic site.3. Anodal stimuli of 150 V commonly produced a descending volley containing a single peak at both recording sites. Modest increases in stimulus intensity to 225-375 V produced a peak 0-8 ms in advance of the wave of lowest threshold in thirteen subjects and, in seven subjects, further increases produced an additional peak 1-7 ms in advance of the first-recruited wave. The early peaks increased in size with stimulus intensity, replacing the first-recruited wave. These results suggest that the site of impulse initiation with electrical stimulation of the motor cortex shifts from superficial cortex to deep structures, approximately 5 and 10-11 cm below the cortex. These sites are probably the internal capsule and the cerebral peduncle.4. With cathode at the vertex and anode over the 'hand area', the response of lowest threshold occurred at the latency of the anodal D wave but could not be recorded at the low-thoracic site, suggesting that it was generated by the anode over the 'hand area'. Slightly higher intensities induced a 'cathodal D wave' and still higher intensities produced late peaks at latencies of anodal I waves. These cathodal D and I waves involved axons innervating lumbar segments. There was no evidence that cathodal stimulation preferentially produced I waves. Cathodal stimulation at the vertex with the anode 7 cm anteriorly produced similar results: D waves were produced at relatively low intensities, but I waves appeared at relatively high stimulus intensities, if at all.

The Incidence of Unacceptable Movement with Motor Evoked Potentials During Craniotomy for Aneurysm Clipping

“…Too high of a stimulus intensity can activate the deep subcortical motor pathways and bypass higher cortical levels, which can lead to generation of myogenic MEPs from contralateral limbs despite possible cortical ischemia. Subcortical motor pathway ischemia also could be missed if corticospinal tract activation occurred even more caudally (2,5,6,13,20). Transcranial MEPs are made more accurate by decreasing stimulation intensity, and it is recommended that the lowest possible stimulation intensity be used (5,8,20,28).…”

Transcranial electrical stimulation of the motor cortex in man: further evidence for the site of activation.

“…The subjects gave informed consent to the operative, anaesthetic and experimental procedures, which were performed with the approval of the appropriate institutional ethics committee. Most of the stimulating and recording techniques have been described in previous papers (Burke et al 1990;Ugawa, Rothwell, Day, Thompson & Marsden, 1991;Hicks, Burke, Stephen, Woodforth & Crawford, 1992).…”

“…As the intensity of stimulation increased, the site of activation of corticospinal fibres shifted deeper into the brain in all subjects, in two stages, giving rise to two earlier waves, D2 and D3, with threshold latencies of, on average, 0f8 ms (range 0 5-10 ms) and 1P6 ms (range 11-2 0 ms) in advance of D1, respectively. The waves which follow the original D1 wave are thought to be I waves set up indirectly by transsynaptic activation of corticospinal neurones (see Burke et al 1990;Hicks et al 1992). …”

Task dependence of responses in first dorsal interosseous muscle to magnetic brain stimulation in man.

“…We would argue that the responses to electrical stimulation cannot be considered to be uninfluenced by the level of cortical excitability, since only the early D wave in the descending corticospinal volley originates from direct activation of axons, and recordings of this volley following electrical stimulation in both animal and human studies have shown substantial indirect (I) wave activity (Boyd et al 1986;Burke et al 1990;Edgley et al 1990). This I wave activity is certainly influenced by the 13-2 375 D. FLAMENT AND OTHERS level of cortical excitability (Hicks et al 1992) and contributes to the short-latency responses recorded from hand muscles (Day et al 1987;.…”

“…A number of studies have attempted to assess the contribution of cortical excitability by comparing responses to magnetic and scalp electrical stimulation (Datta et al 1989;Day et al 1991;Maertens de Noordhout et al 1992). Since one of the effects of scalp electrical stimulation is to directly excite corticospinal axons deep to the cortex (Burke, Hicks & Stephen, 1990;Edgley et al 1990Edgley et al , 1992. one might predict that any contribution to the muscle response evoked by magnetic stimulation that results from changes in the level of cortical excitability should not be present when these responses are elicited by electrical stimuli, since the direct action of these stimuli on subeortical axons should be uninfluenced by changes in the excitability of the overlying cortex.…”