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

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

Journal of Physiology 425: 283-299

1. 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 cortiospinal 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 he vertex rather than 7 cm anterior. There was 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 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. 5. It is concluded that the D and I waves hypothesis' adequately explains the EMG responses to anodal stimulation of the leg areas of the motor cortex, although the explanation for latency shortenings with voluntary effort on increased stimulus intensity must take into account different sites of D wave initiation in addition to temporal summation of D and I waves in the target motoneurone pool. The present results do not support the D and I wave hypothesis when the leg areas of motor cortex are stimulated by cathodal pulses delivered through the intact skull.

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Accession: 007165084

Download citation: RISBibTeXText

PMID: 2213580

DOI: 10.1113/jphysiol.1990.sp018103

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