Gating of somatosensory evoked potentials during different kinds of movement in man.

Abstract

1. Somatosensory evoked potentials from electrical stimulation of the digital nerves of the right thumb have been recorded during the performance of various motor tasks in eight normal subjects. 2. The N20-P30 primary cortical response is only moderately affected by task context, while the P45-N55 secondary response is markedly 'gated' by movement of the stimulated digit. The late N140 vertex is variable. 3. In most subjects, active and passive movements are about equally effective in suppressing the secondary complex; but in one, passive movement has come to be rather less effective than active. 4. Secondary response suppression occurs in proportion to the velocity of the movement of the thumb, up to a velocity of 20 deg/s. 5. Secondary response suppression is unrelated to load in the range 0 to 0.16 Nm. 6. When the stimulus is timed to occur at various points in movement, secondary complex suppression occurs at all stages; but there is little or no suppression when stimulation is timed at 200 ms before the start of or 500 ms after the end of a movement. 7. Secondary response suppression is maximal when the same digit is both moving and shocked. When the right index or little finger are moved instead, the right thumb being stimulated, suppression is less; when the left thumb moves, no suppression is seen. 8. Secondary response suppression is reduced but not lost if the skin and interphalangeal joint of the thumb are anaesthetized distal to the stimulating electrodes. 9. Secondary response suppression is unimpaired when the radial nerve is anaesthetized, paralysing the finger extensors. 10. In an attempt to identify the course taken by the afferent volley between the primary and secondary responses, and to identify the gating site, we recorded the responses in six patients with Parkinson's disease who had undergone thalamotomy. Their secondary responses were present, and gated in the normal way. 11. We are unable to confirm whether the secondary response represents the re-arrival at cortical level of a volley that has traversed the cerebrocerebellar loop. 12. We confirm that the secondary complex is located a little anterior to the primary cerebral response. 13. We conclude that a gating action is exerted in the brain on somatosensory afferent activity, after it first reaches the cortex, and that this gating action associated with movement is controlled by other afferent signals from the stimulated limb, and particularly from the stimulated digit.