CORTICAL EVOKED POTENTIAL AND SUBJECTIVE SENSATION TO ELECTRIC STIMULATION OF THE SKIN: EFFECTS OF POSTURE AND RESPIRATORY MOVEMENT

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Abstract
Cortical evoked potentials were recorded in man in response to percutaneous electric stimulation of the median nerve. In bipolar recording between the sensory area and the midfrontal region, evoked potentials are composed of several deflections. The amplitude of the evoked potentials is not a simple function of the stimulus intensity, but a saturation phenomenon was observed. During voluntary movements of the figers or the rubbing of the forearm, all components of the evoked potentials were inhibited. In a drowsy state, the early components were depressed and the later wave was abolished. When lying on the side stimulated, the early components of the evoked potential were inhibited and the later wave was augmented. Thereby, subjects felt the stimulus to be a "widespread and stronger stimulus". Similar changes of evoked potential and sensation were obtained when the stimuli were given at the top of expiration. In contrast, stimulus was sensed to be a "localized and weaker stimulus" when the subject was lying on the opposite side or if the stimuli were given at the top of inspiration, in which case the early components of the evoked potentials were augmented and the later wave was depressed. The time threshold for successive stimulation was lowest at the stimulus intensity of about the threshold of the motor responses. Bipolar and monopolar recordings were compared and a discussion was made on the correlation between evoked potential and subjective sensation.