The initial positive component of the scalp-recorded somatosensory evoked potential in normal subjects and in patients with neurological disorders.

The initial positive component of the scalp-recorded somatosensory evoked potential to median nerve stimulation was studied in 12 patients with sensory impairment for all modalities of the unilateral extremities due to lesions at or above or below the thalamic level, taking the potentials obtained from eight normal subjects as control. In normal subjects, this potential could be easily obtained in ear reference recordings with a peak latency of 12--15 msec. This finding and positive polarity of this potential indicate that the earlobe could be active for this potential. The wide distribution of this potential was different from the subsequent negative-positive diphasic components. In patients with lesions at or above the thalamic level the p15 potential was of normal configuration and latency to stimulation of the affected side, whilst in patients with lesions in the brain stem or in the cervical cord it could not be obtained to stimualtion of the affected side. These findings and short latency of the p15 potential suggest that it may be the result of activity of the medial lemniscal systems from the medulla to the thalamus.