Spinal components of the cerebral somatosensory evoked response in normal man: The “S wave”

Abstract
Responses evoked over the scalp and the neck by median nerve (or finger I) stimulation were concurrently recorded in 10 subjects. The first component of the cortical SEP consists of a small amplitude polyphasic wave (S wave) which could be recorded bilaterally upon unilateral stimulation. The polarity of the S wave varied according to the reference electrode position, at variance with the P15 [small positive potential] component which remained constantly positive. Different generators are responsible for these 2 potentials. The synchrony between the S wave and the cervical response, which is largely spinal in origin, and some pertinent experimental data, suggest that the S wave is a far field reflection of activity generated mainly in the cervical dorsal columns. These findings might be relevant to the diagnosis of neurological disorders.

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