The evoked potential, a bioelectric response to a purposeful physical stimulus, has been a standard tool of physiology since the demonstration of animal electricity by Luigi Galvani. With technological advancements it has emerged as an important clinical tool for the assessment of neurologic dysfunction. Despite a rich literature, the peripheral and central neural mechanisms of the somatosensory evoked potential (SSEP) are in dispute.

This study was undertaken to determine the peripheral, spinal, and cranial substrates of the SSEP. The inter- and intrasubject stability and stationarity were also evaluated. Studies were conducted in 50 macaque monkeys (Macaca arctoides and Mucaca mulatta) and 25 normal adult human volunteers. Additionally, more than 200 clinical records and concomitant evoked potentials were used.

The major, findings were:​ 1) with closely spaced electrodes over the parietal region of the scalp, the response originates in the primary sensorimotor cortex. With widely spaced electrodes, an earlier component is observed which reflects activity in brainstem nuclei;​ 2) the SSEP represents rapidly adapting peripheral receptors in joints, muscle, and skin;​ 3) the primary scalp-recorded responses are probably conducted by group II afferents through the dorsal column/medial lemniscal system;​ 4) the SSEP is sufficiently well defined and stationary (reproducible) for use as a clinical neurologic indicator;​ 5) the SSEP in man and nonhuman primates and the sensitivity of the responses to mechanical, pharmacologic and electrical alterations in the central nervous system were similar;​ 6) mathematical models of volume conduction show that the responses from various points in the neural pathway are best recorded with closely-spaced or bipolar electrodes. These methods produce improved signal to noise ratios and reduce the ambiguity due to the summation of responses from distant generators.