Potentials evoked in human and monkey cerebral cortex by stimulation of the median nerve. A review of scalp and intracranial recordings.
Somatosensory evoked potentials (SEPs) are generated in afferent pathways, subcortical structures and various regions of cerebellar and cerebral cortex by stimulation of somatic receptors or electrical stimulation of peripheral nerves. This review summarizes current knowledge of SEPs generated in cerebral cortex by stimulation of the median nerve, the most common form of stimulation for human research and clinical investigations. Major sources of data for the review are intracranial recordings obtained from patients during diagnostic or neurosurgical procedures, and similar recordings in monkeys. Short-latency cortical SEPs in the 20-40 ms latency range consist of P20 and N30, recorded from motor cortex and frontal scalp; P25 and N35, recorded from cortex near the central sulcus and central scalp; and N20 and P30, recorded from somatosensory cortex and parietal scalp. Several lines of evidence including cortical surface and intracerebral recordings, neuromagnetic recordings and lesion studies in humans and monkeys, strongly support the conclusion that these potentials are generated in contralateral somatosensory cortex in areas 3b and 1, in contrast to the conclusion of many previous studies that SEPs recorded from the frontal scalp are generated in motor cortex and other frontal lobe areas. These potentials are primarily mediated by cutaneous afferents of the dorsal column-medial lemniscal system; the contribution of muscle afferents has not been completely resolved but appears to be small. There is currently no evidence that short-latency SEPs are generated in cortex other than primary somatosensory cortex. Recordings from the vicinity of the second somatosensory area, from the supplementary motor and sensory areas and from surface cortex other than sensorimotor cortex have not detected reliable short-latency activity, although some of these regions generate long-latency potentials. Consequently, short-latency SEPs recorded from the scalp are similar to those recorded from the surface of sensorimotor cortex. Old World monkeys such as Macaca mulatta and M. fascicularis provide an excellent model for human short-latency SEPs. All the potentials described above in humans have corresponding monkey analogues, with similar distributions over the cortical surface. The squirrel monkey, a New World species, exhibits the same potentials, but due to the different morphology of sensorimotor cortex, the surface distribution of SEPs is quite different.