Human somatosensory evoked potentials to mechanical pulses and vibration: contributions of SI and SII somatosensory cortices to P50 and P100 components

Somatosensory evoked potentials (SEPs) were measured to short tactile pulses and vibratory stimuli applied to the fingertip to determine the characteristics and scalp topography of different early and late SEP components to these types of stimulus. The measurements were obtained from 3 homologous contra- and ipsilateral locations and from the vertex. In 2 subjects the SEPs were measured from 23 recording locations. The subjects were reading during the experiments. The first distinct contralateral response was an anteriorly negative and centrally as well as posteriorly positive peak at about 50 msec latency (P50). Largest P50 responses with shortest peak latencies were measured to single tactile pulses. We suggest that P50 is probably generated in the contralateral SI cortex. The P50 was followed by a distinct negative deflection (N70) in the middle and posterior recording locations on the contralateral hemisphere, which reversed its polarity in the frontal records. This peak was also seen ipsilaterally. At about 100 msec latency a distinct bilateral positive P100 peak was obtained. This peak was most prominent to vibratory stimuli, and especially to high frequency vibration. Comparisons with recent intracortical SEP studies in primates and MEG studies in humans suggest that P100 might be best accounted for by bilateral generators in SII cortices. The early components were followed by a negative N140 wave and by a slow, positive wave with a maximum at about 300 msec. Both waves had an asymmetrical distribution. The N140 wave occurred bilaterally, but was largest contralaterally, and often had two peaks at posterior recording locations. The slow positivity was largest at the vertex and at mid-posterior recording sites.