Different contribution of joint and cutaneous inputs to early scalp somatosensory evoked potentials.

To elucidate whether the frontal components of scalp somatosensory evoked potentials (SEPs) depend on the type of peripheral input, we compared scalp SEPs in response to electrical stimuli applied to: (i) the proximal phalanx of the thumb, involving both deep and cutaneous afferents; and (ii) the distal phalanx of the thumb, involving cutaneous afferents, but excluding joint inputs coming from the interphalangeal articulation. We applied the same dipolar model that we built to explain the scalp SEP distribution to median nerve stimulation in previous investigations. Cortical SEPs after proximal stimulation were generated by three dipolar sources, one of which was likely to account for the frontal scalp N30. When we analyzed SEPs for distal (purely cutaneous) stimulation, the frontal and central recordings showed a clear reduction in amplitude of the negative responses having a latency of about 30 ms. Moreover, when applying the dipole model derived from analysis of responses to proximal stimulation to SEPs to distal stimulation, the source corresponding to the N30 distribution showed no activity, suggesting a strong relationship between joint and tendinous inputs and the activity of the N30 generator.