Human evoked responses to potentially noxious tactile stimulation, i

Primary somatosensory evoked responses to tapping and pinprick of contralateral central palmar skin, recorded from human scalp, yielded significant differences of peak latency but not of amplitude, suggesting afferent conduction by two populations of primary fibers, Group II and Group III respectively. Pinprick first positives were always later for all conscious subjects, and sometimes larger. A model for cortical discrimination of signal phase and frequency is related to these data.