Latencies of peripheral nerve and cerebral evoked responses to air-puff and electrical stimuli

This study examined the latency relationship between mechanically and electrically elicited sensory nerve action potentials (SNAPs) and the somatosensory-evoked potentials (SEPs) they produce. Brief air-puff and electrical stimuli were applied to the tip of the index finger in separate trials and SNAPs from the median nerve at the wrist and SEPs from the scalp were recorded for each stimulus presentation. Air-puff evoked SNAPs were polyphasic, usually consisting of 2 to 4 separate waves, unlike triphasic activity elicited by electrical stimulation. The SEPs produced by these 2 distinct forms of inputs, however, were similar in morphology. The latencies of the initial components of SNAPs and SEPs were longer for air-puff stimulation. The conduction time, however, of the fastest afferent volleys from the wrist to cortex was not significantly different for air-puff (20.52 +/- 1.06 ms, mean +/- SD) and electrical stimulation (20.17 +/- 0.66 ms). It is therefore concluded that the latency delays for air-puff evoked SNAPs and SEPs are due solely to a transduction time at the skin receptors and not due to differences in conduction velocities as suggested in the previous literature.