A comparison between the discharges of human nociceptive nerve fibres and the subject's ratings of his sensations.

Publisher Website
Google Scholar
Abstract
Impulses in cutaneous nerve fibers were recorded percutaneously with tungsten micro-electrodes from the superficial radial nerve of adult human subjects. Eight units studied had conduction velocities below 1.5 m/s and belong to the class of C fibers. On the basis of their responsiveness to mechanical and to thermal stimuli the units were classified as polymodal nociceptors. Units were tested with 12 s heat pulses starting from a base line temperature of 43.0-43.5.degree. C. Heat stimuli reaching 3 different maximal levels were applied in randomized order, the subjects being blind with respect to stimulus size. Each of the 8 units was tested with more than 20 stimuli and 4 of them were 80-125 stimulus repetitions. After each stimulus the subject had to rate his sensations on a 6-point rating scale extending from just noticeable to very hot and painful. Discrimination between the 3 stimulus levels by the integrated spike discharges and by the ratings of the subject was compared using the P(A) [non-parametric measure of discrimination] measure of the Signal Detection Theory. The neurophysiological and the psychophysical measurements provided equal discrimination. Spike discharges and ratings apparently share a common variance beyond their common dependence on the stimulus level. Among the factors contributing to this interdependence, a temporal position effect was the most significant. Despite this interdependence between discharge rates and subjective ratings, the latter gave a better estimation of the stimulus size than of the discharge rates of the individual C fiber under study. The polymodal C-nociceptors might be instrumental for the quantitative aspects of heat pain sensation. Under the conditions of the experiments, the loss of information in the course of central processing might be about equal to the gain by the parallel processing in a population of nociceptors excited by a stimulus.