Role of thermoreceptive afferents in behavioral reaction times to warming temperature shifts applied to the monkey's face

Abstract

Behavioral reaction times (RTs) to warming stimuli were found to co-vary with stimulus parameters and provided an index of warm detection in two rhesus monkeys. In separate electrophysiological experiments, the latency and pattern of discharge in trigeminal thermoreceptive afferents to similar stimuli were investigated and their role in the detection of warming stimuli was assessed by comparing the behavioral and electrophysiological data. The stimulus parameters investigated were the adapted temperature of the skin (25–40 °C), the rate of stimulus change (0.3–5.0 °C/sec), the intensity of the temperature shift (0.4–5.0 °C), and the interstimulus interval (5–20 sec). Interstimulus intervals of less than 20 sec produced increases in RTs. The rate of temperature change was a significant variable determining average RTs: a slow rate of change produced slower and more variable RTs than a faster rate of change. For both monkeys the relationship between average RT and rate of temperature change was monotonic over the range of rates from 0.5 °C to 5.0 °C/sec. With the exception of one monkey at 25 °C, RTs also were faster at the higher adapted temperatures. However, over the range of adapted temperatures and rates of temperature change studied, stimulus intensity was not a significant variable contributing to average RTs. The response of both ‘warm’ and ‘cold’ thermoreceptive afferents was sensitive to the rate of warming the skin. The sensitivity of warm fibers was enhanced by adapting the skin at higher temperatures whereas at any particular rate of warming the response of cold fibers was similar at different adapted skin temperatures. We conclude that the detection of small temperature shifts presented from 25 °C must depend upon activity in cold fibers, since warm fibers were inactive at temperatures below 28 °C. On the other hand, cold fibers conveyed little information to the central nervous system to account for the faster RTs at adapted temperatures above 30 °C while warm fibers were increasingly sensitive in this range. Thus, at higher adapted temperatures, detection by the monkeys of an increase in skin temperature must be dependent upon information relayed by warm fibers.