ELECTROPHYSIOLOGICAL AND PSYCHOPHYSICAL QUANTIFICATION OF TEMPORAL SUMMATION IN THE HUMAN NOCICEPTIVE SYSTEM

Animal experiments have shown that the nociceptive reflex can be used as an indicator of central temporal integration in the nociceptive sys tem. The aim of the present study on humans was to investigate whether the nociceptive reflex, evoked by repetitive strong electrical sural nerve stimuli, increased when summation was reported by the volunteers . The reflexes were recorded from the biceps femoris and rectus femori s muscles in eight volunteers following a series of stimulations at 0. 1, 1, 2, and 3 Hz. Each series consisted of five consecutive stimuli. Using 0.1- and 1-Hz stimulation, the reflex was not facilitated in the course of the five consecutive stimuli. Following 2- and 3-Hz stimula tion, the reflex size (root mean square amplitude) increased significa ntly during the course of the fifth stimulus. This reflex facilitation was followed by a significant increase (summation) in the pain magnit ude when compared with 1- and 0.1-Hz stimulation. Furthermore, the thr eshold for psychophysical summation could be determined. This threshol d (stimulus intensity) decreased when the stimulus frequency (1-5 Hz) of the five consecutive stimuli was increased. The nociceptive reflex and the psychophysical summation threshold might be used to clarify an d quantify aspects of temporal summation within the human nociceptive system.