Quantitative evaluation of hypnotically suggested hyperaesthesia and analgesia by painful laser stimulation.

The ability to reduce both clinically and experimentally induced pain by hypnotic suggestion of analgesia is well known. However, the nature of hypnotic analgesia still remains uncertain. Attempts to demonstrate and identify specific psychophysiological mechanisms have, so far, been unsatisfactory. Methodological problems in inducing pain and monitoring physiological responses may be the reason for this lack of success. In the present study, we have attempted to eliminate some of these methodological problems. The sensory and pain thresholds to laser stimulation were determined, and the laser-evoked brain potentials were measured for 8 highly hypnotically susceptible subjects in 3 conditions: (1) waking state, (2) suggestion of hyperaesthesia, (3) suggestion of analgesia. The thresholds were reduced during induced hyperaesthesia and increased during analgesia. During hyperaesthesia sensations could be evoked by laser intensities which were below intensities that could be perceived in the awake state. The amplitude of the evoked brain potentials increased during hyperaesthesia and decreased during analgesia. The latency of the potential remained constant. The perception of pain during hypnosis can change very fast, indicating that slow endogenous mechanisms may play only a minor role in suggested hyperaesthesia/analgesia.