Hypnotic hypo- and hyperalgesia: divergent effects on pain ratings and pain-related cerebral potentials

Pain ratings and pain-related cerebral potentials in response to noxious stimuli were investigated under hypnotic hypo- or hyperalgesia. Out of a sample of 50 subjects the 10 most highly hypnotizable were selected using the Stanford Hypnotic Susceptibility Scale. Phasic pain was induced by brief electrical stimuli intracutaneously applied to the subject's left middle finger. The subjects took part in three experimental sessions. The first session was without hypnosis for familiarization with the experimental surroundings. In the two other sessions, the subjects were hypnotized and given a suggestion of analgesia or hyperalgesia with respect to pain sensation in the left hand. The sequence of hypnosis was matched within and between sessions. Pain ratings and late cerebral somatosensory evoked potentials (SEP) were used to quantify pain reactions. In addition, auditory evoked potentials (AEP) and spontaneous EEG were evaluated to differentiate between pain-specific and unspecific effects of hypnosis. Only the subjects' verbal reports of pain were drastically influenced by hypnosis: suggestion of analgesia diminished the mean pain ratings, suggestion of hyperalgesia enhanced them (P < 0.001). In contrast, the amplitudes of the late somatosensory potentials evoked by the pain-inducing stimuli were not modified in either of the suggestive states. Furthermore, no effects of hypnosis were found on AEPs and on the power spectra of the spontaneous EEG. The results are discussed on the basis of a dissociation of sensory and affective components of pain under hypnosis.