Frequency of occurrence of the F wave in distal flexor muscles as a function of hypnotic susceptibility and hypnosis.

The aim of the experiment was to assess whether the membrane excitability of flexor cervical and/or lumbar motoneurons is related to hypnotic susceptibility (measured with the Stanford Hypnotic Susceptibility Scale, Form A) and hypnosis. During the experimental sessions, hypnotized subjects were given only suggestions of relaxation (neutral hypnosis) while awake subjects were given instructions to be quiet and relaxed (simple relaxation). F waves were recorded from the abductor digiti minimi and abductor hallucis of both sides after electrical stimulation of the ulnar or tibial nerve, respectively, and were used as an index of motoneuron membrane excitability in three groups of subjects: highly susceptible under neutral hypnosis, highly susceptible and non-susceptible during simple relaxation sessions. In lower limbs, there was no difference among the groups in the frequency of occurrence of F waves. In upper limbs, hypnosis selectively reduced F frequency of occurrence in flexor motoneurons on the right side. This reduction persisted after the end of hypnosis and also occurred during the last period of relaxation in highly susceptible non-hypnotized subjects. Results support the idea that hypnotic phenomena develop along a continuum in which some trait differences can be more easily revealed by the induction of hypnosis.