Auditory hallucinations and passivity experiences are associated with an abnormality in the self-monitoring mechanism that normally allows us to distinguish self-produced from externally produced sensations. It is unclear if chronic central pain disorders such as fibromyalgia and somatoform pain disorders also involve a defect of the self-monitoring mechanism.
Responses to tactile stimulation were assessed in four groups of subjects (N = 40): patients with fibromyalgia, patients with somatoform pain disorder, patients with schizophrenia with auditory hallucinations and/or passivity experiences, and normal control subjects. The subjects were asked to rate the perception of a tactile sensation on their left and right hands. The tactile stimulation was either self-produced by movement of the subject's right or left hand or externally produced by the experimenter.
Normal control subjects experienced self-produced stimuli as less intense than identical, externally produced tactile stimuli. In contrast, patients with fibromyalgia, patients with somatoform pain disorder, and patients with schizophrenia with auditory hallucinations and/or passivity experiences gave the same perceptual ratings for tactile stimuli produced by themselves as those produced by the experimenter (intergroup difference, p = .043; 95% confidence interval [CI], 0.16-0.68). Post hoc tests revealed that this significance was mainly caused by the fibromyalgia (p = .046; 95% CI, -1.66-0.13) and the somatoform pain disorder group (p = .033; 95% CI, -1.71-0.06).
We conclude that central pain disorders such as fibromyalgia and somatoform pain disorders interfere with the correct functioning of the self-monitoring mechanism that normally allows us to distinguish self-produced from externally produced tactile stimuli.