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Introduction
At the close of the 19th century, the French neurologist Jean-Martin Charcot proposed the radical hypothesis that similar brain processes were responsible for the unexplained neurological symptoms of hysteria and the pseudo-neurological behaviours commonly produced by hypnosis. This hypothesis stemmed from many years of work in which hypnosis was used to produce or resolve symptoms in patients with hysteria, including post-traumatic cases. 1 Even in a modern context, Kirsch 2 notes that "Hypnotised subjects are asked to experience paralysis, amnesia, anaesthesia, involuntary movements and hallucinations. In fact, hypnotisability is measured as the number of conversion and dissociation symptoms that the person is able to display. 2 Central to Charcot's explanation was the concept that symptoms could derive from unconscious 'fixed' ideas based on suggestions or autosuggestions "remaining isolated from the rest of the mind and expressing themselves outwardly through corresponding motor phenomena" (quoted in Ellenberger 1 ). Recent advances in cognitive neuroscience provide an opportunity to evaluate Charcot's hypothesis.
This review focuses on 'hysteria' which classically describes the presentation of medical symptoms without evidence of tissue pathology that can adequately explain the impairment, and its relationship to the effects of hypnosis. While the concept of hysteria has historically varied, 3 this review focuses on aspects of the condition which have been modelled using hypnosis; to date, these typically include conditions that would be diagnosed as conversion disorder and affect voluntary motor or sensory function, or would be diagnosed as psychogenic, functional or dissociative amnesia.
These remain controversial and curious diagnoses characterised by putative psychological mechanisms rather than any specific symptom or pathognomonic finding. The review begins by considering the historical links between the two proposed psychological constructs implicated in hysteria-namely 'dissociation' and 'conversion'-before examining the parallels between hysteria and hypnosis.
Dissociation and conversion
Although not the first to use the term, the philosopher and psychiatrist Pierre Janet wrote extensively about dissociation, and it is Janet's conception of dissociation as a 'narrowing of the field of consciousness' resulting in the compartmentalisation of normally integrated mental functions that continues to define the condition today. 1 Janet cited unresolved traumatic memories as the cause of fixed ideas and of dissociation in hysteria, and felt a tendency to dissociate was an innately pathological process whichever form it took....