This review will describe early observational studies of conversion symptoms, neurophysiological studies of conversion disorder, and recent findings from the use of functional neuroimaging to investigate both conversion disorder and feigning. While this is not a systematic review of all research in this area we have attempted to be as comprehensive as possible. We conducted multiple searches in Medline, EMBASE, and PsycINFO databases using the keywords “conversion”, “hysteria”, “dissociative”, “medically unexplained”, or “non-organic illness”. We obtained additional articles through our knowledge of research in this area and from cross-referencing between papers. Although related disorders including nonepileptic seizures and psychogenic movement disorders may share a similar etiology, the vast majority of research in this area has focused on motor and sensory conversion symptoms. We are confident that our discussion on functional imaging includes all published studies of conversion disorders. The study of conversion disorder from a neuropsychiatric perspective should provide not only a greater understanding of the etiology and management of this disorder, but also valuable insights into normal cognitive processes, including volition and attention. Early observational findings Ever since the late 19th century observers have commented that hysterical hemianesthesia and hemiplegia tend to be seen more commonly on the left than the right (Briquet 1859; Jones 1908; Purves-Stewart 1924). This observation appeared to be confirmed in 1977 when Stern found a significantly higher proportion of left-sided conversion symptoms in both left-handed and right-handed patients (Stern 1977). Three hypotheses have been proposed to explain this apparent laterality of conversion symptoms (Axelrod et al 1980). Psychodynamic theorists suggested that neurotic symptoms tend to be seen on the left because of a psychological association between the right and left sides and good and evil, respectively (Axelrod et al 1980). There has, however, been little empirical support for this so-called “evaluative hypothesis” (Roelofs et al 2000). The “convenience hypothesis” suggested that patients would tend to develop symptoms on the side that caused them the least inconvenience; however, the predominance of left-sided symptoms in left-handed patients argues against this (Stern 1977). The “hemispheric specialization” theory proposed a neurobiological explanation for this apparent lateralization (Roelofs et al 2000). It was suggested that the right Neuropsychiatric Disease and Treatment 2006:2(1) Neurobiology of conversion disorder hemisphere has a particular role in emotional processing, making it more likely to mediate affectively determined symptoms (Stern 1977). However, recent neuroimaging studies do not support the simplistic notion of right-sided dominance for emotional processing (Wager et al 2003). Recent studies have questioned whether conversion symptoms are in fact lateralized at all (Roelofs et al 2000). In response to this ongoing debate, Stone and colleagues recently undertook a systematic review looking at more than 100 studies that addressed this issue (Stone et al 2002). They found a small but significant increase in the incidence of symptoms on the left side of the body (58% of patients) but felt that the possibility of reporting bias made this finding inconclusive (Stone et al 2002). Another early approach to understanding the neural mechanisms of conversion disorder was the study of patients with conversion disorder associated with organic lesions. Patients with nonepileptic seizures are more likely to have right hemisphere pathology than those with epileptic seizures, supporting possible right hemisphere involvement in conversion symptoms (Devinsky et al 2001). In contrast there have been case reports of conversion symptoms occurring after injury or infarction of the left cerebral hemisphere (Drake 1993). Other case studies have found left-sided symptoms of probable hysterical origin to be ameliorated by right anterior thalamotomy (Andy 1973). A study of patients referred to a unit for treating behavioral disorders after brain injury found that more than 30% of patients had symptoms suggestive of a hysterical cause (Eames 1992). Diffuse insults such as anoxia or hypoglycemia were much more common in those with hysterical symptoms and indeed were almost invariably present in these cases (Eames 1992). The authors suggested that these findings may point to a role for the basal ganglia and diencephalon (structures particularly vulnerable to such insults) in the emergence of conversion symptoms. However, patients with diffuse and localized brain injury may differ in other neurological and psychological respects, suggesting alternative explanations for this finding. In summary, clinical studies have implicated various brain regions in conversion disorders but many of these findings are based only on case reports and have not been consistently replicated. There is no strong evidence to support the hypothesis of predominant right hemisphere involvement. More recently, research has turned to other techniques to investigate the neural basis of conversion symptoms. 15