Hypnosis has had a long history, perhaps dating back to antiquity with Asclepian dream healing (1). In the modern era, the work of the 18th-century “healer” Franz Anton Mesmer represents the first introduction of a form of therapeutic intervention involving trance states. His work utilized the then current theory of an abnormal distribution of a “universal fluid.” This fluid was believed to be redistributed by the use of magnetic energy. By the use of eye gaze and hand movements, Mesmer would re-equilibrate these fluids (2). Subjects eventually developed perceived involuntary behaviors that included convulsions, altered sensory and motor functioning, as well as other phenomena that were controlled by the magnetizer. Most of these patients were women and, interestingly, many of these behaviors developed before they were exposed to magnetism (1).

The Benjamin Franklin commission in 1784 discredited magnetism. In 1847, Braid developed a procedure called “monoideism.” By the use of an induced trance state, the behavior of a receptive subject could be modulated. Braid attributed these phenomena to neural inhibition (3). Charcot continued this focus. He believed the hypnotic state contributed to neurophysiologic changes. In addition, he attributed susceptibility to the hypnotic state to mental dysfunction. This view was shared by Janet but opposed by Bernheim (4). Nevertheless, the association of hypnotic susceptibility with mental weakness has persisted to this day (1).

Behaviors that were originally attributed to possession and magnetism eventually began to be understood as manifestations of hysteria. In addition, the modern concepts of somatization and conversion were originally conceived of as expression of hysteria. It has been postulated that the development of the field of psychiatry as a new subdivision of medical specialties relied on the existence of hysteria as a unique disease entity (1).

In the middle of the 18th century, Pierre Briquet developed some of the fundamental concepts regarding conversion disorder. He conceptualized the components of this disorder as manifestations of a central nervous system dysfunction. In contrast, Russel Reynolds subsequently stressed the importance of psychological factors when considering conversion disorders (3).

In 1880, Freud and Breuer coined the term “conversion” to describe the clinical findings observed in their case of Anna O. The patient demonstrated a wide variety of somatic complaints including pseudocyesis. These symptoms were felt to be manifestations of unconscious conflicts (5). Hypnosis, learned through their work with Charcot, was pivotal in the treatment of Anna O. Furthermore, it was also a central concept in their published work, Studies in Hysteria, which appeared in 1895. Freud eventually abandoned hypnosis because of the “mysterious work behind hypnotism” and the possible problematic intensification of the transference process (4).

At this point in history, the hysteric was conceptualized as being ill rather than the victim of possession. Patients, most of whom were women, reported somatic illness consisting of convulsions, paralysis, and chronic pain. Their “illness” could also be explained in psychosocial terms with prominent primary (resolution of a psychic conflict) and secondary gain (accruing external benefits).

Charcot, along with Fredrick Myers and Gilles de la Tourette, introduced the concept of the fragmentation of psychic processes, so-called “dissociation.” Janet emphasized the place of trauma in the development of memory fragmentation or dissociation. He also developed the concept of posttraumatic stress disorder (PTSD). Janet felt that the past experiences of a patient set up cognitive and emotional traces that predisposed to future dissociation of emotional experiences. He also noted that the trauma might be re-experienced in the form of flashbacks and behavioral re-enactments (6). This fragmentation of psychological experience could be understood as a manifestation of dissociation and is a critical component of the process of hypnosis. Additionally, such people as Simmel have utilized hypnosis to treat the psychological effects of trauma during World War II. It has also become a useful technique in the treatment of acute and chronic pain, anxiety disorders, weight control, smoking cessation, but most importantly, in the treatment of dissociative disorders in general and PTSD in particular (4). A further discussion of the history of hysteria has been documented elsewhere (7).

It appears that trauma and hysteria share a common etiology. Trauma appears to have a significant effect on the central nervous system. The pleomorphic physiologic effects of trauma on the nervous system suggest a possible neurobiologic phenomenon associated with hysteria. Van der Kolk (8) has reviewed the psychobiologic effects of trauma. These include psychophysiologic and neurohormonal fluctuations in levels of norepinephrine, glucocorticoid, serotonin, and endogenous opioids. In addition, trauma may induce neuroanatomic changes with its primary effects on hippocampal volume, amygdala activation, and right-hemispheric activation (8).

Trauma can also have profound effects on the motor system. This observation was noted in soldiers suffering the effects of continued stress. They were found to show evidence of tremors, hyperkinesis, excessive startle, and mutism (9). Patients, who have experienced trauma from any cause, especially in childhood, may be especially prone to the later development of abnormal motor movements under repeated stress. In addition, some authors (10) have emphasized the association of depression in the etiology of somatization. These proclivities may represent a fundamental biologic substrate and be particularly relevant to the phenomenon of nonepileptic seizures. However, others have questioned the validity of the causative link between trauma and PTSD. These issues have been discussed by Trimble (11).

Hypnosis can be considered to consist of three primary components (4). The first, absorption, is the ability of participants to focus their attention while partially excluding awareness of peripheral events. The second, dissociation, is complementary to absorption and is that state of the participant that separates or fragments perceptions or consciousness from the surrounding world. Information is processed in a variety of systems. With dissociation, one of these systems appears to be acting separately from awareness and influences affect, cognition, or behavior. Finally, suggestibility is the ability to listen and respond to information from the hypnotist with a suspension of critical evaluation (4).

Hypnotic ability has been measured by a variety of psychometric tools. The Stanford Hypnotic Suggestibility Scale has excellent test-retest reliability, but is rather lengthy and more suitable for research purposes. Other, more clinically useful scales include the Stanford Hypnotic Clinical Scale, the Stanford Profile Scales of Hypnotic Ability, and the Hypnotic Induction Profile (HIP) (2). Spiegel et al. (12) standardized the HIP on a clinical population and attempted to measure all three components of the hypnotic process as well as the degree of involuntariness experienced by the subject. Hypnotic ability appears to be both a trait and state phenomena. Hypnotizability varies slightly over time with a peak in late childhood and declining somewhat into adulthood. However, individual hypnotic potential appears to be relatively stable over time (4). This observation was quantified in a study reviewing 50 subjects
evaluated over a period of 25 years. A statistically significant stability coefficient of 0.71 was found with a nonsignificant change in mean scores (13).

It has been postulated that hypnotic ability can be used to differentiate varying psychopathologic states. As a corollary to this, those states characterized by high levels of dissociation, for example, PTSD, would be expected to display high levels of hypnotizability. In contrast, pathologic states like schizophrenia, where ability to focus and maintain attention is impaired, would be expected to have lower levels of hypnotizability (14). Frischholz et al. (15) evaluated this concept. They confirmed these expectations and found that higher hypnotic scores on the HIP and the Stanford Hypnotic Susceptability Scale Form C were found in those patients diagnosed with dissociative disorders and were lowest in those with schizophrenia. Patients with mood and anxiety disorders had intermediary scores (15). In a review of this issue, it was noted that controversy exists regarding the association of hypnotizability and psychopathology in general, but there appears to be agreement in respect to dissociative disorders (14).

An explanation of these findings focuses on the role of trauma, dissociation, and hypnotizability. It has been postulated that trauma, especially early in life, contributes to the persistent use of dissociation as a defense mechanism. The result of this method of processing experience is a fragmentation of the self to varying degrees. It would be expected that an individual who had a predilection to memory fragmentation would display a high level of dissociative ability. This, in fact, seems to be the case. Some researchers have even hypothesized a state of autohypnosis in those patients showing high levels of pathologic dissociation, that is, dissociative identity disorder (16).

As discussed earlier, Janet first discussed the interaction between trauma, both past and present, and dissociation. Recently, the roles of early childhood abuse and dissociation have been presented by several authors (17, 18, 19). In addition, trauma of other kinds has also been implicated, including those from combat (20), burn injury (21), and general acute stress (22). Causal links between self-reported trauma have been critically reviewed and questioned by others (23,24). It does appear, however, that people who have a premorbidly higher level of dissociability have a greater likelihood to develop PTSD with severe trauma (21,25).

The neurologic basis of hypnosis has been the subject of many studies. Hypnosis has no physiologic similarities to somnolence or sleep, despite the fact that hypnos in Greek refers to a sleep state. A particular electroencephalographic (EEG) signature for hypnosis has not been found (4), possibly because of contextual influences, that is, mood states (26). However, studies have suggested the presence of EEG changes in highly hypnotizable patients (27,28). In contrast to the EEG, event-related potentials (ERPs) may offer a more sensitive evaluation tool. ERPs may also be altered by the hypnotic state and appear to offer more characteristic physiologic changes. Patients who are highly hypnotizable have demonstrated specific psychophysiologic responses compared to those patients who have low hypnotizability (26,29,30). For example, more P300 alterations were seen in highly hypnotizable patients in response to positive obstructive and negative obliterating instructions than in poorly hypnotizable patients (26). These findings appear to lend credence to the trait conceptualization of hypnosis.

Finally, positron emission tomography (PET) has also been used to evaluate the physiologic basis of hypnosis. Two modes of investigation have been pursued. The first involves evaluating hypnosis as it is being used to alter other somatic states. Secondly, it has been evaluated independently in highly hypnotizable individuals.