Dissociation is the splitting off of specific mental activities from the rest of normal consciousness, such as the splitting of thoughts or feelings from behavior (e.g., to daydream through a boring lecture and yet end with a complete set of notes without being aware of having taken them) (1). Minor dissociation is a common human phenomenon (2). Dissociative disorders demonstrate severe dissociation that produces significant and diverse symptoms and impairs functioning. Such disorders are fairly common (10% lifetime risk), often occur within the context of childhood physical or sexual abuse or both or associated with acute trauma, and are frequently comorbid with major depression, somatization disorder, affective spectrum disorder (ASD), posttraumatic stress disorder (PTSD), substance abuse, borderline personality disorder, conduct disorder, and antisocial personality disorder (3).

A biologic basis for dissociation is suggested by data such as a variety of functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) studies of dissociative patients who display increased activity compared with normal persons in the right frontal cortex and parietal association areas, whereas others show decreased activity bilaterally in the orbitofrontal region and perhaps the hippocampi. Neurotransmitters involved in dissociation are just being identified, with a focus on increased glutamate (4), as well as others. Much work remains to be done.