This chapter reviews phenomenological, functional,and anatomical similarities and differences of the three main forms of visual reduplication: out-of-body experience, autoscopic hallucination, and heautoscopy. Illusory reduplications of the patient's own body refer to complex manifestations during which patients experience a second own body or self in their environment. Further, the chapter reviews the rarer forms of autoscopic phenomena including multiple visual doubles (polyopic heautoscopy) and inner visual doubles (inner heautoscopy). The description of the visual doubles is followed by a discussion of sensorimotor doubles (feeling of a presence), auditory doubles (hearing of a presence), and negative doubles (negative heautoscopy) due to neurological disease. Although several patients with the feeling of a presence due to focal brain damage are described, the feeling of a presence as an autoscopic phenomenon is considered because it is characterized by a nonvisual body reduplication as opposed to the three main forms of autoscopic phenomena that are all characterized by a visual body reduplication. It is believed that the investigation of the phenomenological, functional, and neural mechanisms leading to the experience of a double in neurological patients (and healthy subjects) is likely to improve self-related neuroscientific models of embodiment, selfhood, and subjectivity.
