A false perception occurring without external stimulation: seeing, hearing, tasting, smelling or feeling things that are not there. Transient hallucinations may occur in healthy persons in the form of dreams, and in the “hypnagogic state” between sleeping and waking. Children who lead a particularly active fantasy life may occasionally think they hear voices or see persons or objects that are not there, especially when they are undergoing emotional difficulties.Both positive and negative hallucinations can be produced by hypnotic suggestion. In response to command some subjects will see a person who is not present, or fail to see one who is present. Hallucinations may also be experienced by normal individuals when they are subjected to special conditions, such as sensory deprivation, solitary confinement, extreme exhaustion or hallucinogenic drugs. In most of these cases the hallucinations are poorly formed and may lack a full sense of reality. They also have limited significance as expressions of the personality and are not considered symptomatic of persistent or deep-seated mental disorder.Psychiatric hallucinations may be associated with either psychogenic or organic disorders. In either case the individual accepts these false perceptions as reality, and responds accordingly. If he hears a voice accusing him of sin, he may cower in the comer; if he sees himself as a saint, a beatific expression will spread over his face; if he smells poison in the surrounding air, he may borrow a nurse’s mask. Hallucinations of this type are considered by many authorities to be perceptual expressions of underlying wishes, feelings, and needs. They may reflect, for example, a need to escape from reality, to enhance self-esteem, to relieve a sense of guilt, or to fulfill aspirations. Personality needs of this kind may influence the content not only of psychogenic hallucinations, but of hallucinations produced by physiological conditions such as toxic or organic states. Moreover, the type of hallucination itself seems to be determined to some extent by psychological factors. A person who is ridden by guilt generally hears condemnatory voices; one who is beset by fear may have visions of terrifying animals; and a victim of sexual anxieties usually sees himself surrounded by sexual symbols.Hallucinations fall into six categories, of which the auditory type is by far the most common:(1) Auditory hallucinations. Sometimes the patient hears strange noises, muffled voices, disconnected words, or his own thoughts in speech (“echo des pensees”), but most frequently he clearly hears remarks addressed to him. The voices are usually attributed to particular persons, to God or simply to “enemies.” They may appear to come from external objects, such as light fixtures, the heating system, or an imaginary telephone. They may also emanate from various parts of the patient’s own body, or from all directions at once. In some cases the remarks are of a pleasant nature, even to the point of assuring the patient that he has been given extraordinary powers or has been appointed by God to save all mankind; but more often he is berated or accused of heinous crimes and unforgivable sins, often in obscene language. Sometimes the patient answers back or conducts a lengthy conversation with an imagined interlocutor. Occasionally the voices may issue commands which lead to atempted suicide or violence against others.(2) Visual hallucinations. Visual images are sometimes of a pleasant nature—angels or circus animals, for example—but in most instances they are disgusting or frightening. The most vivid examples are found in delirium tremens, in which tiny, fast-moving animals terrify the alcoholic patient. These are termed Lilliputian hallucinations. See DELIRIUM TREMENS.(3) Olfactory hallucinations. Most hallucinated odors are strongly objectionable or downright repulsive; for example, decaying flesh, garbage, poison gas, carbolic acid. Such hallucinations seem to be associated with unconscious feelings of guilt, and may be accompanied by accusatory voices.(4) Gustatory hallucinations. The patient tastes poison in his food, or complains that his mouth is full of acid or lye. Hallucinations of taste are usually associated with hallucinations of smell.(5) Tactile or haptic hallucinations. The patient feels electric impulses in various parts of his body, or insects crawling on or under his skin (“formication hallucinations”). Sexual sensations are also hallucinated in some cases of schizophrenia. See FORMICATION.(6) Kinesthetic hallucinations. Various parts of the body feel as if they are changing their shape, size or movement. In schizophrenic reactions and toxic states, an arm may feel twisted, the legs six feet long, or the inside of the body completely hollow. Hallucinations of nonexistent body parts also occur in patients who have undergone amputation. See PHANTOM REACTION.All types of hallucinations occur in the paranoid form of schizophrenia. The visual type predominates in psychoses associated with acute infectious diseases and toxic disorders produced by alcohol (delirium tremens), barbiturates, and metallic poisoning. The auditory type is found primarily in acute alcoholic hallucinosis, senile psychosis (paranoid type)and affective psychoses (involutional psychotic reaction, and the depressive phase of manic-depressive reaction). Hallucinations also occur in epileptic disorders, psychoses with brain tumor, symbiotic psychosis, syphilitic disorders, cocaine addiction, Pick’s disease, and psychosis with cerebral arteriosclerosis