A hallucination is a perception in the absence of external stimulus that has qualities of real perception. Hallucinations are vivid, substantial, and are perceived to be located in external objective space. They are distinguishable from several related phenomena, such as dreaming, which does not involve wakefulness; pseudohallucination, which does not mimic real perception, and is accurately perceived as unreal; illusion, which involves distorted or misinterpreted real perception; and imagery, which does not mimic real perception and is under voluntary control. Hallucinations also differ from delusional perceptions, in which a correctly sensed and interpreted stimulus (i.e., a real perception) is given some additional (and typically absurd) significance.Hallucinations can occur in any sensory modalityvisual, auditory, olfactory, gustatory, tactile, proprioceptive, equilibrioceptive, nociceptive, thermoceptive and chronoceptive.A mild form of hallucination is known as a disturbance, and can occur in most of the senses above. These may be things like seeing movement in peripheral vision, or hearing faint noises or voices. Auditory hallucinations are very common in schizophrenia. They may be benevolent (telling the subject good things about themselves) or malicious, cursing the subject, etc. Auditory hallucinations of the malicious type are frequently heard, for example people talking about the subject behind their back. Like auditory hallucinations, the source of the visual counterpart can also be behind the subject's back. Their visual counterpart is the feeling of being looked or stared at, usually with malicious intent. Frequently, auditory hallucinations and their visual counterpart are experienced by the subject together.Hypnagogic hallucinations and hypnopompic hallucinations are considered normal phenomena. Hypnagogic hallucinations can occur as one is falling asleep and hypnopompic hallucinations occur when one is waking up. Hallucinations can be associated with drug use (particularly deliriants), sleep deprivation, psychosis, neurological disorders, and delirium tremens.The word hallucination itself was introduced into the English language by the 17th-century physician Sir Thomas Browne in 1646 from the derivation of the Latin word alucinari meaning to wander in the mind. For Browne, hallucination means a sort of vision that is depraved and receive[s] its objects erroneously.Encyclopedia Entry for Hallucinations :Hallucinations. Common hallucinations can include: Feeling sensations in the body, such as a crawling feeling on the skin or the movement of internal organs. Hearing sounds, such as music, footsteps, windows or doors banging. Hearing voices when no one has spoken (the most common type of hallucination). These voices may be positive, negative, or neutral. They may command someone to do something that may cause harm to themselves or others. Seeing patterns, lights, beings, or objects that are not there. Smelling an odor. Sometimes, hallucinations are normal. For example, hearing the voice of or briefly seeing a loved one who recently died can be a part of the grieving process.There are many causes of hallucinations, including: Being drunk or high, or coming down from such drugs like marijuana , LSD , cocaine (including crack), PCP, amphetamines, heroin, ketamine, and alcohol Delirium or dementia (visual hallucinations are most common) Epilepsy that involves a part of the brain called the temporal lobe (odor hallucinations are most common) Fever, especially in children and the older people Narcolepsy (disorder that causes a person to fall into periods of deep sleep) Mental disorders, such as schizophrenia and psychotic depression Sensory problem, such as blindness or deafness Severe illness, including liver failure, kidney failure , HIV/AIDS , and brain cancer.A person who begins to hallucinate and is detached from reality should get checked by a health care professional right away. Many medical and mental conditions that can cause hallucinations may quickly become emergencies. The person should not be left alone. Call your health care provider, go to the emergency room, or call your local emergency number (such as 911). A person who smells odors that are not there should also be evaluated by a provider. These hallucinations may be caused by a serious medical condition.Your provider will do a physical examination and take a medical history. They will also ask you questions about your hallucinations. For example, how long the hallucinations have been happening, when they occur, or whether you have been taking medicines or using alcohol or illegal drugs. Your provider may take a blood sample for testing. Treatment depends on the cause of your hallucinations.Sensory hallucinations.American Psychiatric Association. Schizophrenia spectrum and other psychotic disorders.