Hallucination
A hallucination is a perception in the absence of an external context stimulus that has the compelling sense of reality. 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 mental 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 is given some additional significance.
Hallucinations can occur in any sensory modality—visual, auditory, olfactory, gustatory, tactile, proprioceptive, equilibrioceptive, nociceptive, thermoceptive and chronoceptive. Hallucinations are referred to as multimodal if multiple sensory modalities occur.
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 or malicious. 55% of auditory hallucinations are malicious in content, for example, people talking about the subject, not speaking to them directly. Like auditory hallucinations, the source of the visual counterpart can also be behind the subject. This can produce a 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, sleep deprivation, psychosis, neurological disorders, and delirium tremens. Many hallucinations happen also during sleep paralysis.
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 its objects erroneously".
Classification
Hallucinations may be manifested in a variety of forms. Various forms of hallucinations affect different senses, sometimes occurring simultaneously, creating multiple sensory hallucinations for those experiencing them.Auditory
hallucinations are the perception of sound without outside stimulus. Auditory hallucinations can be divided into elementary and complex, along with verbal and nonverbal. These hallucinations are the most common type of hallucination, with auditory verbal hallucinations being more common than nonverbal. Elementary hallucinations are the perception of sounds such as hissing, whistling, an extended tone, and more. In many cases, tinnitus is an elementary auditory hallucination. However, some people who experience certain types of tinnitus, especially pulsatile tinnitus, are actually hearing the blood rushing through vessels near the ear. Because the auditory stimulus is present in this situation, it does not qualify it as a hallucination.Complex hallucinations are those of voices, music, or other sounds that may or may not be clear, may or may not be familiar, and may be friendly, aggressive, or among other possibilities. A hallucination of a single individual person of one or more talking voices is particularly associated with psychotic disorders such as schizophrenia, and hold special significance in diagnosing these conditions.
In schizophrenia, voices are normally perceived coming from outside the person, but in dissociative disorders they are perceived as originating from within the person, commenting in their head instead of behind their back. Differential diagnosis between schizophrenia and dissociative disorders is challenging due to many overlapping symptoms, especially Schneiderian first rank symptoms such as hallucinations. However, many people who do not have a diagnosable mental illness may sometimes hear voices as well. One important example to consider when forming a differential diagnosis for a patient with paracusia is lateral temporal lobe epilepsy. Despite the tendency to associate hearing voices, or otherwise hallucinating, and psychosis with schizophrenia or other psychiatric illnesses, it is crucial to take into consideration that, even if a person does exhibit psychotic features, they do not necessarily have a psychiatric disorder on its own. Disorders such as Wilson's disease, various endocrine diseases, numerous metabolic disturbances, multiple sclerosis, systemic lupus erythematosus, porphyria, sarcoidosis, and many others can present with psychosis.
Musical hallucinations are also relatively common in terms of complex auditory hallucinations and may be the result of a wide range of causes ranging from hearing-loss, lateral temporal lobe epilepsy, arteriovenous malformation, stroke, lesion, abscess, or tumor.
The Hearing Voices Movement is a support and advocacy group for people who hallucinate voices, but do not otherwise show signs of mental illness or impairment.
High caffeine consumption has been linked to an increase in likelihood of one experiencing auditory hallucinations. A study conducted by the La Trobe University School of Psychological Sciences revealed that as few as five cups of coffee a day could trigger the phenomenon.
Visual
A visual hallucination is "the perception of an external visual stimulus where none exists". A separate but related phenomenon is a visual illusion, which is a distortion of a real external stimulus. Visual hallucinations are classified as simple or complex:- Simple visual hallucinations are also referred to as non-formed visual hallucinations and elementary visual hallucinations. These terms refer to lights, colors, geometric shapes, and indiscrete objects. These can be further subdivided into phosphenes which are SVH without structure, and photopsias which are SVH with geometric structures.
- Complex visual hallucinations are also referred to as formed visual hallucinations. CVHs are clear, lifelike images or scenes such as people, animals, objects, places, etc.
Command
Command hallucinations are hallucinations in the form of commands; they appear to be from an external source, or can appear coming from the subject's head. The contents of the hallucinations can range from the innocuous to commands to cause harm to the self or others. Command hallucinations are often associated with schizophrenia. People experiencing command hallucinations may or may not comply with the hallucinated commands, depending on the circumstances. Compliance is more common for non-violent commands.Command hallucinations are sometimes used to defend a crime that has been committed, often homicides. In essence, it is a voice that one hears and it tells the listener what to do. Sometimes the commands are quite benign directives such as "Stand up" or "Shut the door." Whether it is a command for something simple or something that is a threat, it is still considered a "command hallucination." Some helpful questions that can assist one in determining if they may have this includes: "What are the voices telling you to do?", "When did your voices first start telling you to do things?", "Do you recognize the person who is telling you to harm yourself ?", "Do you think you can resist doing what the voices are telling you to do?"
Olfactory
Phantosmia, smelling an odor that is not actually there, and parosmia, inhaling a real odor but perceiving it as different scent than remembered, are distortions to the sense of smell, and in most cases, are not caused by anything serious and will usually go away on their own in time. It can result from a range of conditions such as nasal infections, nasal polyps, dental problems, migraines, head injuries, seizures, strokes, or brain tumors. Environmental exposures can sometimes cause it as well, such as smoking, exposure to certain types of chemicals, or radiation treatment for head or neck cancer. It can also be a symptom of certain mental disorders such as depression, bipolar disorder, intoxication, substance withdrawal, or psychotic disorders. The perceived odors are usually unpleasant and commonly described as smelling burned, foul, spoiled, or rotten.Tactile
Tactile hallucinations are the illusion of tactile sensory input, simulating various types of pressure to the skin or other organs. One subtype of tactile hallucination, formication, is the sensation of insects crawling underneath the skin and is frequently associated with prolonged cocaine use. However, formication may also be the result of normal hormonal changes such as menopause, or disorders such as peripheral neuropathy, high fevers, Lyme disease, skin cancer, and more.Gustatory
This type of hallucination is the perception of taste without a stimulus. These hallucinations, which are typically strange or unpleasant, are relatively common among individuals who have certain types of focal epilepsy, especially temporal lobe epilepsy. The regions of the brain responsible for gustatory hallucination in this case are the insula and the superior bank of the sylvian fissure.Sexual
Sexual hallucinations are the perception of erogenous or orgasmic stimuli. They may be unimodal or multimodal in nature and frequently involve sensation in the genital region, though it is not exclusive. Frequent examples of sexual hallucinations include the sensation of being penetrated, experiencing orgasm, feeling as if one is being touched in an erogenous zone, sensing stimulation in the genitals, feeling the fondling of one's breasts or buttocks and tastes or smells related to sexual activity. Visualizations of sexual content and auditory voices making sexually explicit remarks may sometimes be included in this classification. While it features components of other classifications, sexual hallucinations are distinct due to the orgasmic component and unique presentation.The regions of the brain responsible differ by the subsection of sexual hallucination. In orgasmic auras, the mesial temporal lobe, right amygdala and hippocampus are involved. In males, genital specific sensations are related to the postcentral gyrus and arousal and ejaculation are linked to stimulation in the posterior frontal lobe. In females, however, the hippocampus and amygdala are connected. Limited studies have been done to understand the mechanism of action behind sexual hallucinations in epilepsy, substance use, and post-traumatic stress disorder etiologies.