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A hallucination, in the broadest sense, is a visual perception in the absence of a visual stimulus. In a stricter sense, hallucinations are defined as perceptions in a conscious and awake state in the absence of external stimuli which have qualities of real perception, in that they are vivid, substantial, and located in external objective space. These definitions distinguish hallucinations from the related phenomena of dreaming, which does not involve consciousness; illusion, which involves distorted or misinterpreted real perception; imagery, which does not mimic real perception and is under voluntary control; and pseudohallucination, which does not mimic real perception, but is not under voluntary control. Hallucinations also differ from "delusional perceptions", in which a correctly sensed and interpreted genuine perception is given some additional (and typically bizarre) significance.
A mild form of hallucination is known as a disturbance, and can occur in any of the senses above. These may be things like seeing movement in peripheral vision, or hearing faint noises and voices.
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 may also be associated with drug or alcohol use (particularly deliriants), sleep deprivation, psychosis, neurological disorders, and delirium tremens.
Studies have shown that hallucinatory experiences take place worldwide. One study from as early as 1894 reported that approximately 10% of the population experienced hallucinations. A 1996-1999 survey of over 13,000 people reported a much higher figure, with almost 39% of people reporting hallucinatory experiences, 27% of which were daytime hallucinations, mostly outside the context of illness or drug use. From this survey, olfactory (smell) and gustatory (taste) hallucinations seem the most common in the general population.
Types of hallucinations
Hallucinations may be manifested in a variety of forms. Various forms of hallucinations affect the different senses, sometimes occurring simultaneously, creating multiple sensory hallucinations for the patient.
Auditory hallucinations (also known as Paracusia),are the perception of sound without outside stimulus. Auditory hallucinations can be divided into two categories: elementary and complex. 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, and are frequently misdiagnosed.
Complex hallucinations are those of voices, music, or other sounds which may or may not be clear, may be familiar or completely unfamiliar, and friendly or aggressive, among other possibilities. Hallucinations of one or more talking voices, are particularly associated with psychotic disorders such as schizophrenia, and hold special significance in diagnosing these conditions. However, many people not suffering from 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 suffer from a psychiatric disorder on its own. Disorders such as Wilson's disease, various endocrinological disorders, numerous metabolic disturbances, multiple sclerosis, systemic lupus erythematosis, 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 (such as in Musical Ear Syndrome), lateral temporal lobe epilepsy, arteriovenous malformation, stroke, lesion, 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.
Phantosmia is the phenomenon of smelling odors that aren't really present. The most common odors are unpleasant smells such as rotting flesh, vomit, urine, feces, smoke, etc. Phantosmia often results from damage to the nervous tissue in the olfactory system. The damage can be caused by viral infection, brain tumor, trauma, surgery, and possibly exposure to toxins or drugs. Phantosmia can also be induced by epilepsy, particularly medial temporal lobe epilepsy, because of the excitability of the temporal lobes and the amount of olfactory input received by the amygdala, entorhinal cortex, and piriform cortex. Phantosmia is not the same as parosmia, in which a stimulus is present but the perception and/or interpretation of the smell is distorted.
- Main article: Hypnagogia
These hallucinations occur just before falling asleep, and affect a surprisingly high proportion of the population. The hallucinations can last from seconds to minutes, all the while the subject usually remains aware of the true nature of the images. These are usually associated with narcolepsy, but can also affect normal minds. Hypnagogic hallucinations are sometimes associated with brainstem abnormalities, but this is rare.
Peduncular means pertaining to the peduncle, which is a neural tract running to and from the pons on the brain stem. These hallucinations usually occur in the evenings, but not during drowsiness, as in the case of hypnagogic hallucination. The subject is usually fully conscious and can interact with the hallucinatory characters for extended periods of time. As in the case of hypnagogic hallucinations, insight into the nature of the images remains intact. The false images can occur in any part of the visual field, and are rarely polymodal.
- Main article: Peduncular hallucinosis
- Main article: Delirium tremens
One of the more enigmatic forms of visual hallucination is the highly variable, possibly polymodal delirium tremens. Individuals suffering from delirium tremens may be agitated and confused, especially in the later stages of this disease. Insight is gradually reduced with the progression of this disorder. Sleep is disturbed and occurs for a shorter period of time, with REM overflow.
Parkinson's disease and Lewy body dementia
Parkinson's disease is linked with Lewy body dementia for their similar hallucinatory symptoms. The symptoms strike during the evening in any part of the visual field, and are rarely polymodal. The segue into hallucination may begin with illusions where sensory perception is greatly distorted, but no novel sensory information is present. These typically last for several minutes, during which time the subject may be either conscious and normal or drowsy/inaccessible. Insight into these hallucinations is usually preserved and REM sleep is usually reduced. Parkinson's disease is usually associated with a degraded substantia nigra pars compacta, but recent evidence suggests that PD affects a number of sites in the brain. Some places of noted degradation include the median raphe nuclei, the noradrenergic parts of the locus coeruleus, and the cholinergic neurons in the parabrachial and pedunculopontine nuclei of the tegmentum.
This type of hallucination is usually experienced during the recovery from a comatose state. The migraine coma can last for up to two days, and a state of depression is sometimes comorbid. The hallucinations occur during states of full consciousness, and insight into the hallucinatory nature of the images is preserved. It has been noted that ataxic lesions accompany the migraine coma.
Charles Bonnet syndrome
Charles Bonnet syndrome is the name given to visual hallucinations experienced by blind patients. The hallucinations can usually be dispersed by opening or closing the eyelids until the visual images disappear. The hallucinations usually occur during the morning or evening, but are not dependent on low light conditions. These prolonged hallucinations usually do not disturb the patients very much, as they are aware that they are hallucinating. A differential diagnosis are opthalmopathic hallucinations .
Visual hallucinations due to focal seizures differ depending on the region of the brain where the seizure occurs. For example, visual hallucinations during occipital lobe seizures are typically visions of brightly colored, geometric shapes that may move across the visual field, multiply, or form concentric rings and generally persist from a few seconds to a few minutes. They are usually unilateral and localized to one part of the visual field on the ipsilateral side of the seizure focus, typically the temporal field. However, unilateral visions moving horizontally across the visual field begin on the contralateral side and move towards the ipsilateral side.
Temporal lobe seizures, on the other hand, can produce complex visual hallucinations of people, scenes, animals, and more as well as distortions of visual perception. Complex hallucinations may appear real or unreal, may or may not be distorted with respect to size, and may seem disturbing or affable, among other variables. One rare but notable type of hallucination is heautoscopy, a hallucination of a mirror image of one's self. These "other selves" may be perfectly still or performing complex tasks, may be an image of a younger self or the present self, and tend to be only briefly present. Complex hallucinations are a relatively uncommon finding in temporal lobe epilepsy patients. Rarely, they may occur during occipital focal seizures or in parietal lobe seizures.
thumb|400px|right|Some examples of the dolly zoom effect simulating a type of visual illusion that a focal seizure may produce. Distortions in visual perception during a temporal lobe seizure may include size distortion (micropsia or macropsia), distorted perception of movement (where moving objects may appear to be moving very slowly or to be perfectly still), a sense that surfaces such as ceilings and even entire horizons are moving farther away in a fashion similar to the dolly zoom effect, and other illusions. Even when consciousness is impaired, insight into the hallucination or illusion is typically preserved.
Other types of hallucinations create the sensation of tactile sensory input, simulating various types of pressure to the skin or other organs. This type of hallucination is often associated with substance use, such as someone who feels bugs crawling on them (known as formication) after a prolonged period of cocaine or amphetamine use.
Various theories have been put forward to explain the occurrence of hallucinations. When psychodynamic (Freudian) theories were popular in psychiatry, hallucinations were seen as a projection of unconscious wishes, thoughts and wants. As biological theories have become orthodox, hallucinations are more often thought of (by psychologists at least) as being caused by functional deficits in the brain. With reference to mental illness, the function (or dysfunction) of the neurotransmitter dopamine is thought to be particularly important. The Freudian interpretation may have an aspect of truth, as the biological hypothesis explains the physical interactions in the brain, while the Freudian deals with the origin of the flavor of the hallucination. Psychological research has argued that hallucinations may result from biases in what are known as metacognitive abilities. These are abilities that allow us to monitor or draw inferences from our own internal psychological states (such as intentions, memories, beliefs and thoughts). The ability to discriminate between internal (self-generated) and external (stimuli) sources of information is considered to be an important metacognitive skill, but one which may break down to cause hallucinatory experiences. Projection of an internal state (or a person's own reaction to another's) may arise in the form of hallucinations, especially auditory hallucinations. A recent hypothesis that is gaining acceptance concerns the role of overactive top-down processing, or strong perceptual expectations, that can generate spontaneous perceptual output (that is, hallucination).
Stages of a hallucination
- Emergence of surprising or warded-off memory or fantasy images 
- Frequent reality checks 
- Last vestige of insight as hallucinations become "real" 
- Fantasy and distortion elaborated upon and confused with actual perception 
- Internal-external boundaries destroyed and possible pantheistic experience 
- Apparitional experience
- Closed-eye hallucination
- Focal seizures
- Folie à deux
- Form constant
- Hallucinations in the sane
- Imaginary friend
- Lucid dream
- Phantom eye syndrome
- Psychedelics, dissociatives and deliriants
- Simulated reality
References & Bibliography
- ↑ Leo P. W. Chiu (1989). Differential diagnosis and management of hallucinations. Journal of the Hong Kong Medical Association 41 (3): 292–7.
- ↑ Sidgwick H, Johnson A, Myers FWH et al (1894). Report on the census of hallucinations. Proceedings of the Society for Psychical Research 34: 25–394.
- ↑ Ohayon MM (Dec 2000). Prevalence of hallucinations and their pathological associations in the general population. Psychiatry Res 97 (2-3): 153–64.
- ↑ Medical dictionary.
- ↑ Thompson, Andrea Hearing Voices: Some People Like It. LiveScience.com. URL accessed on 2006-11-25.
- ↑ Murat Ozsarac, Ersin Aksay, Selahattin Kiyan, Orkun Unek, F. Feray Gulec, De Novo Cerebral Arteriovenous Malformation: Pink Floyd's Song 'Brick in the Wall' as a Warning Sign, The Journal of Emergency Medicine, In Press, Corrected Proof, Available online 13 August 2009, ISSN 0736-4679, DOI: 10.1016/j.jemermed.2009.05.035.
- ↑ Phantom smells
- ↑ 8.0 8.1 8.2 8.3 8.4 8.5 Manford M, Andermann F (Oct 1998). Complex visual hallucinations. Clinical and neurobiological insights. Brain 121 ((Pt 10)): 1819–40.
- ↑ Mark Derr (2006) Marilyn and Me, "The New York Times" February 14, 2006
- ↑ Engmann B (2008). Phosphenes and photopsias - ischaemic origin or sensorial deprivation? - Case history. Z Neuropsychol. 19 (1): 7–13.
- ↑ 11.0 11.1 Panayiotopoulos, Chrysostomos P. A clinical guide to epileptic syndromes and their treatment: based on the ILAE classification and practice parameter guidelines. 2. ed. London: Springer, 2007.
- ↑ Bien CG, Benninger FO, Urbach H, Schramm J, Kurthen M, Elger CE (2000) Localizing value of epileptic visual auras. Brain 123:244–253 PubMed
- ↑ Kapur S (Jan 2003). Psychosis as a state of aberrant salience: a framework linking biology, phenomenology, and pharmacology in schizophrenia. Am J Psychiatry 160 (1): 13–23.
- ↑ Bentall RP (Jan 1990). The illusion of reality: a review and integration of psychological research on hallucinations. Psychol Bull 107 (1): 82–95.
- ↑ Grossberg S (Jul 2000). How hallucinations may arise from brain mechanisms of learning, attention, and volition. J Int Neuropsychol Soc 6 (5): 583–92.
- ↑ 16.0 16.1 16.2 16.3 16.4 Horowitz MJ (1975). "Hallucinations: An Information Processing Approach" West LJ, Siegel RK Hallucinations; behavior, experience, and theory, New York: Wiley.
- Johnson FH (1978). The anatomy of hallucinations, Chicago: Nelson-Hall Co.
- Bentall RP, Slade PD (1988). Sensory deception: a scientific analysis of hallucination, London: Croom Helm.
- Larøi F, Aleman A (2008). Hallucinations: The Science of Idiosyncratic Perception, American Psychological Association (APA).
- INTERVOICE Online, website of the international hearing voices movement
- Hearing Voices Network
- "Anthropology and Hallucinations", chapter from The Making of Religion
- "The voice inside: A practical guide to coping with hearing voices"
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