A paracusia, or auditory hallucination, is a form of hallucination that involves perceiving sounds without auditory stimulus.

A common form of auditory hallucination involves hearing one or more talking voices. This may be associated with psychotic disorders, and holds special significance in diagnosing these conditions. However, individuals without any psychiatric disease whatsoever may hear voices.

There are three main categories into which the hearing of talking voices often fall: a person hearing a voice speak one's thoughts, a person hearing one or more voices arguing, or a person hearing a voice narrating his/her own actions. These three categories do not account for all types of auditory hallucinations.

Other types of auditory hallucination include exploding head syndrome and musical ear syndrome. In the latter, people will hear music playing in their mind, usually songs they are familiar with. This can be caused by: lesions on the brain stem (often resulting from a stroke); also, sleep disorders such as narcolepsy, tumors, encephalitis, or abscesses. This should be distinguished from the commonly experienced phenomenon of getting a song stuck in one's head. Reports have also mentioned that it is also possible to get musical hallucinations from listening to music for long periods of time. Other reasons include hearing loss and epileptic activity.

In the past, the cause of auditory hallucinations has been attributed to cognitive suppression by way of executive function failure of the fronto-parietal sulcus. Newer research has found that they coincide with the left superior temporal gyrus, suggesting that they are better attributed to speech misrepresentations. It is assumed through research that the neural pathways involved in normal speech perception and production, which are lateralized to the left temporal lobe, also underlie auditory hallucinations . Auditory hallucinations correspond with spontaneous neural activity of the left temporal lobe, and the subsequent primary auditory cortex. The perception of auditory hallucinations correspond to the experience of actual external hearing, despite the absence of physical acoustic output .

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 modality—visual, 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 and/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 his/her 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".

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.

In 73 individual cases reviewed by Evers and Ellger, 57 patients heard tunes that were familiar, while 5 heard unfamiliar tunes. These tunes ranged from religious pieces to childhood favorites, and also included popular songs from the radio. Vocal and instrumental forms of classical music were also identified in some patients. Keshavan found that the consistent feature of musical hallucinations was that it represented a personal memory trace. Memory traces refer to anything that may seem familiar to the patient, which indicated why certain childhood or familiar songs were heard.

Investigators have successfully narrowed down the major factors that are associated with musical hallucinations. Evers and Ellgers compiled a significant portion of musical hallucination articles, case studies etc. and were able to categorize five major etiologies:

- Hypoacusis

- Psychiatric disorders

- Focal brain lesion

- Epilepsy

- Intoxication

In 2015 a small survey reported voice hearing in persons with a wide variety of DSM-5 diagnoses, including:

- Bipolar disorder

- Borderline personality disorder

- Depression (mixed)

- Dissociative identity disorder

- Generalized anxiety disorder

- Major depression

- Obsessive compulsive disorder

- Post-traumatic stress disorder

- Psychosis (NOS)

- Schizoaffective disorder

- Schizophrenia

However, numerous persons surveyed reported no diagnosis. In his popular 2012 book "Hallucinations", neurologist Oliver Sacks describes voice hearing in patients with a wide variety of medical conditions, as well as his own personal experience of hearing voices.

Tactile hallucination is the false perception of tactile sensory input that creates a hallucinatory sensation of physical contact with an imaginary object. It is caused by the faulty integration of the tactile sensory neural signals generated in the spinal cord and the thalamus and sent to the primary somatosensory cortex (SI) and secondary somatosensory cortex (SII). Tactile hallucinations are recurrent symptoms of neurological diseases such as schizophrenia, Parkinson's disease, Ekbom's syndrome and delerium tremens. Patients who experience phantom limb pains also experience a type of tactile hallucination. Tactile hallucinations are also caused by drugs such as cocaine and alcohol.

The occurrence of MES has been suggested to be very high among the hearing impaired through acquired deafness or the ear condition known as tinnitus. Though exact causation is uncertain, it has been theorized that the "release phenomenon" is taken into effect. The "release phenomenon" says that individuals with acquired deafness may experience musical hallucinations because the lack of stimulation, which can give room for the brain to interpret internal sounds as being external.

Sufferers typically hear music or singing and the condition is more common in women. The hallucinatory experiences differ from that commonly experienced in psychotic disorders although there may be some overlap. The most important distinction is the realization that the hallucinations are not real. Delusional beliefs associated with the hallucinations may occur, but some degree of insight should be preserved. There should not be any other psychotic symptoms present, especially hallucinations in other modalities.

The hallmark sign of Alice in Wonderland syndrome (AIWS) is a migraine, and AIWS may in part be caused by the migraine. AIWS affects the sense of vision, sensation, touch, and hearing, as well as one's own body image.

A prominent and often disturbing symptom are experiences of altered body image. The person may find that they are confused as to the size and shape of parts of (or all of) their body. They may feel as though their body is expanding or getting smaller. Alice in Wonderland syndrome also involves perceptual distortions of the size or shape of objects. Other possible causes and signs of the syndrome include migraines, use of hallucinogenic drugs, and infectious mononucleosis.

Patients with certain neurological diseases have experienced similar visual hallucinations. These hallucinations are called "Lilliputian," which means that objects appear either smaller or larger than they actually are.

Patients may experience either micropsia or macropsia. Micropsia is an abnormal visual condition, usually occurring in the context of visual hallucination, in which affected persons see objects as being smaller than those objects actually are. Macropsia is a condition where the individual sees everything larger than it actually is.

A relationship between the syndrome and mononucleosis has been suggested.

One 17-year-old male, Michael Huang, described his odd symptoms. He said, "quite suddenly objects appear small and distant (teliopsia) or large and close (peliopsia). I feel as I am getting shorter and smaller 'shrinking' and also the size of persons are not longer than my index finger (a lilliputian proportion). Sometimes I see the blind in the window or the television getting up and down, or my leg or arm is swinging. I may hear the voices of people quite loud and close or faint and far. Occasionally, I experience attacks of migrainous headache associated with eye redness, flashes of lights and a feeling of giddiness. I am always conscious to the intangible changes in myself and my environment."

The eyes themselves are normal, but the person will often 'see' objects as the incorrect size, shape or perspective angle. Therefore, people, cars, buildings, houses, animals, trees, environments, etc., look smaller or larger than they should be, or that distances look incorrect; for example, a corridor may appear to be very long, or the ground may appear too close.

The person affected by Alice in Wonderland Syndrome may also lose the sense of time, a problem similar to the lack of spatial perspective. In other words, time seems to pass very slowly, akin to an LSD experience. The lack of time, and space, perspective leads to a distorted sense of velocity. For example, one could be inching along ever so slowly in reality, yet it would seem as if one were sprinting uncontrollably along a moving walkway, leading to severe, overwhelming disorientation. This can then cause the person to feel as if movement, even within his or her own home, is futile.

In addition, some people may, in conjunction with a high fever, experience more intense and overt hallucinations, seeing things that are not there and misinterpreting events and situations.

Other minor or less common symptoms may include loss of limb control and general dis-coordination, memory loss, lingering touch and sound sensations, and emotional experiences.

Musical hallucinations and MES have only become widely recognizable in the last few decades of research, but there are indications throughout history that have described symptoms of musical hallucinations. The Romantic composer Robert Schumann was said to have heard entire symphonies in his head from which he drew as inspiration for his music, but later in his life this phenomenon had diminished to just a note that played ceaselessly within his head. An alternative explanation is that his symptoms were caused by syphilis or mercury poisoning used for its treatment. The Russian composer Dmitri Shostakovich was also recorded as experiencing music hallucinations after some shrapnel was removed from his skull.

About 7% of individuals with Parkinson's disease also experience mild or severe types of tactile hallucinations. Most of these hallucinations are based on the sensation of a particular kind of animal. Several case studies were conducted by Fénelon and his colleagues on parkinson's patients that had tactile hallucinations. One of his patients described that he sensed "spiders and cockroaches chewing on his lower limb" which was rather painful. Several other patients felt that there was a parasitic infestation of their skin which caused lesions on their skins due to the obsessive need of itching. Fénelon also analyzed the particular types of tactile hallucinations experienced, the timing of such experience and certain drugs that could eliminate such experience. It was concluded that patients with both Parkinson's disease and tactile hallucinations not only experienced sensations elicited by insects under their skin but also by vivid tactile sensations of people. These hallucinations were aggravated during evening times due to altered arousal states and were alleviated by dopaminergic treatment such as the intake of clozapine. The study also explains that the pathophysiology of tactile hallucinations is uncertain, however, such hallucinations can be attributed to narcoleptic rapid eye movement sleep disorders due to its concordance with visual hallucinations. Moreover, it emphasizes that individuals who have had Parkinson's for a longer period of time have a more severe form of tactile hallucinations than with individuals who have succumbed to this disease for just a short period of time.

Clinical drugs used as an antiparkinsonian agent such as Trihexyphenidyl are known to create tactile hallucinations in parkinson patients.

Alice in Wonderland syndrome is a disorienting neuropsychological condition that affects perception. People experience size distortion such as micropsia, macropsia, pelopsia, or teleopsia. Size distortion may occur of other sensory modalities.

It is often associated with migraines, brain tumors, and the use of psychoactive drugs. It can also be the initial symptom of the Epstein–Barr virus (see mononucleosis). AiWS can be caused by abnormal amounts of electrical activity causing abnormal blood flow in the parts of the brain that process visual perception and texture.

Anecdotal reports suggest that the symptoms are common in childhood, with many people growing out of them in their teens. It appears that AiWS is also a common experience at sleep onset, and has been known to commonly arise due to a lack of sleep.

A pseudohallucination is an involuntary sensory experience vivid enough to be regarded as a hallucination, but recognised by the patient not to be the result of external stimuli. Unlike normal hallucinations, which occurs when one sees, hears, smells, tastes or feels something that is not there, with a compelling feeling or thought that it is real, pseudohallucinations are recognised by the person as unreal.

In other words, it is a hallucination that is recognized as a hallucination, as opposed to a "normal" hallucination which would be perceived as real. An example used in psychiatry is the hearing of voices which are "inside the head" according to the patient; in contrast, a hallucination would be indistinguishable to the patient from a real external stimulus, e.g. "people were talking about me".

The term is not widely used in the psychiatric and medical fields, as it is considered ambiguous; the term "nonpsychotic hallucination" is preferred. Pseudohallucinations, then, are more likely to happen with a hallucinogenic drug. But "the current understanding of pseudohallucinations is mostly based on the work of Karl Jaspers".

A further distinction is sometimes made between pseudohallucinations and "parahallucinations", the latter being a result of damage to the peripheral nervous system.

They are considered a feature of conversion disorder, somatization disorder, and dissociative disorders. Also, pseudohallucinations can occur in people with visual/hearing loss, with the typical such type being Charles Bonnet syndrome.

Chronic hallucinatory psychosis is a psychosis subtype, classified under "Other nonorganic psychosis" by the . Other abnormal mental symptoms in the early stages are, as a rule, absent. The patient is most usually quiet and orderly, with a good memory.

It has often been a matter of the greatest difficulty to decide under which heading of the recognized classifications individual members of this group should be placed. As the hallucinations give rise to slight depression, some might possibly be included under melancholia. In others, paranoia may develop. Others, again, might be swept into the widespread net of dementia praecox. This state of affairs cannot be regarded as satisfactory, for they are not truly cases of melancholia, paranoia, dementia praecox or any other described affection.

This disease, as its name suggests, is a hallucinatory case, for it is its main feature. These may be of all senses, but auditory hallucinations are the most prominent. At the beginning, the patient may realize that the hallucination is a morbid phenomenon and unaccountable. They may claim to hear a "voice" speaking, though there is no one in the flesh actually doing so. Such a state of affairs may last for years and possibly, though rarely, for life, and the subject would not be deemed insane in the ordinary sense of the word.

It's probable, however, that this condition forms the first stage of the illness, which eventually develops on definite lines. What usually happens is the patient seeks an explanation for the hallucinations. As none is forthcoming he/she tries to account for their presence and the result is a delusion, and, most frequently, a delusion of persecution. Also, it needs to be noted that the delusion is a comparatively late arrival and is the logical result of the hallucinations.

People with significant vision loss may have vivid, complex recurrent visual hallucinations (fictive visual percepts). One characteristic of these hallucinations is that they usually are "lilliputian" (hallucinations in which the characters or objects are smaller than normal). The most common hallucination is of faces or cartoons. Sufferers understand that the hallucinations are not real, and the hallucinations are only visual, that is, they do not occur in any other senses, e.g. hearing, smell or taste. Among older adults (> 65 years) with significant vision loss, the prevalence of Charles Bonnet syndrome has been reported to be between 10% and 40%; a 2008 Australian study found the prevalence to be 17.5%. Two Asian studies, however, report a much lower prevalence. The high incidence of non-reporting of this disorder is the greatest hindrance to determining the exact prevalence; non-reporting is thought to be a result of sufferers being afraid to discuss the symptoms out of fear that they will be labelled insane.

People suffering from CBS may experience a wide variety of hallucinations. Images of complex colored patterns and images of people are most common, followed by animals, plants or trees and inanimate objects. The hallucinations also often fit into the person's surroundings.

A hallucination is defined as sensory perception in the absence of external stimuli. Hallucinations are different from illusions, or perceptual distortions, which are the misperception of external stimuli. Hallucinations may occur in any of the senses and take on almost any form, which may include simple sensations (such as lights, colors, tastes, and smells) to experiences such as seeing and interacting with fully formed animals and people, hearing voices, and having complex tactile sensations.

Auditory hallucinations, particularly experiences of hearing voices, are the most common and often prominent feature of psychosis. Hallucinated voices may talk about, or to, the person, and may involve several speakers with distinct personalities. Auditory hallucinations tend to be particularly distressing when they are derogatory, commanding or preoccupying. However, the experience of hearing voices need not always be a negative one. One research study has shown that the majority of people who hear voices are not in need of psychiatric help. The Hearing Voices Movement has subsequently been created to support voice hearers, regardless of whether they are considered to have a mental disorder or not.

They are normally colorful, vivid images and occur during wakefulness, and predominately at night. Lilliputian hallucinations (also called Alice in Wonderland syndrome), hallucinations in which people or animals appear smaller than they would be in real life, are common in cases of peduncular hallucinosis. Most patients exhibit abnormal sleep patterns characterized by insomnia and daytime drowsiness. Peduncular hallucinosis has been described as a “release phenomenon” due to damage to the ascending reticular activating system, which is supported by the sleep disturbance characteristic of this syndrome. In most cases, people are aware that the hallucinations are not real. However, some people experience agitation and delusion and mistake their hallucinations for reality.

People with psychosis normally have one or more of the following:

- hallucinations

- delusions

- catatonia

- thought disorder.

Impairments in social cognition also occur.

Peduncular hallucinosis (PH), or Lhermitte's peduncular hallucinosis, is a rare neurological disorder that causes vivid visual hallucinations that typically occur in dark environments, and last for several minutes. Unlike some other kinds of hallucinations, the hallucinations that patients with PH experience are very realistic, and often involve people and environments that are familiar to the affected individuals. Because the content of the hallucinations is never exceptionally bizarre, patients can rarely distinguish between the hallucinations and reality.

In 1922, the French neurologist Jean Lhermitte documented the case of a patient who was experiencing visual hallucinations that were suggestive of localized damage to the midbrain and pons. After other similar case studies were published, this syndrome was labeled "peduncular hallucinosis."

The accumulation of additional cases by Lhermitte and by others influenced academic medical debate about hallucinations and about behavioral neurology.

Lhermitte provided a full account of his work in this area in his book "Les hallucinations: clinique et physiopathologie," which was published in Paris in 1951 by Doin publishing.

Contemporary researchers, with access to new technologies in medical brain imaging, have confirmed the brain localization of these unusual hallucinations.

The main symptoms of paraphrenia are paranoid delusions and hallucinations. The delusions often involve the individual being the subject of persecution, although they can also be erotic, hypochondriacal, or grandiose in nature. The majority of hallucinations associated with paraphrenia are auditory, with 75% of patients reporting such an experience; however, visual, tactile, and olfactory hallucinations have also been reported. The paranoia and hallucinations can combine in the form of “threatening or accusatory voices coming from neighbouring houses [and] are frequently reported by the patients as disturbing and undeserved". Patients also present with a lack of symptoms commonly found in other mental disorders similar to paraphrenia. There is no significant deterioration of intellect, personality, or habits and patients often remain clean and mostly self-sufficient. Patients also remain oriented well in time and space.

Paraphrenia is different from schizophrenia because, while both disorders result in delusions and hallucinations, individuals with schizophrenia exhibit changes and deterioration of personality whereas individuals with paraphrenia maintain a well-preserved personality and affective response.

A religious delusion is any delusion involving religious themes or subject matter. Though a small minority of psychologists have characterized all or nearly all religion as delusion, others focus solely on a denial of any spiritual cause of symptoms exhibited by a patient and look for other answers relating to a chemical imbalance in the brain, although there is actually no evidence of pathology in any psychiatric illness which means a diagnosis is made purely on opinions of professionals based on symptoms the person exhibits.

Individuals experiencing religious delusions are preoccupied with religious subjects that are not within the expected beliefs for an individual's background, including culture, education, and known experiences of religion. These preoccupations are incongruous with the mood of the subject. Falling within the definition also are delusions arising in psychotic depression; however, these must present within a major depressive episode and be congruous with mood.

Researchers in a 2000 study found religious delusions to be unrelated to any specific set of diagnostic criteria, but correlated with demographic criteria, primarily age. In a comparative study sampling 313 patients, those with religious delusion were found to be aged older, and had been placed on a drug regime or started a treatment programme at an earlier stage. In the context of presentation, their global functioning was found to be worse than another group of patients without religious delusions. The first group also scored higher on the Scale for the Assessment of Positive Symptoms (SAPS), had a greater total on the Brief Psychiatric Rating Scale (BPRS), and were treated with a higher mean number of neuroleptic medications of differing types during their hospitalization.

Religious delusion was found in 2007 to strongly correlate with "temporolimbic overactivity". This is a condition where irregularities in the brain's limbic system may present as symptoms of paranoid schizophrenia.

In a 2010 study, Swiss psychiatrists found religious delusions with themes of spiritual persecution by malevolent spirit-entities, control exerted over the person by spirit-entities, delusional experience of sin and guilt, or delusions of grandeur.

Religious delusions have generally been found to be less stressful than other types of delusion. A study found adherents to new religious movements to have similar delusionary cognition, as rated by the Delusions Inventory, to a psychotic group, although the former reported feeling less distressed by their experiences than the latter.

Visual agnosia is a broad category that refers to a deficiency in the ability to recognize visual objects. Visual agnosia can be further subdivided into two different subtypes: apperceptive visual agnosia and associative visual agnosia.

Individuals with apperceptive visual agnosia display the ability to see contours and outlines when shown an object, but they experience difficulty if asked to categorize objects. Apperceptive visual agnosia is associated with damage to one hemisphere, specifically damage to the posterior sections of the right hemisphere.

In contrast, individuals with associative visual agnosia experience difficulty when asked to name objects. Associative agnosia is associated with damage to both the right and left hemispheres at the occipitotemporal border. A specific form of associative visual agnosia is known as prosopagnosia. Prosopagnosia is the inability to recognize faces. For example, these individuals have difficulty recognizing friends, family and coworkers. However, individuals with prosopagnosia can recognize all other types of visual stimuli.

Neuroscientists have learned a lot about the role of the brain in numerous cognitive mechanisms by understanding corresponding disorders. Similarly, neuroscientists have come to learn a lot about music cognition by studying music-specific disorders. Even though music is most often viewed from a "historical perspective rather than a biological one" music has significantly gained the attention of neuroscientists all around the world. For many centuries music has been strongly associated with art and culture. The reason for this increased interest in music is because it "provides a tool to study numerous aspects of neuroscience, from motor skill learning to emotion".

Auditory agnosia is a form of agnosia that manifests itself primarily in the inability to recognize or differentiate between sounds. It is not a defect of the ear or "hearing", but a neurological inability of the brain to process sound meaning. It is a disruption of the "what" pathway in the brain. Persons with auditory agnosia can physically hear the sounds and describe them using unrelated terms, but are unable to recognize them. They might describe the sound of some environmental sounds, such as a motor starting, as resembling a lion roaring, but would not be able to associate the sound with "car" or "engine", nor would they say that it "was" a lion creating the noise. Auditory agnosia is caused by damage to the secondary and tertiary auditory cortex of the temporal lobe of the brain.