Phantosmia (olfactory hallucinations), smelling an odor that is not actually there, and parosmia (olfactory illusions), inhaling a real odor but perceiving it as different scent than remembered, are distortions to the sense of smell (olfactory system) that, in most cases, are not caused by anything serious and 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 are sometimes the cause as well, such as smoking, exposure to certain types of chemicals (e.g., insecticides or solvents), or radiation treatment for head or neck cancer. It can also be a symptom of certain mental disorders such as depression, bipolar disorder, intoxication or withdrawal from drugs and alcohol, or psychotic disorders (e.g., schizophrenia). The perceived odors are usually unpleasant and commonly described as smelling burned, foul spoiled, or rotten.

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.

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.

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

- hallucinations

- delusions

- catatonia

- thought disorder.

Impairments in social cognition also occur.

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.

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.

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.

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.

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.

Musical hallucinations fall under the category of auditory hallucinations and describe a disorder in which a sound is perceived as instrumental music, sounds, or songs. It is a very rare disorder, reporting only 0.16% in a cohort study of 3,678 individuals.

Disruption of sleep-wake cycle is almost invariably present in delirium and often predates the appearance of a full-blown episode. Minor disturbances with insomnia or excessive daytime somnolence may be hard to distinguish from other medically ill patients without delirium, but delirium typically involves more substantial alterations with sleep fragmentation or even complete sleep-wake cycle reversal that reflect disturbed circadian rhythm regulation. The relationship of circadian disturbances to the characteristic fluctuating severity of delirium symptoms over a 24-hour period or to motor disturbance is unknown.

Motor activity alterations are very common in delirium. They have been used to define clinical subtypes (hypoactive, hyperactive, mixed) though studies are inconsistent as to the prevalence of these subtypes. Cognitive impairments and EEG slowing are comparable in hyperactive and hypoactive patients though other symptoms may vary. Psychotic symptoms occur in both although the prevailing stereotype suggests that they only occur in hyperactive cases. Hypoactive cases are prone to non detection or misdiagnosis as depression. A range of studies suggest that motor subtypes differ regarding underlying pathophysiology, treatment needs, and prognosis for function and mortality though inconsistent subtype definitions and poorer detection of hypoactives impacts interpretation of these findings.

Psychotic symptoms occur in up to 50% of patients with delirium. While the common non-medical view of a delirious patient is one who is hallucinating, most people who are medically delirious do not have either hallucinations or delusions. Thought content abnormalities include suspiciousness, overvalued ideation and frank delusions. Delusions are typically poorly formed and less stereotyped than in schizophrenia or Alzheimer’s disease. They usually relate to persecutory themes of impending danger or threat in the immediate environment (e.g. being poisoned by nurses). Misperceptions include depersonalisation, delusional misidentifications, illusions and hallucinations. Hallucinations and illusions are frequently visual though can be tactile and auditory. Abnormalities of affect which may attend the state of delirium may include many distortions to perceived or communicated emotional states. Emotional states may also fluctuate, so that a delirious person may rapidly change between, for example, terror, sadness and jocularity.

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 .

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.

Delirious patients have diminished comprehension as evidenced by reduced ‘grasp’ of their surroundings and difficulties in connecting with their immediate environment, executive dysfunction affecting abstraction, initiation/perseveration, switching mental sets, working memory, temporal sequencing and organization, insight and judgment. Though none of these cognitive deficits is specific to delirium, the array and pattern is highly suggestive.

Language disturbances in delirium include anomic aphasia, paraphasias, impaired comprehension, agraphia, and word-finding difficulties. Incoherent or illogical / rambling conversation is reported commonly. Disorganised thinking includes tangentiality, circumstantiality and a proneness to loose associations between elements of thought which results in speech that often makes limited sense with multiple apparent irrelevancies. This aspect of delirium is common but often difficult for non-experts to assess reliably.

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.

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.

Clinical lycanthropy is defined as a rare psychiatric syndrome that involves a delusion that the affected person can transform into, has transformed into, or is a non-human animal. Its name is associated with the mythical condition of lycanthropy, a supernatural affliction in which humans are said to physically shapeshift into wolves. It is purported to be a rare disorder.

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.

Other visual hallucinations tend to stem from psychological disorders. Whereas a person with a psychological disorder thinks their hallucinations are real, people with peduncular hallucinosis normally know that the visual hallucinations they see are not real. Peduncular hallucinations are independent of seizures, unlike some other visual hallucinations.

Schizophrenia is often described in terms of positive and negative (or deficit) symptoms. "Positive symptoms" are those that most individuals do not normally experience, but are present in people with schizophrenia. They can include delusions, disordered thoughts and speech, and tactile, auditory, visual, olfactory and gustatory hallucinations, typically regarded as manifestations of psychosis. Hallucinations are also typically related to the content of the delusional theme. Positive symptoms generally respond well to medication.

"Negative symptoms" are deficits of normal emotional responses or of other thought processes, and are less responsive to medication. They commonly include flat expressions or little emotion, poverty of speech, inability to experience pleasure, lack of desire to form relationships, and lack of motivation. Negative symptoms appear to contribute more to poor quality of life, functional ability, and the burden on others than positive symptoms do. People with greater negative symptoms often have a history of poor adjustment before the onset of illness, and response to medication is often limited.

The validity of the positive and negative construct has been challenged by factor analysis studies observing a three dimension grouping of symptoms. While different terminology is used, a dimension for hallucinations, a dimension for disorganization, and a dimension for negative symptoms are usually described.

Alice in Wonderland syndrome is a disturbance of perception rather than a specific physiological change to the body's systems. The diagnosis can be presumed when other causes have been ruled out and if the patient presents symptoms along with migraines and complains of onset during the day (although it can also occur at night).

Another symptom of Alice in Wonderland syndrome is sound distortion, such as every little movement making a clattering sound.

Individuals with schizophrenia may experience hallucinations (most reported are hearing voices), delusions (often bizarre or persecutory in nature), and disorganized thinking and speech. The last may range from loss of train of thought, to sentences only loosely connected in meaning, to speech that is not understandable known as word salad. Social withdrawal, sloppiness of dress and hygiene, and loss of motivation and judgment are all common in schizophrenia.

Distortions of self-experience such as feeling as if one's thoughts or feelings are not really one's own to believing thoughts are being inserted into one's mind, sometimes termed passivity phenomena, are also common. There is often an observable pattern of emotional difficulty, for example lack of responsiveness. Impairment in social cognition is associated with schizophrenia, as are symptoms of paranoia. Social isolation commonly occurs. Difficulties in working and long-term memory, attention, executive functioning, and speed of processing also commonly occur. In one uncommon subtype, the person may be largely mute, remain motionless in bizarre postures, or exhibit purposeless agitation, all signs of catatonia. People with schizophrenia often find facial emotion perception to be difficult. It is unclear if the phenomenon called "thought blocking", where a talking person suddenly becomes silent for a few seconds to minutes, occurs in schizophrenia.

About 30 to 50 percent of people with schizophrenia fail to accept that they have an illness or comply with their recommended treatment. Treatment may have some effect on insight.

People with schizophrenia may have a high rate of irritable bowel syndrome but they often do not mention it unless specifically asked. Psychogenic polydipsia, or excessive fluid intake in the absence of physiological reasons to drink, is relatively common in people with schizophrenia.

Zou huo ru mo (走火入魔) or "qigong deviation" (气功出偏), is a Chinese term traditionally used to indicate that something has gone wrong in spiritual or martial arts training, applied to describe a physiological or psychological disorder believed to result during or after qigong practice, believed by the qigong community to result from "improper practice" of qigong and other self-cultivation techniques, and highlighted in the social and political context of mass popularization of qigong in China.

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.