Hallucinations can be caused by a number of factors. Hypnagogic hallucination These hallucinations occur just before falling asleep and affect a high proportion of the population: in one survey 37% of the respondents experienced them twice a week. The hallucinations can last from seconds to minutes; all the while, the subject usually remains aware of the true nature of the images. These may be associated with narcolepsy. Hypnagogic hallucinations are sometimes associated with brainstem abnormalities, but this is rare. Peduncular hallucinosis 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 then 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. 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 rapid eye movement sleep. Parkinson's disease and Lewy body dementia Parkinson's disease is linked with Lewy body dementia for their similar hallucinatory<unk> . The<unk>  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 area and pedunculopontine nuclei of the tegmentum. Migraine coma 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 a partially or severely sight impaired person. The hallucinations can occur at any time and can distress people of any age, as they may not initially be aware that they are hallucinating. They may fear for their own mental health initially, which may delay them sharing with carers until they start to understand it themselves. The hallucinations can frighten and disconcert as to what is real and what is not. The hallucinations can sometimes be dispersed by eye movements, or by reasoned logic such as, "I can see fire but there is no smoke and there is no heat from it" or perhaps, "We have an infestation of rats but they have pink ribbons with a bell tied on their necks." Over elapsed months and years, the hallucinations may become more or less frequent with changes in ability to see. The length of time that the sight impaired person can suffer from these hallucinations varies according to the underlying speed of eye deterioration. A differential diagnosis are ophthalmopathic hallucinations. Focal epilepsy 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 contralateral 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 toward 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 to be 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 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. 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. Drug-induced hallucination Drug-induced hallucinations are caused by hallucinogens, dissociatives, and deliriants, including many drugs with anticholinergic actions and certain stimulants, which are known to cause visual and auditory hallucinations. Some psychedelics such as lysergic acid diethylamide (LSD) and psilocybin can cause hallucinations that range in the spectrum of mild to intense. Hallucinations, pseudohallucinations, or intensification of pareidolia, particularly auditory, are known side effects of opioids to different degrees—it may be associated with the absolute degree of agonism or antagonism of especially the kappa opioid receptor, sigma receptors, delta opioid receptor and the NMDA receptors or the overall receptor activation profile as synthetic opioids like those of the pentazocine, levorphanol, fentanyl, pethidine, methadone and some other families are more associated with this side effect than natural opioids like morphine and codeine and semi-synthetics like hydromorphone, amongst which there also appears to be a stronger correlation with the relative analgesic strength. Three opioids, Cyclazocine (a benzormorphan opioid/pentazocine relative) and two levorphanol-related morphinan opioids, Cyclorphan and Dextrorphan are classified as hallucinogens, and Dextromethorphan as a dissociative. These drugs also can induce sleep (relating to hypnagogic hallucinations) and especially the pethidines have atropine-like anticholinergic activity, which was possibly also a limiting factor in the use, the psychotomimetic side effects of potentiating morphine, oxycodone, and other opioids with scopolamine (respectively in the Twilight Sleep technique and the combination drug Skophedal, which was eukodal (oxycodone), scopolamine and ephedrine, called the "wonder drug of the 1930s" after its invention in Germany in 1928, but only rarely specially compounded today) (q.q.v.). Sensory deprivation hallucination Hallucinations can be caused by sensory deprivation when it occurs for prolonged periods of time, and almost always occurs in the modality being deprived (visual for blindfolded/darkness, auditory for muffled conditions, etc.) Experimentally-induced hallucinations Anomalous experiences, such as so-called benign hallucinations, may occur in a person in a state of good mental and physical health, even in the apparent absence of a transient trigger factor such as fatigue, intoxication or sensory deprivation. The evidence for this statement has been accumulating for more than a century. Studies of benign hallucinatory experiences go back to 1886 and the early work of the Society for Psychical Research, which suggested approximately 10% of the population had experienced at least one hallucinatory episode in the course of their life. More recent studies have validated these findings; the precise incidence found varies with the nature of the episode and the criteria of "hallucination" adopted, but the basic finding is now well-supported. Non-celiac gluten sensitivity There is tentative evidence of a relationship with non-celiac gluten sensitivity, the so-called "gluten psychosis".
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