[8] Hallucinations can occur in any sensory modality—visual, auditory, olfactory, gustatory, tactile, proprioceptive, equilibrioceptive, nociceptive, thermoceptive and chronoceptive.
Hallucinations can be associated with drug use (particularly deliriants), sleep deprivation, psychosis, neurological disorders, and delirium tremens.
[22] Complex hallucinations are those of voices, music,[20] 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.
[25] One important example to consider when forming a differential diagnosis for a patient with paracusia is lateral temporal lobe epilepsy.
[26] 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, the auditory version of Charles Bonnet syndrome), lateral temporal lobe epilepsy,[27] arteriovenous malformation,[28] stroke, lesion, abscess, or tumor.
[31] A study conducted by the La Trobe University School of Psychological Sciences revealed that as few as five cups of coffee a day (approximately 500 mg of caffeine) could trigger the phenomenon.
One subtype of tactile hallucination, formication, is the sensation of insects crawling underneath the skin and is frequently associated with prolonged cocaine use.
[41] 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.
The regions of the brain responsible for gustatory hallucination in this case are the insula and the superior bank of the sylvian fissure.
[44] 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.
[47][48] In males, genital specific sensations are related to the postcentral gyrus and arousal and ejaculation are linked to stimulation in the posterior frontal lobe.
[50][51] Limited studies have been done to understand the mechanism of action behind sexual hallucinations in epilepsy, substance use, and post-traumatic stress disorder etiologies.
The multiple sensory modalities can occur at the same time (simultaneously) or with a delay (serial), be related or unrelated to each other, and be consistent with reality (congruent) or not (incongruent).
Multimodal hallucinations are correlated to poorer mental health outcomes, and are often experienced as feeling more real.
[3] 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.
[57] Peduncular means pertaining to the peduncle, which is a neural tract running to and from the pons on the brain stem.
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.
[62] Charles Bonnet syndrome is the name given to visual hallucinations experienced by a partially or severely sight impaired person.
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."
The length of time that the sight impaired person can have these hallucinations varies according to the underlying speed of eye deterioration.
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.
Some psychedelics such as lysergic acid diethylamide (LSD) and psilocybin can cause hallucinations that range in the spectrum of mild to intense.
[67][68][69] 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.).
)[71] 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.
Reduced grey matter in regions of the superior temporal gyrus/middle temporal gyrus, including Broca's area, is associated with auditory hallucinations as a trait, while acute hallucinations are associated with increased activity in the same regions along with the hippocampus, parahippocampus, and the right hemispheric homologue of Broca's area in the inferior frontal gyrus.
[77] One proposed model of hallucinations posits that over-activity in sensory regions, which is normally attributed to internal sources via feedforward networks to the inferior frontal gyrus, is interpreted as originating externally due to abnormal connectivity or functionality of the feedforward network.
[79] Thalamocortical circuits, composed of projections between thalamic and cortical neurons and adjacent interneurons, underlie certain electrophysical characteristics (gamma oscillations) that are associated with sensory processing.
However, for those hallucinations caused by mental disease, a psychologist or psychiatrist should be consulted, and treatment will be based on the observations of those doctors.
Prevalence of hallucinations varies depending on underlying medical conditions,[84][9] which sensory modalities are affected,[10] age[85][84] and culture.