Hallucinations are abnormal sensory perceptions of stimuli that occur in the absence of external stimuli. Hallucinations can be visual, auditory, tactile, olfactory, or gustatory. There are numerous disorders that are associated with hallucinations, including, but not limited to, schizophrenia, posttraumatic stress disorder (PTSD), substance use and withdrawal, and mood disorders. To determine appropriate treatment, the etiology of the hallucinations must be ascertained by conducting a thorough medical history, psychological assessment, and, if warranted, imaging studies.
Hallucinations can be defined as conscious abnormal sensory perceptions that do not have a source in the outside world. Hallucinations can involve one or more senses, including visual, auditory, gustatory, olfactory, or tactile. People who experience hallucinations report experiences such as seeing things that aren’t there, hearing voices that no one else can hear, feeling that there is something crawling on their skin, smelling things that no one else can smell, or tasting things that other people cannot taste.
In one study, Johns and colleagues found that prevalence of hallucinations in a community sample ranged from 2% to 10%. There is currently no evidence that hallucinations occur more frequently in some racial or ethnic groups than in others, and gender does not appear to affect the presence or frequency of hallucinations.
Disorders Associated With Hallucinations
Hallucinations are associated with numerous disorders, illnesses, and states. Currently, there does not appear to be a single underlying cause that can explain all types of hallucinations. Several explanations and hypotheses have been put forth for various disorders, but to date, the causes of hallucinations are not completely understood.
Hallucinations are most commonly associated with schizophrenia, a mental illness characterized by disordered perceptions, thoughts, and behaviors. According to the National Institute of Mental Health, approximately 75% of individuals with a diagnosis of schizophrenia experience auditory hallucinations, visual hallucinations, or both. The auditory hallucinations may be command hallucinations, in which the person hears voices ordering him or her to do something.
Other disorders are that are less frequently associated with hallucinations include eye disorders such as macular degeneration or glaucoma; high fever, particularly in children and the elderly; late-stage Alzheimer’s disease; migraine headaches; intoxication or withdrawal from alcohol or drugs; severe medical illness such as liver or kidney failure or brain cancer; severe mood disorders such as bipolar disorder and depression; post traumatic stress disorder; and temporal lobe epilepsy. In addition, hallucinations are also associated with normal sleep-wake cycles. Approximately one third of adults experience hypnagogic hallucinations, which occur as a person passes from wakefulness into sleep; another 10% to 12% of adults report hypnopompic hallucinations, which occur as the person is waking up.
Hallucinations and Violence
The relationship between hallucinations and violent behavior has been the subject of debate. Some research has found a modest positive relationship between hallucinations and violence, whereas other studies found no immediate relationship. Dale E. McNeil and colleagues studied the relationship between command hallucinations and violence in a sample of 130 inpatients who were diagnosed with schizophrenia. They found that 30% of the inpatients reported that they had experienced command hallucinations to hurt someone else in the past year, while 22% of the patients reported that they complied with those command hallucinations. These findings suggest that patients who experienced command hallucinations were almost twice as likely to engage in violent behavior as patients who did not experience command hallucinations. Other studies have reported compliance for command hallucinations of violence ranging from 39.2% to 88.5%. Compliance with command hallucinations has been found to be related to whether or not the person recognizes the hallucinated voice, with those recognizing the voice being more likely to obey the command.
Hallucinations and Schizophrenia
Hallucinations are most commonly associated with schizophrenia. Patients with schizophrenia may experience auditory and/or visual hallucinations. Some research suggests that auditory hallucinations can be caused by high levels of the neurotransmitter dopamine in the patient’s brain. Researchers have found evidence, however, both to support and to refute the dopamine hypothesis. The evidence that most strongly supports the dopamine hypothesis comes from the effects of drugs such as amphetamines and cocaine. These drugs are known to increase the levels of dopamine in the brain and can result in psychotic symptoms, including hallucinations, when large doses are consumed over long periods. Several studies have found that when patients with schizophrenia were administered drugs that produce increased dopamine levels, up to 75% of them had significant increases in their hallucinations and psychotic symptoms, while control subjects without schizophrenia showed no effects on being administered the same drugs. Further evidence supporting the dopamine hypothesis was found following the discovery of a class of drugs known as phenothiazines, which include antipsychotic medications. These drugs bind to dopamine receptors and have been found to decrease the positive symptoms of schizophrenia, including hallucinations.
With the advent of more sophisticated brain imaging techniques such as positron emission tomography (PET) scanning, newer findings challenged the dopamine hypothesis. In PET studies with schizophrenic patients, researchers found that in some patients, more than 90% of the dopamine receptors were blocked by the antipsychotic drugs, yet there was no observed diminution in psychotic symptoms, including hallucinations. However, the patients in this study had been receiving treatment with antipsychotic medications for more than 30 years. In another study, researchers found that 90% to 95% of patients who were only recently diagnosed with schizophrenia responded to antipsychotic medications, and scans of their brains revealed that only 60% to 70% of the dopamine receptors were blocked. Finally, in recent years, atypical antipsychotic medications have been developed to treat schizophrenia. While equally as effective as the typical antipsychotic medications, these atypical antipsychotic medications block fewer of the dopamine receptors (about 60-70%). Thus, confronted by some evidence that supports and other evidence that refutes the dopamine hypothesis, research continues into the etiology of schizophrenia.
There has also been a great deal of research investigating the structural and functional abnormalities in the brains of patients with schizophrenia. Researchers have found that some people with schizophrenia have changes in the density of the brain’s gray matter in the frontal and temporal lobes. If these differences in brain structure were present since birth, then they could result in dopamine hypersensitivity as described above, resulting in psychotic symptoms such as hallucinations.
Researchers have also noted abnormal patterns in brain activity among patients with schizophrenia. More specifically, abnormalities were found in the corollary discharge mechanism, which enables people to distinguish between internal and external stimuli. Studies with electroencephalograms (EEGs) of the brains of patients with schizophrenia that were taken while the patients were talking found that the corollary discharges from the frontal cortex of the brain (the area where thoughts are produced) did not provide information to the auditory cortex (the area in which sounds are interpreted) that the sounds that were detected were self-generated. Therefore, this dysfunction would lead patients with schizophrenia to perceive internal stimuli as being generated by external sources, thereby producing auditory hallucinations.
Finally, there is some evidence that auditory hallucinations may be related to tissue loss in the primary auditory cortex. The receptors in the auditory cortex process information and then send it to the thalamus, which filters the information before sending it to be decoded in the brain. These complex processes transform abstract sensory information such as sound and light waves into recognizable images and voices of the world around us. While dysfunctions in any of these structures alone would not explain the presence of hallucinations, it is possible that patients with schizophrenia may experience the malfunction of several of these neurotransmitter and receptors networks simultaneously. None of these defects alone would cause schizophrenia or trigger a psychotic episode; however, they do confer a predisposition for developing schizophrenia. Thus, individuals with these defects would be more likely to experience auditory or visual misperceptions, which would present themselves as auditory or visual hallucinations.
Hallucinations and Posttraumatic Stress Disorder
Trauma survivors who develop PTSD often report visual and auditory hallucinations. Hallucinations in trauma survivors are often referred to as flashbacks. During these flashbacks, the person relives the traumatic experience as if they were really there. Although these flashbacks can be described as hallucinations, they are nonpsychotic in nature. It is believed that flashbacks in patients with PTSD occur following abnormal memory formation patterns that occur during the traumatic experience. In cases of trauma, it is hypothesized that instead of being processed in the hippocampus, where memories are described using language, traumatic memories are stored in the amygdala, which stores the memory as an emotional experience. As a consequence, the traumatic memories are stored in the amygdala without words but only with intense emotions, and the memories are associated with vivid sensations and sensory perceptions that can manifest themselves as hallucinations during stressful situations.
Hallucinations and Substance Abuse
Hallucinations can be caused by overdoses of prescription drugs, illegal drugs, and alcohol or drug withdrawal. Substance-induced hallucinations seem to occur because of blocking of the action of serotonin, while phencyclidine induces hallucinations by blocking glutamate receptors. Interestingly, individuals who have used lysergic acid diethylamide (LSD) have reported flashbacks, or spontaneous hallucinations, which occur when the person is no longer taking the drug. This phenomenon is referred to as hallucinogen persisting perception disorder.
Withdrawal from alcohol can also result in hallucinations. These types of hallucinations usually occur if a chronic alcoholic suddenly stops drinking alcohol. Initially, on withdrawal, patients report auditory hallucinations, such as hearing threatening or accusatory voices. After several days of withdrawal, patients can experience delirium tremens, a condition in which they feel disoriented, depressed, and feverish and experience visual hallucinations.
Hallucinations and Mood Disorders
Hallucinations have also been associated with mood disorders. Approximately 20% of patients in the manic phase of bipolar disorder and almost 10% of patients with major depressive disorder experience auditory hallucinations. It is not clear what causes patients with mood disorders to experience hallucinations. There appears to be a genetic link, as psychotic mood states have been found to run in families. Additionally, elevated levels of the hormone cortisol have been found in patients who experience depression with psychosis.
Assessment of Hallucinations
To assess hallucinations, the general physician, psychiatrist, or psychologist should conduct a thorough medical and psychosocial examination to rule out possible organic, environmental, or psychological causes. Depending on the patient’s symptoms and medical history, such an evaluation may also involve laboratory tests and imaging studies. If a psychological cause such as schizophrenia is suspected, a psychologist will typically conduct an interview with the patient and his or her family and administer one of several clinical inventories, or tests, to evaluate the mental status of the patient. This could include the Mini-Mental Status Exam (MMSE), the Psychotic Symptom Rating Scales, the Positive and Negative Syndrome Scale, or the Scale for Assessment of Positive Symptoms. A total score of 20 or lower on the MMSE is generally indicative of delirium, dementia, schizophrenia, or severe depression.
Treatment of Hallucinations
If hallucinations are related to schizophrenia or another psychotic disorder, then the patient should be under the care of a psychiatrist. For schizophrenia-related hallucinations, the patient should be prescribed antipsychotic medication such as thioridazine (Mellaril), haloperidol (Haldol), chlorpromazine (Thorazine), clozapine (Clozaril), or risperidone (Risperdal). Treatment for hallucinations that are not related to schizophrenia are dependent on the disorder associated with the onset of hallucinations and could include anticonvulsant or anti-depressant medications, psychotherapy, brain or ear surgery, or therapy for drug dependence. Hallucinations related to normal sleeping and waking are considered normal and do not require intervention.
- American Psychiatric Association. (2000). Diagnostic and statistical manual of mental disorders (4th ed., text revision). Washington, DC: Author.
- Behrendt, R. (2006). Dysregulation of thalamic sensory “transmission” in schizophrenia neurochemical vulnerability to hallucinations. Journal of Psychopharmacology, 20(3), 356-372.
- Johns, L. C., Nazroo, J. Y., Bebbington, P., & Kuipers, E. (2002). Occurrence of hallucinatory experiences in a community sample and ethnic variations. British Journal of Psychiatry, 180, 174-178.
- McNeil, D. E., Eisner, J. P., & Binder, R. L. (2000). The relationship between command hallucinations and violence. Psychiatric Services, 51, 1288-1292.
- Ohayon, M. M. (2002). Prevalence of hallucinations and their pathological associations in the general population. Psychiatry Research, 97, 153-164.