A hallucination is an unreal and misleading sensory event that an individual experiences with any of the five senses. They can consist of just one sensory experience or multiple. For example, someone could hallucinate only the sound of a flock of geese flying overhead but not "see" the geese. Alternatively, that person could both see and hear the birds. Most hallucinations occur in individuals who are mentally ill or under the influence of drugs or alcohol. Delusions are persistent, powerful thoughts that are not based on facts or truths. People with delusions remain firm in their beliefs regardless of evidence that refutes it. Associated with mental illnesses like bipolar disorder and PTSD, delusions primarily emerge from imbalances of serotonin, dopamine, norepinephrine, and other brain chemicals.
The five main types of hallucinations are visual, auditory, taste, olfactory, and tactile. Auditory hallucinations or hearing voices is the most common type reported by people with mental illness. Taking drugs or drinking too much alcohol is more likely to cause visual hallucinations. People who have substance abuse issues and are going through withdrawal without medical intervention sometimes experience tactile hallucinations. They may feel like insects are crawling on or under their skin or scalp, a phenomenon called formication. They may think their skin is on fire and insist on remaining in a cold shower for hours. The least common hallucinations involve taste and smell. If someone tastes or smells something that does not exist, this may indicate a brain tumor or lesion affecting specific parts of the brain.
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