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Visual hallucinations are visual sensations which are distorted in nature or outright false, occurring with no stimulus to explain the visions. There are a number of reasons why visual hallucinations can occur, ranging from dementia to drug abuse, and they can be a sign that someone is experiencing a serious underlying medical problem if they occur with no clear explanation. It is also possible to experience other types of sensory hallucinations during a visual hallucination. Thus, people may feel like they can touch objects in the vision or may report smells which are not linked with any known stimuli.
In some types of visual hallucinations, vision is simply distorted. People may see halos, streaks of color, tracers, and other artifacts in their vision which do not actually exist. There types of visual hallucinations are often associated with recreational drug use, and can also be caused by some prescription drugs, or medical issues such as a high fever. Distortion of vision may last a few seconds to several days, depending on the cause.
In other visual hallucinations, visions are entirely false. People may see people, animals, or objects which are not there, or may be transported into scenes which don't actually exist. A common type of visual hallucination seen among elderly patients is a hallucination of a dead loved one; people may claim to be able to see a dead spouse, for example, and may even carry on conversations with the hallucination.
Stress, fatigue, and mental illness can all cause visual hallucinations, as can neurological conditions, medications, and eye conditions. Finding the cause of visual hallucinations is important, as it may be treatable. For example, if someone is experiencing visual hallucinations due to impingement on the optic nerve, surgery could be performed to free the nerve and restore vision. In cases where the condition is not treatable, being aware of the fact that a patient may experience hallucinations can be important for care providers. For example, patients on medications which cause hallucinations may appear to act erratically or irrationally, but this is because of things which only they can see.
In some cultures historically, people deliberately cultivated visual hallucinations, believing that they were sent by higher powers to provide information or religious insight. Sometimes a specific member of society was designated as a recipient of such visions, while in other cases groups would ingest hallucinogenic drugs for the purpose of experiencing visions together. Some cultures still retain the practice of using such drugs in a religious context.