What Is Hallucination?

Lucy Bell-Young

by Lucy Bell-Young

31st March 2021

When talking about hallucination, we usually think of it as a visual experience as they involve seeing something that isn’t there. But a hallucination can actually involve any number of your five senses: it can be tactile, olfactory, gustatory, somatic, or a combination of these. 

While hallucinations are known to be triggered by chemicals or drugs that have hallucinogenic properties, they can also be triggered by underlying medical conditions, like epilepsy, and even by sensory deprivation or stimulation.

Hallucination concept, delirium in EKW tunnel

What Chemicals Can Cause Hallucination?

Chemicals that cause hallucinations are called hallucinogens or hallucinogenic drugs. Although many of these are illicit drugs, a hallucination can also be caused by anaesthetics or by mushrooms and plants containing naturally occurring biochemicals. Whatever the vehicle, the two main effects of hallucinogenic chemicals are the same: they either reduce or enhance sensory inputs.

Hallucinogenic chemicals interrupt the normal metabolic functions related to the way the brain receives information from sensory inputs or external stimuli. Hallucinogens work by suppressing neural signals from the senses while a person is awake. As a result, a disjunct is created. The conscious part of the brain then accesses the memories for sensory analogues, using experiences from the recent past as a replacement for the sensory inputs in the present. Information becomes jumbled in the process, but the feeling or sense of being stimulated feels realistic.

This is common not only with illicit hallucinogens, like the drug LSD, but also with some anaesthetics, like sodium amobarbital. When injected with this, patients may experience hallucinations before surgery, when the anaesthesia is taking effect, or after surgery, when the drug is waning in its effect on the conscious brain.

All that said, some hallucinogenic chemicals cause hallucinations by increasing or amplifying the sensory inputs, rather than suppressing them. This effect is common among many types of psychoactive fungi, like magic mushrooms, and it’s caused by a naturally occurring hallucinogenic compound called psilocybin.

But hallucinogenic chemicals, be they naturally occurring or manufactured, aren’t the only things that cause hallucination. Aside from hallucinogens, the chemical balance in the brain can also be temporarily disrupted through mechanical means. These include both sensory deprivation, such as being placed in a dark and soundproof room, and sensory overstimulation, such as being subjected to loud noises and bright, flashing lights.

Medic hands in blue protective gloves with anaesthesia syringe
Anaesthetics, like sodium amobarbital, often make patients hallucinate before or after surgery

What Causes A Person To Hallucinate?

Hallucinations occur when there’s a disjunct between sensory inputs (external stimuli) and the way the brain processes information. They can be triggered either by decreasing the sensory inputs so that the brain has to pull sensory analogues from memories, or by increasing the sensory inputs to a point that the brain is overwhelmed. Drugs, physical stimuli, medical conditions, and mental illnesses can all cause hallucinations.

We’ve already looked at how drugs cause hallucination, so here’s a look at how they can be caused by other means:

  1. Sensory deprivation or stimulation

Sensory deprivation or stimulation is a mechanical way of triggering hallucinations; no hallucinogens are involved. Whether it’s placing someone in a sensory deprivation chamber or exposing them to flashing lights and loud noises, the point is to either suppress or overwhelm their sensory inputs.

When this happens, a sort of short-circuit in the information processing of the brain occurs, and this is what triggers hallucinations. 

  1. Medical and physical conditions

Another way that hallucinations are triggered is through pre-existing medical conditions, like epilepsy, that disrupt the normal electro-chemical balance in the brain.

Epileptic hallucinations depend on which part of the brain is affected by the seizures or misfires. Epileptic patients may feel detached from their bodies, and they may also experience gustatory, auditory, or visual hallucinations. Before or during epileptic seizures, a patient may also see light spots or flashes of sharp light.

Hallucinations can also be caused by a high fever, which is the body’s response to inflammation and can cause mental confusion. Hallucinations caused by fevers is most common among young children who experience convulsions due to having a very high temperature.

Other medical and physical conditions that can cause hallucinations are:

  • Migraines
  • Social isolation, especially among the elderly
  • Deafness, blindness or vision problems
  • Terminal illnesses, such as stage 3 HIV (AIDS), brain cancer or kidney and liver failure
  1. Mental illnesses

Various types of mental illnesses have accompanying hallucinations, which may include auditory, visual, olfactory, gastutory, tactile, or somatic hallucinations. 

For instance, people living with schizophrenia may experience auditory or visual hallucinations. Research indicates that this is due to the modulation of thalamocortical gamma activity by external sensory input becoming impaired. When this happens, the brain uses attentional mechanisms to supplement the lack of sensory input, similar to how hallucinogenic chemicals in drugs work. 

Hallucinations are different from delusions. Typically, people who hallucinate are aware that they’re hallucinating. In comparison, delusional people don’t know that they’re delusional. Delusions may or may not involve hallucinations.

Medical illustration of a brain with epilepsy
People with epilepsy experience bursts of electrical activity in their brain, which can affect sensory systems and cause hallucinations

What Drugs Are Hallucinogens?

Hallucinogens are typically illicit drugs, but some are regulated drugs with medical applications. Some examples of hallucinogens are:

  • Sodium amobarbital: Hallucination is common with the injection of this drug, which is a type of general anaesthetic. Surgery patients report hallucinations before falling asleep or after waking up.
  • LSD from ergot fungus that grows on rye: Chemically known as lysergic acid diethylamide with the chemical formula C20H25N3O, LSD is one of the most powerful hallucinogens. It blocks serotonin and acts on the sympathetic nervous system in the midbrain.
  • Psilocybin from mushrooms: This is a psychedelic drug that can be derived from more than 200 species of fungus. It has the chemical formula C12H17N2O4P. The most powerful sources of this psychedelic are from the various species of the fungal genus Psilocybe, such as P. azurescens, P. semilanceata, and P. cyanescens.
  • Tetrahydrocannabinol (THC) from marijuana: This is one of the 113 cannabinoids found in cannabis. It’s the psychoactive constituent that causes hallucinations, though these are quite rare. It has multiple isomers or molecular forms, but the chemical formula is the same: C₂₁H₃₀O₂. However, the active ingredient refers to the Delta-9-THC isomer, which has the chemical name (−)-trans-Δ⁹-tetrahydrocannabinol.
  • Ayahuasca: This is known by many other names such as hoasca, aya, and yage. Ayahuasca is consumed as tea, and is processed through brewing plants that contain dimethyltryptamine (DMT), a fast and potent psychoactive, together with Amazonian vine, which prevents the breakdown of DMT in the digestive system. 
  • Dimethyltryptamine, or DMT: Commonly known as Dimitri, this is a naturally occurring organic chemical found in some species of Amazonian vine plants. It can also be artificially synthesized. The final product is a white, crystalline powder that is vaporized and smoked in a pipe or bong. Causing powerful hallucinations that last for around 15 minutes, some researchers believe that the brain produces DMT when we dream, but the exact role that DMT plays in the body has evaded scientists for years, so the verdict is still out on that one.
Chemical formula of psilocybin on top of mushroom photo
Psilocybin is a naturally occuring psychedelic drug derived from over 200 species of mushrooms and fungi

Drugs that are considered hallucinogens are usually non-addicting. However, they can potentially damage the brain and can be toxic at certain levels. Therefore, they’re still either banned or strictly controlled for medical use.


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