An external hallucination is the perception of a visual hallucination that displays itself seamlessly into the external environment as if it were physically present.   This is in stark contrast to internal hallucinations, such as dreams, that occur exclusively within an imagined environment and can typically only be viewed with closed eyes.
This effect is capable of manifesting itself across the 4 different levels of intensity described below:
Vaguely defined hallucinations
Partially defined hallucinations
Fully defined hallucinations
Alongside a specific levelling system, there are also environmental factors that directly alter both the likelihood of external hallucinations manifesting themselves and the level of detail they are rendered at. For example, the more unfamiliar with the external environment a person is, the more likely it is that this effect will manifest itself. Cluttered areas tend to produce more external hallucinations. The amount of light within a room is inversely proportional to the intensity of them, with less light leading to significantly more hallucinations and more light leading to fewer, although more detailed, hallucinations.
The content within these external hallucinations can be further broken down into four distinct subcomponents. These are described and documented within their own dedicated articles, each of which are listed below:
- Autonomous entities
- Object activation
- Perspective hallucinations
- Scenarios and plots
- Settings, sceneries, and landscapes
- Shadow people
It is worth noting that the content, style, and general behaviour of an external hallucination is often largely dependent on the emotional state of the person experiencing it. For example, a person who is emotionally stable and generally happy will usually be more prone to experiencing neutral, interesting, or positive hallucinations. In contrast, a person who is emotionally unstable and generally unhappy will usually be more prone to experiencing sinister, fear-inducing, and negative hallucinations.
External hallucinations are often accompanied by other coinciding effects such as delirium, internal hallucinations and delusions. They are most commonly induced under the influence of heavy dosages of deliriant compounds, such as DPH, datura, and benzydamine. However, they can also occur less commonly under the influence of psychedelics, dissociatives, stimulant psychosis, and sleep deprivation.
The specific differences between each potential style of external hallucination can be broken down into the following variations:
Lucid vs Delirious
A person experiencing a hallucinatory state can maintain a consistent level of awareness regarding the fact that none of these events are actually occurring and that the effects are simply the result of a drug-induced hallucination. In contrast with this, hallucinations may also become completely believable, no matter how nonsensical they may be. This is exactly the same way that people do not have any problem accepting absurd and non-linear plots within their dreams.
Fixed vs Interactive
Hallucinations can present themselves in a manner that prevents one from touching or conversing with the hallucination in any way. For example, a person’s hand may pass directly through a hallucinated object and cause it to disappear, or a hallucinated person may ignore any attempts at conversation. In contrast, certain hallucinations may present themselves in a manner that a person could potentially interact with through touch or speech.
New experiences vs Old experiences
In terms of their subject matter, hallucinations can either be entirely new experiences or they can be old, everyday experiences in the form of replayed memories.
Controllable vs Autonomous
Hallucinations can be partially to completely controllable or seemingly random in nature. When controllable, this can be described as their content always seeming to perfectly follow and fit the general subject matter of one’s current thought stream, with varying levels of control. In contrast, autonomous hallucinations are completely spontaneous in their subject matter and entirely uncontrollable.
Geometry-based vs Solid
Hallucinations can be comprised of condensed geometry or they can appear to be made from realistic and/or lifelike materials. This is usually dependent upon the type of substance consumed.
-  Gauntlett-Gilbert, J., & Kuipers, E. (2003). Phenomenology of visual hallucinations in psychiatric conditions. The Journal of nervous and mental disease, 191(3), 203-205. | https://doi.org/10.1097/01.NMD.0000055084.01402.02
- Obreshkova, D., Kandilarov, I., Angelova, V. T., Iliev, Y., Atanasov, P., & Fotev, P. S. (2017). Pharmaco-toxicological aspects and analysis of phenylalkylamine and indolylalkylamine hallucinogens. Pharmacia, 64(1), 32-47. | http://bsphs.org/wp-content/uploads/2017/04/Angelova.pdf