At the lowest level, external hallucinations generally consist of movement within the peripheral vision and ill-defined, fleeting hallucinations that disappear once a person double takes.
Vaguely defined hallucinations
At this level, the hallucinations are visible within one’s direct line of sight, but are not fully defined in their appearance. This means that, although visible, they do not look completely detailed and are often extremely blurry or semi-translucent with little to no colour.
Partially defined hallucinations
At this level, the hallucinations become distinct enough in their detail and vividness to extend beyond transparent, colourless, or blurry manifestations. However, they still remain unconvincing and do not quite live up to the detail of everyday life.
Fully defined hallucinations
At this level, the hallucinations have become completely realistic and will rarely disappear due to a double take. They are now capable of a completely convincing and photorealistic appearance and their behaviour becomes far more lifelike. Additionally, they may become numerous enough to fully engulf the entirety of the person’s environment.
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 psychedelic 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