So, the way I've understood it, "sensory disturbances" can be categorized as follows:

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Any sensory experience that isn't real goes under "sensory disturbances" in this diagram. If this unreal sensory experience is a distortion or misunderstanding of real, external stimuli, then it falls under "illusions". If the unreal sensory experience is generated in the absence of external stimuli, it is a hallucination. If the person having the hallucination believes in it, it is a true hallucination. If not, then it is just a hallucination.

So, if a person perceives an object to be a different color, whether it is due to an altered state of consciousness, or if it is due confusing circumstances, is this an illusion or a hallucination? The object is real, it does have a color (which in a sense isn't real and potentially experienced differently by different people), and the person sees the color differently than what they normally would. Is this external stimuli being distorted/misinterpreted, or is it a hallucination being superimposed upon or merged with an external piece of stimuli. What if the subject doesn't see a different color, but rather a different shade of the same color?

If a taste is changed from e.g. sweet to salty is it an illusion or a hallucination? If an object is perceived to move when it is not moving, is it an illusion or a hallucination.

At the heart of these questions really is the question: does in the absence of external stimuli necessitate that the hallucination not involve external stimuli, or not depend on it? If the object is falsely perceived to be moving, it involves it (and in a sense depends on it too), however, it isn't the nature of the object that causes the sensory disturbance, but rather an internally created disturbance (the perception of motion) instilled into the object. But since that percept of motion, although originating from within, modified an external stimuli in order to give the final experience, is that experience then an illusion due to it coming in the form of distorted, external stimuli?

Basically, I'm wondering about the nuanced border between illusions and hallucinations, if there even is one. If there isn't one, and there's a large grey area between them, then how does one differentiate between the two different categories proposed for the class of drugs that are hallucinogens? The broadest definition includes all drugs that sensory disturbances, and the most narrow definition is limited to those that produce hallucinations. How can that discussion be had in the event that there is a large gray area between illusions and hallucinations?


1 Answer 1


The differences between illusion and hallucinations are quite clear and have been known for over 100 years.

For example, hallucinations are when you see things in total darkness or hear things when there is no sound. It is perception WITHOUT an external stimuli.

Regarding illusions, there IS something to be seen, heard, tasted or felt, an external stimulis, but the impression received is misinterpreted.

Therefore, when you ask about the distortion of taste, that is in response to an external stimuli (food, drink etc.) and it is being misinterpreted. A more intense flavor, or changed flavor (salty to sweet) are still misinterpretations of an external stimuli.

What matters here is not how the taste is changed, but the fact that it HAS changed.

Reference - https://doi.org/10.1176/ajp.58.3.443

  • $\begingroup$ @ChrisRogers Hi Chris. I didn't make any claims at all in fact, I simply used the OP's own provided explanations to the sensory disturbances diagram. Perhaps it is the OP who should provide references to the claims he makes as a basis for his questions. $\endgroup$
    – NetServOps
    Jun 20, 2021 at 8:14
  • $\begingroup$ I figured that 1. and 2. where categorized as illusions, I just included them to see if my base understanding was correct. You didn't really say if my base understanding was correct, but rather pointed to the fact that according to my base understanding, 1. and 2. are illusions. 3. however, is a bit more tricky. It does not follow from my explanation of the categories that 3. is an illusion. It depends on the semantics of the categorizer. If a taste is enhanced, is it the same taste, only with a different intensity? Most would say so. If a taste goes from sweet to salt? [continued] $\endgroup$
    – A. Kvåle
    Jun 20, 2021 at 13:48
  • $\begingroup$ [continued] Is it the same taste when it has gone from sweet to salt? A lot of people would say no. Well if its not the same taste, can one then say it is a distortion of a taste, or the generation of a new taste? Was a taste distorted to the point of the subject experiencing a new taste (illusion), or, was a taste ignored, and a new, hallucinated taste replaced it in the mind of the subject (hallucination). As I elaborate these questions, you see that they came from an uncertainty of the mechanisms behind certain sensory disturbances as well as the semantics underpinning the categorization. $\endgroup$
    – A. Kvåle
    Jun 20, 2021 at 13:53
  • $\begingroup$ @A.Kvåle I don't think 3. is tricky at all. If an illusion is a distortion or misunderstanding of external stimuli, then food on your tongue which then triggers your taste buds IS external stimuli. Your brain then interprets the taste as something other than what it really is, therefore, its an illusion. $\endgroup$
    – NetServOps
    Jun 20, 2021 at 23:38
  • $\begingroup$ Isn't that a somewhat arbitrary distinction? The signals that travel towards you brain carry the information about the experience, which the circuitry is "designed" to interpret in a specific, correct way. If that circuitry is altered, or if the signal is of an illusory nature, then the circuitry isn't able to interpret it the way it's supposed to. This is an illusion, and is obviously distinct from when the signal originates from within the brain, causing the experience of something unreal (hallucination). However, what if the signal is somehow altered by the brain: [continued] $\endgroup$
    – A. Kvåle
    Jun 22, 2021 at 0:07

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