As far as I understand, there is a compelling sense of reality to sensory hallucinations. Do hallucinations (as, e.g. in the DSM) substitute for the sense and experience of a real reality?

For example, if a "hallucination" involves someone's face to be covered in maggots, would that entail a completely life-like, 'opaque', and entirely believable experience of that perception?

  • $\begingroup$ if so, is this not a_lot stranger than delusional belief? $\endgroup$ – user3293056 Sep 12 '15 at 23:12

Elliot et al. (2008) define a hallucination as:

A sensory experience which occurs in the absence of corresponding external stimulation of the relevant sensory organ, has sufficient sense of reality resemble a veridical perception (i.e. the perception seems to be "real"), over which the subject does not feel direct and voluntary control, and which occurs in the awake state.

They define illusions as:

These are false perceptions of a real external stimulus, for example a change in shape, size, color or texture. In some cases, where the external stimulus is minimal, the differentiation nosologically (nosology: the classification of diseases) from hallucination can be difficult, although illusions carry different aetiological and diagnostic implications.

and delusions as:

[A]bnormalities of thought rather than perception (although they may develop from the latter) and may be defined as ‘fixed false beliefs, strongly held and immutable in the face of refuting evidence, that are not consonant with the person’s education, social and cultural background.' [I]ts exact meaning and usage have evolved continuously, reflecting trends in psychology. Delusional themes commonly include: guilt, worthlessness, ill-health, persecution, reference, grandeur, love, jealousy, poverty, infestation, and religion. [...]

Regarding the difference between a hallucination and an illusion, which are the most closely related ones among the three, the Stanford Encyclopedia gives a clear-cut example, namely:

For example, when one has a visual experience as of a red object, it may be that one is really seeing an object and its red color (veridical perception), that one is seeing a green object (illusion), or that one is not seeing an object at all (hallucination).

- Elliot et al. Epilepsy Research (2009); 85: 162-71

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  • $\begingroup$ so what is the difference between a hallucination and something imagined with a compelling sense of reality, that seems to be real in that sense? $\endgroup$ – user3293056 Jul 8 '18 at 10:27
  • $\begingroup$ @user3293056 I would believe the difference lies in the fact that when one is hallucinating, they are unaware that they are experiencing perception in the absence of a stimulus, i.e., they believe an external sensory stimulus to be present over which they do not have voluntary control, as quoted by AliceD. However, when one is imagining something, there seems to be no doubt by the person imagining it that it's "all in my head", i.e., awareness of lack of external stimulus and voluntary control. $\endgroup$ – EMMs2008 Jul 13 '18 at 14:18
  • $\begingroup$ ok. i think in that case then thought alienation is a hallucination, interesting. still not sure if voices have to be auditory, however. i guess my question in the OP asks if hallucinated voices always have to drown out other sounds. i very much doubt they do. cheers $\endgroup$ – user3293056 Jul 14 '18 at 14:29

I really recommend Hallucinations by Oliver Sacks.

In this book, Oliver Sacks defines hallucination as:

percepts arising in the absence of any external reality--seeing things or hearing things that are not there.

In other words, a hallucination mimics perception.

Implanted-electrode studies

Studies have shown that if the region of the brain involved in the perception of faces is abnormally activated, individuals may hallucinate faces.

Similarly, if the region of the brain employed in reading is activated, it may give rise to hallucinations of letters.

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  • $\begingroup$ what about synesthesia? $\endgroup$ – user3293056 Jul 10 '18 at 16:32
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    $\begingroup$ @user3293056 synesthesia may be the result of cross wiring between the brain area responsible for color and the brain area responsible for numbers; these two areas are next to each other. Hence, when you see a number you see a corresponding color. This fresh perspective was given by VS Ramachandran; check out his fascinating ted talk: youtube.com/… $\endgroup$ – user19721 Jul 10 '18 at 23:26

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