A hallucination is defined, in psychiatry, as a sensory perception without the presence of a detectable stimulus: for example seeing objects physically absent, or hearing voices without anyone speaking.
Psychosensory hallucinations can affect all of the senses, such as sight, hearing, smell, taste and superficially touch.
Psychic hallucinations concern only thought: ideas, feelings, representations … imposing themselves on the mind.
The classification of hallucinations varies between authors, and different types of hallucinations may overlap. The hallucinatory phenomenon is heterogeneous, linked to very diverse causes, the limits and mechanism of which remain debated.
Originally, “so-called psychic hallucinations” concerned only thought, in the verbal domain, without sensory modalities, then they were extended to complex psychosensory forms.
For example, they are internal voices (heard in the head and not by the ear) transmitted from the outside as by telepathy. The patient receives by thought information, ideas, music, reveries, memories, representations … which are imposed on him.
We speak of the disappropriation of the interior language, where the subject no longer has the voluntary control of his interior life. The patient experiences this phenomenon as the intrusion of another in his consciousness, what he thinks or feels no longer belongs to him.
In influence syndrome, the subject has the feeling of being possessed (imposed thoughts), and of being directed from the outside (imposed behavior).
In the mental automatism syndrome, psychosensory hallucinations, psychic and influence syndrome are grouped together, with a double phenomenon of possession (parasitism) and mechanization of the psychic life of the subject.
(The hallucinations of Don Quixote. )
Various theories have been proposed, but the mechanism of hallucination is not known with certainty. Neuropsychological theories make the link between hallucinatory activity and known mechanisms of brain function. Psychoanalytic theories make the link with the content of the dream and an infantile regression.
The heterogeneous nature of the hallucinatory phenomenon makes it difficult to provide a unique global explanatory model.
The neurochemical approach is based on the fact that toxic substances and chemicals are capable of producing hallucinations (hallucinogenic substances). Such substances are serotonin agonists, others dopamine, etc. From there, the hallucinations would be linked to disorders of brain neurotransmission.
The neurological approach (clinical, imaging, neurosurgery, etc.) establishes links between the type of hallucination and cerebral structures, such as sensory areas or those involved in dream states. The proposed models bring in disafferentation phenomena; direct excitation of areas of the cerebral cortex (epilepsies), or dissociation of the dream mechanism (cerebral lesion, narcolepsy, etc.).
The cognitive approach also offers several models, including the failure of a self-monitoring function of the distinction between self and the outside, both at the level of perception and at the level of attribution. The subject is no longer able to perceive and judge that an internal statement comes from him: the origin is attributed to the external space, outside of him. Even in the case where it is experienced within, the phenomenon is experienced as external and foreign to itself.
Freud brings hallucinations closer to dreams, as corresponding to a return of the repressed to the conscious level. There would be a regression at an early infantile stage, that which precedes the distinction between representations and perceptions. Hallucination is then a projection of the self into the exterior.
Freud is also at the origin of the problematic of “negative hallucination”, that is to say the fact of not perceiving an existing object, whatever its position, its intensity … There would be a positive hallucination or perception without an object, and a negative hallucination or object without perception, present in the environment but literally “erased” in the consciousness of the subject. This concept remains debated, but Freud made it a key to understanding positive hallucinations.