What is Hallucination? Why Does It Happen?

A hallucination is a perception that occurs without external stimuli and feels real. It is the inability to distinguish between real events and those produced by the imagination. It is one of the features used in the diagnosis of schizophrenia, seen in 60-70% of this disease. The most common form is auditory hallucination.

Who Gets Hallucinations?

Psychiatric disorders, drug side effects, epilepsy, alcoholism, anesthesia, substance abuse, prolonged sleeplessness, brain cancer , terminal diseases such as kidney and liver failure, AIDS, high fever, migraine headaches, social isolation, deafness, blindness, vision problems Many diseases and problems such as stress, grief, depression and dementia ( Alzheimer’s disease, etc.) can trigger hallucinations.

Does It Occur in Healthy People?

Hallucinations can also occur in normal people. The frequency of hallucinations in children and adolescents has been reported as 7-30% in various studies. It has been determined that after the death of one of the spouses, during the mourning period, the other spouse experiences the hallucination of his/her lost spouse at a rate of one-third to one-two. Culture can also be effective in accepting perceived things as imaginary or real. In healthy persons, pseudohallucinations can be produced even with mild sensory deprivation. The neurobiology of hallucinations has been most studied in patients with schizophrenia, but studies in healthy individuals may also be informative. However, we are not sure that hallucinations occur by the same mechanisms in sick and healthy individuals.

Among the diverse and complex symptoms seen in psychotic disorders, hallucinations appear to be a relatively distinct and well-defined complaint. It may therefore be a good target for understanding the associated brain mechanisms. In people who are psychologically normal but have hallucinations due to brain damage, the lesion is usually detected in the brain regions associated with the hallucinated sensation (sight, hearing, touch, etc.). For example, the complex visual hallucinations seen in Charles Bonnet syndrome are usually caused by macular degeneration or lesions in the neural pathways between the visual cortex and the eye.

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Auditory Hallucinations in Schizophrenia

Neuroimaging methods are often used to understand the brain regions involved in producing hallucinations. Numerous studies have been conducted in schizophrenic patients with auditory hallucinations. Brain regions that are activated in auditory hallucinations are the areas responsible for normal hearing, language perception and production. The primary auditory cortex, on the posterior surface of the superior temporal gyrus, senses the pure tone of sound. The secondary auditory attachment areas surrounding it distinguish the phonetic features of speech. In the middle temporal gyrus, there are parts that are responsible for understanding speech. Of the two main language centers, Wernicke’s area is in the posterior temporal and parietal cortex, relating word meanings to objects and concepts. Broca’s area is in the inferior frontal gyrus and takes part in sentence production. The anterior cingulate cortex, which is the region responsible for attention and care, is effective in executing the behaviors required for speech. Dorsolateral prefrontal cortex may provide conscious and unconscious sense of auditory awareness.

In schizophrenic patients with auditory hallucinations, a decrease in gray matter volume has been detected in the superior temporal gyrus, including the primary auditory cortex. Another study reported volume reduction in the dorsolateral prefrontal cortex. Disruption of frontotemporal interaction can cause involuntary hallucination experience. The paracingulate sulcus in the medial frontal cortex may be important in distinguishing between real and imaginary inputs. Functional activation studies with actively hallucinating participants showed increased activity in the language areas and primary auditory cortex.

Distinguishing Internal and External Warnings

One of the theories regarding the origin of hallucinations is that one’s inner voice or sensory stimuli seem to come from an external source. Blakemore et al. suggested that the inability to tickle oneself can shed light on this phenomenon. The tactile and other stimuli produced by the person may be weakened by external stimuli. Because one can predict the results of the senses produced by himself. Healthy subjects are more insensitive to self-tickling, while those with auditory hallucinations or passivity phenomena (loss of sense of separation between self and others) are unable to distinguish between self-generated and extrinsic stimuli. This finding may be due to a lack of connectivity between the brain areas that produce movement and perceive sensation. One of the things needed to distinguish between internal and external sensations is to place the sensory stimulus in the right place in space and time. It has been shown that patients with schizophrenia have difficulty in evaluating time intervals. This situation may be related to the disorder in the organization of information according to time. Knowing the neurobiology of self-distinction and timing can improve our understanding of hallucinations.

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Hypnosis Experiments

An interesting way used in hallucination research is to induce hallucinations in healthy individuals with hypnosis. In hypnotizable individuals, the right anterior cingulate cortex is activated when they hear real external speech and are asked to hallucinate it under hypnosis. However, it is not activated when they dream of speaking. Therefore, the anterior cingulate cortex may play a role in attributing speech to an external source. However, the anterior cingulate cortex was not activated when people who could not hallucinate under hypnosis heard external speech in the experiment. This indicates the complexity of interpreting the model. Perhaps the anterior cingulate cortex is a region associated with susceptibility to hypnotic suggestion.

People awakened during REM sleep may experience hallucinations and delusions. This feature has also been used in the planning of various studies.

The Role of Gamma Oscillations

One of the theories explaining the mechanism of hallucinations deals with gamma oscillations (vibrations) in thalamocortical networks . Accordingly, perceptual experience results from the synchronization of gamma oscillations. Gamma vibrations are limited by sensory inputs and prefrontal and limbic attention mechanisms. There is evidence that there is impaired regulation of thalamocortical gamma activity by external sensory stimuli in schizophrenia. In the absence of external stimuli, attentional mechanisms may predominate and cause hallucinations. Stress, hyperarousal and neurochemical changes in schizophrenia may predispose to this imbalance.

Direction of Research

Data from neuroimaging studies may reveal brain regions that should be targeted in repetitive transcranial magnetic stimulation used for the treatment of hallucinations. Studies in humans can only show a correlation (correlation) between hallucinations and neural processes, but generally cannot prove a cause-effect relationship. Animal models are used for this. On the other hand, trying to understand that animals are hallucinating from their behavior is difficult and may not be reliable enough. As a result, it can be said that we do not know enough about why and how hallucinations occur, but studies are continuing increasingly.

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