Hallucinations – What are Hallucinations? The Neurobiology of Hallucinations
Time to read: 3 minutes A sensory perception that has the compelling sense of reality of a true perception but that occurs without external stimulation of the relevant sensory organ. Hallucinations should be distinguished from illusions, in which an actual external stimulus is mis-perceived or misinterpreted. The person may or may not have insight into the fact that he or she is having a hallucination.
Key characteristics of Hallucinations
- unwilled – not subject to conscious manipulation
- has the same qualities as real perception, i.e. vivid, solid
- perceived as being located in the external world
- ‘perceptions which arise in the absence of any external stimulus’ (Esquirol, 1833)
- ‘a false perception which is not in any way a distortion of a real perception but which springs up alongside it’ (Jaspers)
A recent study looked at the points of similarities and differences across diagnostic classes. They compared the features of hallucinations (auditory and visual) between two or more population groups, one of which is schizophrenia. The study found that 95% of the features of hallucinations in schizophrenia are shared with other psychiatric disorders. 85% with medical/neurological conditions, 66% with drugs and alcohol conditions, and 52% with the nonclinical groups. [Waters et al, 2017]
Key messages:
- No single hallucination feature or characteristic uniquely indicated a diagnosis of schizophrenia, with the sole exception of an age of onset in late adolescence.
- Hallucinations occur with significant frequency in other psychiatric and medical conditions.
- It is inadvisable to give weight to the presence of any featural properties alone in making a schizophrenia diagnosis.
- It is more important to focus instead on the co-occurrence of other symptoms and the value of hallucinations as an indicator of vulnerability.
NEUROBIOLOGY OF HALLUCINATIONS
“The most consistent finding of structural neuroimaging studies of patients with auditory hallucinations is reduced grey matter volume in the superior temporal gyrus, including the primary auditory cortex. One fairly large study also reported volume reduction in the dorsolateral prefrontal cortex, suggesting that faulty frontotemporal interactions may contribute to the experience of hallucinations being involuntary.” (Allen)
Predictive processing models have become the dominant approach to explain hallucinations in Schizophrenia spectrum disorders (SSD) , with self-monitoring theories emphasising the contribution of failures of the brain’s systems for making predictions, and Bayesian approaches arguing that patients with SSD have a prediction error deficit. [Hare, 2021].
A novel hypothesis is put forward that a functional brain network called the “salience network” might play a critical role in selecting and gating sensory representations for higher conscious processing, and that this process may be abnormally selected for conscious processing in individuals with SSD that experience hallucinations. [Hare, 2021].
The Global Workspace (GW) theory may be useful in understanding these phenomena, with a focus on the role of the cortico-thalamic-reticular activating system, lateral fronto-parietal networks, and ignition in conscious perception. [Hare, 2021].
Major neurotransmitters and targets involved are: [Tang & Tang , 2020].
- 5-HT2A
- Dopamine receptors
- GABA receptors
- NMDA receptors
- Cannabinoid receptors
- Opioid receptors
- Cholinergic pathways
RECOMMENDED BOOKS
References
Allen P, Laroi F, McGuire PK, et al. The hallucinating brain: a review of structural and functional neuroimaging studies of hallucinations. Neurosci Biobehav Rev. 2008;32:175–91
