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.