Hallucinations: Possible Mechanisms

Scientists investigate hallucinations as changes in the processes that construct perception. The brain combines sensory input with expectations, memories and attention. One hypothesis proposes that expectations receive too much weight when external information is uncertain. Another concerns source monitoring: difficulty identifying whether an experience originated internally or from the outside world. These explanations may overlap rather than compete.

Hallucinations are sensory experiences that occur without a corresponding external stimulus. They can affect any sense and vary in intensity, frequency and emotional meaning. They occur in several psychiatric, neurological, medical and sleep-related circumstances, so their presence alone does not establish a diagnosis of schizophrenia.

For voice-hearing, studies also examine language networks, auditory processing and internally generated speech. Dopamine, glutamate and other neurotransmitters may influence the wider circuits involved, but no single chemical abnormality explains every hallucination. Findings from groups do not identify the exact mechanism in one person. Different causes, including sleep transitions and neurological disease, can produce superficially similar experiences through different pathways. Research models are therefore tools for investigating perception, not proof that a person’s account is fabricated. Understanding mechanisms may improve treatment, especially when medication alone does not adequately reduce distress or disruption.

Predictive-processing models describe perception as an interaction between incoming information and expectations. Source-monitoring models focus on recognising where experiences originate. Both can contribute useful hypotheses, and different hallucinations may involve different processes. Their value lies in generating testable explanations rather than providing a definitive account that can be read directly from one person’s description of an unusual experience.

Further reading: National Institute of Mental Health (1); National Institute of Mental Health (2).