In the intricate dance between perception and reality, our brains continuously engage in a process of hypothesis testing—weighing sensory input against prior expectations to construct our conscious experience. When this delicate balance shifts too far toward prior beliefs operating at unconscious levels, hallucinations can emerge. This report examines how subpersonal priors—automatic expectation mechanisms operating below conscious awareness—can generate perceptual experiences detached from objective reality.
The Predictive Coding Framework and Strong Priors
The predictive coding theory of perception proposes that the brain actively generates predictions about sensory inputs rather than passively receiving information. Within this framework, hallucinations can be understood as false positive inferences that occur when prior beliefs exert excessive influence over perceptual processes. Recent empirical research demonstrates that strong, overly precise priors can produce hallucinations even in healthy individuals, with hallucination-prone people showing increased susceptibility to these laboratory-induced phenomena15.
The balance between prior beliefs and sensory evidence forms a critical fulcrum for perceptual stability. When this equilibrium tilts excessively toward priors—particularly at the subpersonal, automatic level of neural processing—perception becomes dominated by expectations rather than actual sensory input. This imbalance can manifest as perceiving something that isn’t physically present, which defines a hallucination78.
Empirical Evidence for Prior Overweighting
Multiple independent laboratories have documented the relationship between prior overweighting and hallucination susceptibility. Controlled studies have shown that hallucination-prone individuals exhibit stronger employment of both global (gist) and local (detail) priors during perceptual tasks3. This suggests these individuals rely more heavily on their pre-existing beliefs or expectations when interpreting ambiguous sensory information.
In one particularly illuminating experimental paradigm called the “Conditioned Hallucinations” task, researchers found that participants who experience hallucinations in daily life were more likely to report hearing sounds that weren’t presented during the experiment10. These findings were consistent across both clinical and non-clinical populations, indicating that the overweighting of perceptual priors relative to sensory evidence represents a transdiagnostic mechanism underlying hallucinatory experiences7.
Active Inference and the Generation of False Percepts
The active inference model provides a computational framework for understanding how subpersonal priors influence perception and potentially lead to hallucinations. In this model, perception operates as a process of hypothesis testing, where sensory data help disambiguate between alternative explanations about the world2. Crucially, this inferential process depends on maintaining an appropriate balance between prior beliefs about hidden variables and the sensations they cause.
When applied to auditory verbal hallucinations (AVH), this model suggests that a false inference that a voice is present, despite the absence of corresponding auditory input, indicates the domination of prior beliefs over perceptual inference7. Computer simulations based on this framework demonstrate that hallucinatory percepts can emerge when an agent expects to hear a voice in the presence of imprecise sensory data2.
The Precision Weighting Mechanism
A key mechanism in the relationship between subpersonal priors and hallucinations involves precision weighting—the brain’s assignment of confidence levels to both predictions and prediction errors. Precision can be conceptualized as the inverse of uncertainty; highly precise signals are weighted more heavily in perceptual inference12.
Hallucinations may result from either overly precise prior beliefs or reduced precision of sensory evidence that contradicts expectations. As one study explains, “a down-weighting of the precision of sensations (i.e., silence) that contradict the expected percept (i.e., a voice)” can lead to false perceptions7. This precision imbalance causes the brain to favor its predictions over contradictory sensory information, potentially resulting in the perception of stimuli that aren’t objectively present.
Different Types of Hallucinations Based on Belief Structures
Research distinguishes between different types of hallucinations based on the nature of the underlying belief disturbances. “In-context hallucinations” occur when individuals cannot use sensory information to correct prior beliefs about hearing a voice, but their beliefs about content (such as the sequential order of a sentence) remain accurate8. In contrast, “out-of-context hallucinations” emerge when hallucinating subjects also have inaccurate beliefs about state transitions, leading to disordered hallucinated content reminiscent of the bizarre hallucinations sometimes observed in conditions like schizophrenia8.
This distinction helps explain the spectrum of hallucinatory experiences—from those that seem plausible given the context to more bizarre manifestations disconnected from environmental contingencies. The computational mechanisms underlying these different manifestations involve varying degrees of precision imbalance at different levels of the perceptual hierarchy38.
Clinical Implications and State-Sensitivity
Understanding hallucinations through the lens of subpersonal priors carries significant clinical implications. Research has shown that the relationship between prior overweighting and hallucination propensity is not merely a static trait but rather a state-sensitive marker that can fluctuate with symptom severity10. This dynamic relationship suggests that measuring changes in perceptual prior weighting could potentially serve as a biomarker for tracking hallucination susceptibility or treatment response.
Interestingly, studies have found that patients with psychosis who do not experience hallucinations do not show the same pattern of prior overweighting4, indicating specificity of this abnormality to hallucinations rather than psychotic illness more broadly. This specificity further supports the centrality of subpersonal prior mechanisms in hallucination formation.
Conclusion
The relationship between subpersonal priors and hallucinations represents a compelling example of how automatic brain processes operating below conscious awareness can profoundly influence our perceptual experience. When these priors become too strong or precise relative to sensory evidence, they can generate percepts detached from physical reality—hallucinations.
This predictive coding account of hallucinations offers a unifying framework that spans from normal perception to pathological states, emphasizing the continuum of perceptual experiences rather than categorical distinctions. It encourages a more empathic approach to clinical hallucinations by recognizing them as extreme manifestations of normal perceptual mechanisms rather than entirely alien phenomena5.
As research in this area continues to advance, improved understanding of the computational and neural mechanisms underlying hallucinations may lead to novel interventions targeting the precision balance between prior beliefs and sensory evidence, potentially offering new avenues for treating distressing hallucinations across various clinical conditions.