Hallucinations and Strong Priors
Authors: Philip R. Corlett, Guillermo Horga, Paul C. Fletcher, Ben Alderson-Day, Katharina Schmack, Albert R. Powers
Journal: Trends in Cognitive Sciences (2018)

Recent data establish a role for strong prior beliefs in the genesis of hallucinations. These data are difficult to reconcile with aberrant inner-speech theories, in which 'weaker' predictions about the potential consequences of one's own inner speech drive an inference that speech is emanating from an agent external to oneself. In the predictive-coding view, this failure of self-prediction renders the consequences of one's inner speech surprising. The prediction errors induced are explained away by the strong higher-level priors identified in recent work. The presence and contents of hallucinations can be understood in terms of learning, inference, and a reliability-based trade-off between internal and external information sources, biased toward high-level priors. If priors divorce perception from sensation somewhat, the distinction between hallucination and perception becomes less clear. We hope this explanation renders hallucinations more understandable and less stigmatizing. Hallucinations, perceptions in the absence of objectively identifiable stimuli, illustrate the constructive nature of perception. Here, we highlight the role of prior beliefs as a critical elicitor of hallucinations. Recent empirical work from independent laboratories shows strong, overly precise priors can engender hallucinations in healthy subjects and that individuals who hallucinate in the real world are more susceptible to these laboratory phenomena. We consider these observations in light of work demonstrating apparently weak, or imprecise, priors in psychosis. Appreciating the interactions within and between hierarchies of inference can reconcile this apparent disconnect. Data from neural networks, human behavior, and neuroimaging support this contention. This work underlines the continuum from normal to aberrant perception, encouraging a more empathic approach to clinical hallucinations. Hallucinations, perceptions in the absence of objectively identifiable stimuli, illustrate the constructive nature of perception. Here, we highlight the role of prior beliefs as a critical elicitor of hallucinations. Recent empirical work from independent laboratories shows strong, overly precise priors can engender hallucinations in healthy subjects and that individuals who hallucinate in the real world are more susceptible to these laboratory phenomena. We consider these observations in light of work demonstrating apparently weak, or imprecise, priors in psychosis. Appreciating the interactions within and between hierarchies of inference can reconcile this apparent disconnect. Data from neural networks, human behavior, and neuroimaging support this contention. This work underlines the continuum from normal to aberrant perception, encouraging a more empathic approach to clinical hallucinations. 'Instead of saying that an hallucination is a false exterior percept, one should say that the external percept is a true hallucination.' [1Taine H. De l'intelligence. Librairie Hachette et Cie, 1870Google Scholar] Hallucinations (see Glossary) are percepts without corresponding external stimuli [2Tracy D.K. Shergill S.S. Mechanisms underlying auditory hallucinations-understanding perception without stimulus.Brain Sci. 2013; 3: 642-669Crossref PubMed Google Scholar]. They characterize many serious mental illnesses such as schizophrenia and post-traumatic stress disorder [3McCarthy-Jones S. Longden E. Auditory verbal hallucinations in schizophrenia and post-traumatic stress disorder: common phenomenology, common cause, common interventions?.Front. Psychol. 2015; 6: 1071Crossref PubMed Google Scholar]. 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AVHs occur in around 80% of patients diagnosed with schizophrenia, but they present us with profound clinical problems: since they are so common, their status as diagnostic markers for mental illness is very uncertain [12Powers 3rd, A.R. et al.Varieties of voice-hearing: psychics and the psychosis continuum.Schizophr. Bull. 2017; 43: 84-98Crossref PubMed Scopus (19) Google Scholar]. Moreover, when treatment is indicated, they can, in a significant proportion of people, prove persistent [13Shergill S.S. et al.Auditory hallucinations: a review of psychological treatments.Schizophr. Res. 1998; 32: 137-150Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar]. Thus, there is a real need to understand AVHs. We review work using formal computational models of perception to understand the basis of AVHs [14Halligan P.W. David A.S. Cognitive neuropsychiatry: towards a scientific psychopathology.Nat. Rev. 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If priors are more precise than sensory inputs, we consider them strong. They will dominate inference and prediction errors will be ignored [17Feldman H. Friston K.J. Attention, uncertainty, and free-energy.Front. Hum. Neurosci. 2010; 4: 215Crossref PubMed Google Scholar, 18Friston K. Kiebel S. Predictive coding under the free-energy principle.Philos. Trans. R. Soc. Lond. B Biol. Sci. 2009; 364: 1211-1221Crossref PubMed Scopus (341) Google Scholar, 19Friston K.J. Stephan K.E. Free-energy and the brain.Synthese. 2007; 159: 417-458Crossref PubMed Scopus (194) Google Scholar, 20Teufel C. et al.The role of priors in Bayesian models of perception.Front. Comput. Neurosci. 2013; 7: 25Crossref PubMed Scopus (0) Google Scholar]. By contrast, relatively more precise prediction errors will dominate priors and drive belief updating (changing one's priors for subsequent inference). The powerful contribution of expectation to perception leads us to speculate that hallucinations might arise when prior predictions exert an inordinate influence over perceptual inferences, creating percepts with no corresponding stimuli at all [11Powers III, A.R. et al.Hallucinations as top-down effects on perception.Biol. Psychiatry Cogn. Neurosci. Neuroimaging. 2016; 1: 393-400Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar, 15Friston K.J. Hallucinations and perceptual inference.Behav. Brain Sci. 2005; 28: 764-766Crossref Scopus (51) Google Scholar]. That prior experiences could engender hallucinations in healthy volunteers was first reported by Carl Seashore, working at the Yale Psychological Laboratory in 1895. Seashore reported hallucinations in one modality (touch, sight, taste) conditional on learned cues in another sensory modality and engendered hallucinations by mere suggestion [21Seashore, C.E. (1895) Measurements of Illusions and Hallucinations in normal life. Reprinted from Studies from the Yale Psychological Laboratory, Vol. 3Google Scholar]. A more recent example of hallucination by suggestion: when healthy volunteers were instructed they would hear the song White Christmas, but instead they were played white noise, 5% reported experiencing Bing Crosby's voice [22Barber T.X. Calverley D.S. An experimental study of hypnotic (auditory and visual) hallucinations.J. Abnorm. Psychol. 1964; 68: 13-20Crossref PubMed Scopus (0) Google Scholar]. People who hear hallucinatory voices in their everyday lives are more susceptible to this effect [23Mintz S. Alpert M. Imagery vividness, reality testing, and schizophrenic hallucinations.J. Abnorm. Psychol. 1972; 79: 310-316Crossref PubMed Scopus (119) Google Scholar]. Pavlov's assertion that such inferences were similar to the conditioned reflex [24Pavlov I.P. Natural science and the brain.Lectures on Conditioned Reflexes: Twenty-five Years of Objective Study of the Higher Nervous Activity (Behaviour) of Animals. Liverwright Publishing Corporation, 1928Crossref Google Scholar] has garnered empirical support. Ellson in the 1940s embraced the conditioning paradigm and used more controlled procedures for consistent stimulus delivery. The illumination of a tungsten filament bulb was presented as a predictor of the presentation of a near threshold 1-kHz tone. After learning this association, the tone could be omitted and participants would continue to report tones, conditional on the illumination of the bulb [25Ellson D.G. Hallucinations produced by sensory conditioning.J. Exp. Psychol. 1941; 28: 1-20Crossref Scopus (0) Google Scholar]. These studies underline the role of learning in these hallucinations. The predicting stimulus need not be visual and the hallucination need not be auditory. Tones can likewise be established as predictors of visual stimuli (e.g., dimly illuminated triangles), such that a 1-kHz tone presentation can engender a visual hallucination (of triangles that were never presented). These experiences even transfer out of the laboratory; subjects report seeing triangles on their television screen, conditional on hearing a 1-kHz tone, even when no triangles were presented [26Davies P. et al.An effective paradigm for conditioning visual perception in human subjects.Perception. 1982; 11: 663-669Crossref PubMed Google Scholar]. More recently, visual-auditory conditioning has been used to demonstrate that voice-hearing patients are significantly more susceptible to this effect than patients without hallucinations and controls [27Kot T. Serper M. Increased susceptibility to auditory conditioning in hallucinating schizophrenic patients: a preliminary investigation.J. Nerv. Ment. Dis. 2002; 190: 282-288Crossref PubMed Scopus (27) Google Scholar]. However, these observations (and those that preceded them) could have been driven by the demand characteristics perceived from the experimental situation. More contemporary research has sought to mitigate this concern. Prior knowledge of a visual scene confers an advantage in recognizing a degraded version of that image [28Teufel C. et al.Shift toward prior knowledge confers a perceptual advantage in early psychosis and psychosis-prone healthy individuals.Proc. Natl. Acad. Sci. U. S. A. 2015; 112: 13401-13406Crossref PubMed Scopus (48) Google Scholar]. Patients at risk for psychosis, and, by extension, voice-hearing, were particularly susceptible to this advantage, and its magnitude correlated with hallucination-like percepts. Similarly, there is a version of this effect in audition; voice-hearing participants appear to have an enhanced prior for speech in degraded auditory stimuli even when not explicitly instructed. Indeed, they were able to detect that degraded speech earlier than control participants [29Alderson-Day B. et al.Distinct processing of ambiguous speech in people with non-clinical auditory verbal hallucinations.Brain. 2017; 140: 2475-2489Crossref PubMed Scopus (6) Google Scholar]. It is difficult to attribute these findings to demand characteristics. A bias toward top-down information appears to be the basis for 'sensory conditioning' [27Kot T. Serper M. Increased susceptibility to auditory conditioning in hallucinating schizophrenic patients: a preliminary investigation.J. Nerv. Ment. Dis. 2002; 190: 282-288Crossref PubMed Scopus (27) Google Scholar, 30Ellson D.G. Hallucinations produced by sensory conditioning.J. Exp. 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We recently showed that this effect is mediated by precise prior beliefs, that those priors are more precise in people who hallucinate, and that people with a diagnosed psychotic illness are less likely to update those prior beliefs in light of new evidence [34Powers A.R. et al.Pavlovian conditioning-induced hallucinations result from overweighting of perceptual priors.Science. 2017; 357: 596-600Crossref PubMed Scopus (45) Google Scholar]. Critically, the neural circuit underlying these conditioned phenomena, including superior temporal gyrus and insula, largely overlapped with the circuit engaged when patients report hearing voices in the scanner [34Powers A.R. et al.Pavlovian conditioning-induced hallucinations result from overweighting of perceptual priors.Science. 2017; 357: 596-600Crossref PubMed Scopus (45) Google Scholar, 35Jardri R. et al.Cortical activations during auditory verbal hallucinations in schizophrenia: a coordinate-based meta-analysis.Am. J. 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Visual hallucinations in Parkinson's disease have likewise been related to precise priors [38O'Callaghan C. et al.Visual hallucinations are characterized by impaired sensory evidence accumulation: insights from hierarchical drift diffusion modeling in Parkinson's disease.Biol. Psychiatry Cogn. Neurosci. Neuroimaging. 2017; 2: 680-688Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar], which may be driven by dopaminergic, cholinergic, and serotonergic perturbations [39Factor S.A. et al.The role of neurotransmitters in the development of Parkinson's disease-related psychosis.Eur. J. Neurol. 2017; 24: 1244-1254Crossref PubMed Scopus (0) Google Scholar]. While we focus here on AVHs, we suggest that precise priors in may underwrite hallucinations in other modalities and illnesses. The challenge will be characterizing them and discerning why each specific underlying pathology culminates in voices rather than visions, hallucinations rather than delusions (Box 1).Box 1Hallucinations and Delusions: Commonalities and DifferencesDelusions and hallucinations tend to co-occur, although not always. In this computational psychiatry literature, they have been dissociated. Whereas hallucinations have been consistently related to more precise priors, delusions have been associated with less precise priors.For instance, examining the relation between priors and delusional experiences with ambiguous visual motion stimuli reveals that delusion-proneness in healthy individuals as well as in schizophrenia patients was related to weaker statistical learning of new priors [111Schmack K. et al.Delusions and the role of beliefs in perceptual inference.J. Neurosci. 2013; 33: 13701-13712Crossref PubMed Scopus (54) Google Scholar, 112Schmack K. et al.Perceptual instability in schizophrenia: probing predictive coding accounts of delusions with ambiguous stimuli.Schizophr. Res. Cogn. 2015; 2: 72-77Crossref PubMed Google Scholar, 113Stuke H. et al.Delusion proneness is linked to a reduced usage of prior beliefs in perceptual decisions.Schizophr. Bull. 2018; (Published online January 20, 2018)https://doi.org/10.1093/schbul/sbx189Crossref Scopus (2) Google Scholar]. However, delusion-proneness in healthy individuals as well as in schizophrenia patients is correlated with high-precision priors induced by a high-level cognitive manipulation (a suggestion that 3D glasses will shift their perception of the ambiguous motion [111Schmack K. et al.Delusions and the role of beliefs in perceptual inference.J. Neurosci. 2013; 33: 13701-13712Crossref PubMed Scopus (54) Google Scholar, 114Schmack K. et al.Enhanced predictive signalling in schizophrenia.Hum. Brain Mapp. 2017; 38: 1767-1779Crossref PubMed Scopus (3) Google Scholar]). These apparently contradictory observations might be reconciled by assuming a hierarchy in the brain's predictive machinery: Here, imprecise perceptual priors (or failure to attenuate sensory precision) may lead to the formation of delusional beliefs. At the same time, precise cognitive priors at a higher hierarchical level will sculpt perception, subtending hallucinations and the maintenance of delusions (by engendering delusion consistent percepts [115Corlett P.R. et al.From drugs to deprivation: a Bayesian framework for understanding models of psychosis.Psychopharmacology (Berl.). 2009; 206: 515-530Crossref PubMed Scopus (131) Google Scholar]).This comorbidity of hallucinations and delusions is consistent with the factor structure of psychotic symptoms, a three-factor solution: negative symptoms, disorganization, and positive symptoms (comprising hallucinations and delusions) [116Liddle P.F. The symptoms of chronic schizophrenia. A re-examination of the positive-negative dichotomy.Br. J. Psychiatry. 1987; 151: 145-151Crossref PubMed Google Scholar]. However, there may be structure within the positive symptom factor [117Minas I.H. et al.Positive and negative symptoms in the psychoses: multidimensional scaling of SAPS and SANS items.Schizophr. Res. 1992; 8: 143-156Abstract Full Text PDF PubMed Scopus (0) Google Scholar]. Hallucinations may co-occur more frequently with delusions of parasitosis (that there are insects on the skin) and less frequently with grandiose ideas, religious and referential delusions [117Minas I.H. et al.Positive and negative symptoms in the psychoses: multidimensional scaling of SAPS and SANS items.Schizophr. Res. 1992; 8: 143-156Abstract Full Text PDF PubMed Scopus (0) Google Scholar]. Might there be psychological and biological evidence in favor of such relationships between symptoms?A recent study probed priors for the gist versus the details of ambiguous images in a healthy population with varying degrees of hallucination- and delusion-proneness [118Davies D.J. et al.Anomalous perceptions and beliefs are associated with shifts toward different types of prior knowledge in perceptual inference.Schizophr. Bull. 2017; 44: 1245-1253Crossref Scopus (2) Google Scholar]. Hallucination-proneness correlated with stronger employment of global (gist) and local (detail) priors, whereas delusion-proneness was associated with less reliance on local priors. Thus, differential weighting of specific hierarchical levels drives different psychotic symptoms [119Kwisthout J. et al.To be precise, the details don't matter: on predictive processing, precision, and level of detail of predictions.Brain Cogn. 2017; 112: 84-91Crossref PubMed Scopus (1) Google Scholar].The precision-weighting of priors and prediction errors may be independent at different hierarchical levels and in different modalities [18Friston K. Kiebel S. Predictive coding under the free-energy principle.Philos. Trans. R. Soc. Lond. B Biol. Sci. 2009; 364: 1211-1221Crossref PubMed Scopus (341) Google Scholar]. Neurobiologically, this may be mediated by the higher density of recurrent connections in higher-level association cortices, compared with primary sensory regions, such that a psychotogenic perturbation that impacts excitatory/inhibitory (E/I) balance may have more profound effects higher rather than lower in the hierarchy [120Yang G.J. et al.Functional hierarchy underlies preferential connectivity disturbances in schizophrenia.Proc. Natl. Acad. Sci. U. S. A. 2016; 113: E219-E228Crossref PubMed Scopus (26) Google Scholar]. See [121Jardri R. Deneve S. Circular inferences in schizophrenia.Brain. 2013; 136: 3227-3241Crossref PubMed Scopus (60) Google Scholar] for a detailed exposition of the role of E/I balance in learning, inference, and psychosis. In brief, the E/I relationships may implement exactly the precision-weighting mechanisms that underlie predictive coding. Blocking N-methyl-d-aspartic acid (NMDA) receptors (with ketamine for example) profoundly alters E/I balance [122Murray J.D. et al.Linking microcircuit dysfunction to cognitive impairment: effects of disinhibition associated with schizophrenia in a cortical working memory model.Cereb. Cortex. 2014; 24: 859-872Crossref PubMed Scopus (83) Google Scholar, 123Anticevic A. et al.NMDA receptor function in large-scale anticorrelated neural systems with implications for cognition and schizophrenia.Proc. Natl. Acad. Sci. U. S. A. 2012; 109: 16720-16725Crossref PubMed Scopus (120) Google Scholar], thus altering the balance between priors and prediction errors [115Corlett P.R. et al.From drugs to deprivation: a Bayesian framework for understanding models of psychosis.Psychopharmacology (Berl.). 2009; 206: 515-530Crossref PubMed Scopus (131) Google Scholar], perhaps differently at different hierarchical levels [120Yang G.J. et al.Functional hierarchy underlies preferential connectivity disturbances in schizophrenia.Proc. Natl. Acad. Sci. U. S. A. 2016; 113: E219-E228Crossref PubMed Scopus (26) Google Scholar]. NMDA blockade would decrease the precision of priors facilitating delusion-like inferences [124Corlett P.R. et al.Prediction error, ketamine and psychosis: an updated model.J. Psychopharmacol. 2016; 30: 1145-1155Crossref PubMed Scopus (22) Google Scholar]. However, ketamine does not typically engender hallucinations (except perhaps in circumstances of high environmental uncertainty [125Powers 3rd, A.R. et al.Ketamine-induced hallucinations.Psychopathology. 2015; 48: 376-385Crossref PubMed Scopus (24) Google Scholar]). Hallucinations (and sustained delusions) would entail neuroplastic learned changes in priors, which may arise from dopamine-driven learning mechanisms or serotonergic effects on deep pyramidal cells [115Corlett P.R. et al.From drugs to deprivation: a Bayesian framework for understanding models of psychosis.Psychopharmacology (Berl.). 2009; 206: 515-530Crossref PubMed Scopus (131) Google Scholar]. These effects are not characteristic of acute ketamine. But they may arise with chronic administration of ketamine or amphetamine or the administration of serotonergic hallucinogens [115Corlett P.R. et al.From drugs to deprivation: a Bayesian framework for understanding models of psychosis.Psychopharmacology (Berl.). 2009; 206: 515-530Crossref PubMed Scopus (131) Google Scholar]. Comparing and contrasting the different phenomenologies of these drugs [115Corlett P.R. et al.From drugs to deprivation: a Bayesian framework for understanding models of psychosis.Psychopharmacology (Berl.). 2009; 206: 515-530Crossref PubMed Scopus (131) Google Scholar] as well as psychotic symptoms across different illnesses will be a key test of our hypothesis. We suspect that in each case, where there are hallucinations, there will be high-precision priors. Delusions and hallucinations tend to co-occur, although not always. In this computational psychiatry literature, they have been dissociated. Whereas hallucinations have been consistently related to more precise priors, delusions have been associated with less precise priors. For instance, examining the relation between priors and delusional experiences with ambiguous visual motion stimuli reveals that delusion-proneness in healthy individuals as well as in schizophrenia patients was related to weaker statistical learning of new priors [111Schmack K. et al.Delusions and the role of beliefs in perceptual inference.J. Neurosci. 2013; 33: 13701-13712Crossref PubMed Scopus (54) Google Scholar, 112Schmack K. et al.Perceptual instability in schizophrenia: probing predictive coding accounts of delusions with ambiguous stimuli.Schizophr. Res. Cogn. 2015; 2: 72-77Crossref PubMed Google Scholar, 113Stuke H. et al.Delusion proneness is linked to a reduced usage of prior beliefs in perceptual decisions.Schizophr. Bull. 2018; (Published online January 20, 2018)https://doi.org/10.1093/schbul/sbx189Crossref Scopus (2) Google Scholar]. However, delusion-proneness in healthy individuals as well as in schizophrenia patients is correlated with high-precision priors induced by a high-level cognitive manipulation (a suggestion that 3D glasses will shift their perception of the ambiguous motion [111Schmack K. et al.Delusions and the role of beliefs in perceptual inference.J. Neurosci. 2013; 33: 13701-13712Crossref PubMed Scopus (54) Google Scholar, 114Schmack K. et al.Enhanced predictive signalling in schizophrenia.Hum. Brain Mapp. 2017; 38: 1767-1779Crossref PubMed Scopus (3) Google Scholar]). These apparently contradictory observations might be reconciled by assuming a hierarchy in the brain's predictive machinery: Here, imprecise perceptual priors (or failure to attenuate sensory precision) may lead to the formation of delusional beliefs. At the same time, precise cognitive priors at a higher hierarchical level will sculpt perception, subtending hallucinations and the maintenance of delusions (by engendering delusion consistent percepts [115Corlett P.R. et al.From drugs to deprivation: a Bayesian framework for understanding models of psychosis.Psychopharmacology (Berl.). 2009; 206: 515-530Crossref PubMed Scopus (131) Google Scholar]). This comorbidity of hallucinations and delusions is consistent with the factor structure of psychotic symptoms, a three-factor solution: negative symptoms, disorganization, and positive symptoms (comprising hallucinations and delusions) [116Liddle P.F. The symptoms of chronic schizophrenia. A re-examination of the positive-negative dichotomy.Br. J. Psychiatry. 1987; 151: 145-151Crossref PubMed Google Scholar]. However, there may be structure within the positive symptom factor [117Minas I.H. et al.Positive and negative symptoms in the psychoses: multidimensional scaling of SAPS and SANS items.Schizophr. Res. 1992; 8: 143-156Abstract Full Text PDF PubMed Scopus (0) Google Scholar]. Hallucinations may co-occur more frequently with delusions of parasitosis (that there are insects on the skin) and less frequently with grandiose ideas, religious