Perceiving is believing: a Bayesian approach to explaining the positive symptoms of schizophrenia

In psychosis, false perceptions (hallucinations) and false beliefs (delusions) may arise from reshaping of top-down predictions in attempts to suppress aberrant and persistent exteroceptive prediction errors.

“In psychosis, false perceptions (hallucinations) and false beliefs (delusions) may arise from reshaping of top-down predictions in attempts to suppress aberrant and persistent (exteroceptive) prediction errors [78].”

Delusions may arise as rational (Bayes-optimal) posterior beliefs that explain away precise sensory prediction errors.

“they could arise as rational (Bayes-optimal) posterior beliefs that explain away precise sensory prediction errors: e.g., Fletcher and Frith (2009).”

Disturbances in sensed agency arise not simply from predictive mismatches, but from pathologically imprecise predictions about the sensory consequences of action.

“Following previous models (Fletcher and Frith, 2009; Synofzik et al., 2010), disturbances in sensed agency arise not simply from predictive mismatches, but from pathologically imprecise predictions about the sensory consequences of action.”

Fletcher and Frith propose that, for positive symptoms in psychoses, a Bayesian perspective can accommodate hallucinations and delusions within a common framework.

“Fletcher and Frith (2009) propose that, for positive symptoms in psychoses, a Bayesian perspective can accommodate hallucinations and delusions within a common framework”

Psychosis has been related to a decreased precision of prior beliefs and/or increased precision of sensory data.

“Psychosis has been related to a decreased precision of prior beliefs and/or increased precision of sensory data (13, 24, 58–61).”

According to predictive coding, reduced precision of priors could potentially account for both perceptual and cognitive factors in the genesis of some delusions.

“According to predictive coding, reduced precision of priors could potentially account for both factors, given it would alter perceptual inference and make cognitive explanations for altered percepts less constrained (58).”

The emergence of delusions might result from decreased precision of priors.

“In contrast, the emergence of delusions might result from decreased precision of priors as outlined above (24, 58, 59) (Fig. 1B).”

The process of false prediction errors leading to deep revisions is self-entrenching; false perceptions and bizarre beliefs form an epistemically insulated self-confirming cycle.

“Such a process is self-entrenching. As new generative models take hold, their in fl uence fl ows back down so that incoming data is sculpted by the new (but now badly misinformed) priors so as to ' conform to expectancies ' (Fletcher & Frith 2009, p. 348). False perceptions and bizarre beliefs thus form an epistemically insulated self-con fi rming cycle.”

Recent theories have hypothesised that precision weighting of cortical unsigned prediction errors is mediated by neuromodulators (including dopamine), and link a malfunctioning dopamine system to psychosis through aberrant precision-weighting of these prediction errors.

“Recent theories [3, 4, 6, 9, 28] have hypothesised that precision weighting of cortical unsigned prediction errors is mediated by neuromodulators (including dopamine), and link a malfunctioning dopamine system to psychosis through aberrant precision-weighting of these prediction errors.”

In their account, a shift from hallucination to delusion reflects readjustment of top-down predictions within successively higher levels of cortical hierarchies, in successive attempts to explain away residual prediction errors.

“In their compelling account, a shift from hallucination to delusion reflects readjustment of top-down predictions within successively higher levels of cortical hierarchies, in successive attempts to explain away residual prediction errors”

Aberrant encoding of precision could lead to an abnormally strong weighting of prediction error, which in turn leads to aberrant learning and the formation of delusional beliefs.

“The resulting aberrant encoding of precision could lead to an abnormally strong weighting of PE, which in turn leads to aberrant learning and the formation of delusional beliefs (53, 58, 59, 64).”

Malfunctions in the prediction error economy yield persistent and highly weighted false errors that propagate up the hierarchy, forcing deep revisions in the model of the world.

“The suggestion (Corlett et al. 2009a; 2009b; Fletcher & Frith 2009) is that malfunctions within that complex economy (perhaps fundamentally rooted in abnormal dopaminergic functioning) yield wave upon wave of persistent and highly weighted ' false errors ' that then propagate all the way up the hierarchy forcing, in severe cases (via the ensuing waves of neural plasticity) extremely deep re”

Understanding the positive symptoms of schizophrenia requires understanding disturbances in the generation and weighting of prediction error.

“The key idea, however implemented, is that understanding the positive symptoms of schizophrenia requires understanding disturbances in the generation and weighting of prediction error.”

Positive symptoms (hallucinations and delusions) in schizophrenia can be accounted for as false inference.

“from false inference as an account of positive symptoms (hallucinations and delusions) in schizophrenia [73]”

Neurodevelopmental disorders such as autism spectrum disorder and schizophrenia have been linked to an atypical integration of prior and incoming information.

“neurodevelopmental disorders such as autism spectrum disorder [132,133] and schizophrenia [134,135] have been linked to an atypical integration of prior and incoming information”

Many researchers have assumed that mental states can be orderly or disorderly and have fruitfully pursued predictive coding in the context of psychopathology.

“Many have assumed that this must be so – and indeed have fruitfully pursued predictive coding in this context (Fletcher and Frith, 2009; Edwards et al., 2012; Adams et al., 2013b; Hohwy, 2013).”

If Bayesian-based explanations of psychiatric disorders fail to predict symptom patterns, treatment outcomes, or neural correlates in out-of-sample datasets, the theory's applied value diminishes.

“If Bayesian-based explanations of psychiatric disorders (e.g., schizophrenia as altered precision weighting) fail to predict symptom patterns, treatment outcomes, or neural correlates in out-of-sample datasets, the theory’s applied value diminishes (Fletcher and Frith 2009).”

Similar Bayesian theories have previously been stated in the context of schizophrenia.

“It is of interest to note that similar Bayesian theories have previously been stated in the context of schizophrenia (63, 68, 89–92).”

This model and variants thereof have already found widespread application to other psychiatric disorders with perceptual aberrations, in particular schizophrenia.

“This model, and variants thereof, has already found widespread application to other psychiatric disorders with perceptual aberrations, in particular schizophrenia (63, 89, 90, 92, 107).”

A single core disturbance relating to the balance between bottom-up and top-down processing can explain both the hallucinatory experiences and the bizarre delusional beliefs of psychotic patients.

“For instance, it has been hypothesized that a single core disturbance relating to the balance between bottom-up and top-down processing can explain both the hallucinatory experiences and the bizarre delusional beliefs of psychotic patients (8,11).”

Patients with schizophrenia have deficits in probabilistic reasoning that affect their beliefs.

“In a review and synthesis of the literature “Perceiving is believing: a Bayesian approach to explaining the positive symptoms of schizophrenia”, Fletcher and Frith suggest that patients have deficits in probabilistic reasoning that affect their beliefs (Fletcher & Frith, 2009).”

A normally functioning mechanism that updates context with new information is critical for accurate perception and inference.

“They suggested that a normally functioning mechanism that updates context with new information is critical for accurate perception and inference.”

Disruption of the context-updating mechanism may result in the persistence of false beliefs (delusions) in the face of incontrovertible evidence.

“Disruption of this mechanism may result in the persistence of false beliefs in the face of incontrovertible evidence to the contrary (delusions).”

Psychosis is linked to increased prediction error signaling, which in turn leads to aberrant salience and the formation of delusions.

“It has been proposed that psychosis is linked to increased prediction error signaling 122 which in turn leads to aberrant salience and the formation of delusions, as proposed earlier.123,124,[130][131][132]”

Disrupted predictive coding has been invoked as a mechanism underlying hallucinations.

“Disrupted predictive coding has also been invoked as a mechanism underlying hallucinations per se.122-124”

Recent theories propose that hallucinations could be due to altered inference mechanisms.

“Recent theories propose that hallucinations could be due to altered inference mechanisms.[122][123][124]”

Others have linked hallucinations to a failure to attenuate the sensory consequences of inner speech, analogous to the mechanism thought to underlie delusions of control.

“Others have linked hallucinations to a failure to attenuate the sensory consequences of inner speech, in analogy to the mechanism that is thought to underlie delusions of control.42,124,135”

A growing field of theoretical and experimental approaches relates psychotic features such as hallucinations and delusions to a general disruption in the inferential process.

“A growing field of theoretical and experimental approaches have related psychotic features, such as hallucinations and delusions, to a general disruption in the inferential process (Friston, 2005b; Fletcher and Frith, 2008; Schmack et al., 2013).”

Over-trusted sensory signals become over-salient and unpredictable; they could be transmitted up the hierarchy, but no adjustments could fully resolve the aberrancy.

“Artificially over-trusted, sensory signals become over-salient and unpredictable. They could be transmitted up the hierarchy, but no adjustments could fully resolve the aberrancy (Fletcher and Frith, 2008).”

Hallucinations and delusions are two sides of the same coin.

“This assumption is in line with the idea that hallucinations and delusions are two sides of a same coin (Fletcher and Frith, 2008).”

Schizophrenia involves bizarre pathognomonic clinical symptoms such as passivity experiences, auditory hallucinations, and delusions of reference.

“Third, the theory must account for the mechanism that gives rise to such bizarre yet pathognomonic clinical symptoms as passivity experiences, auditory hallucinations, and delusions of reference.”

Feinberg and Frith first developed the idea that passivity experiences and certain auditory hallucinations could be explained by efference copy/corollary discharge abnormalities disrupting patients' ability to distinguish between internally and externally generated events.

“Feinberg 8 and Frith 9 first developed the idea that many of the most distinctive symptoms of schizophrenia-most notably the passivity experiences and certain types of auditory hallucinations-could be explained by efference copy/corollary discharge abnormalities disrupting patients' ability to distinguish between internally and externally generated events.”

This delayed suppression results in confusion as to the origins of actions, hence passivity experiences for willed motor actions and auditory hallucinations for willed cognitions.

“This, in turn, results in confusion as to the origins of these actions and hence passivity experiences in the case of willed motor actions and auditory hallucinations in the case of willed cognitions.8,9”