Abstract
Delusions are, by popular definition, false beliefs that are held with certainty and resistant to contradictory evidence. They seem at odds with the notion that the brain at least approximates Bayesian inference. This is especially the case in schizophrenia, a disorder thought to relate to decreased – rather than increased – certainty in the brain's model of the world. We use an active inference Markov decision process model (a Bayes-optimal decision-making agent) to perform a simple task involving social and non-social inferences. We show that even moderate changes in some model parameters – decreasing confidence in sensory input and increasing confidence in states implied by its own (especially habitual) actions – can lead to delusions as defined above. Incorporating affect in the model increases delusions, specifically in the social domain. The model also reproduces some classic psychological effects, including choice-induced preference change, and an optimism bias in inferences about oneself. A key observation is that no change in a single parameter is both necessary and sufficient for delusions; rather, delusions arise due to conditional dependencies that create ‘basins of attraction’ which trap Bayesian beliefs. Simulating the effects of antidopaminergic antipsychotics – by reducing the model's confidence in its actions – demonstrates that the model can escape from these attractors, through this synthetic pharmacotherapy.
Highlights
IntroductionDelusions are a puzzling phenomenon for all that encounter them, but for those who believe the brain performs inference according to Bayesian principles (or at least comes close to doing so), they must be especially mysterious
Delusions are a puzzling phenomenon for all that encounter them, but for those who believe the brain performs inference according to Bayesian principles, they must be especially mysterious
The key parts of the model that contribute to these false inferences are: reduced likelihood precision, which reduces the impact of sensory evidence; affect, which biases beliefs towards trusting or distrusting, and a propensity to form overconfident priors over pol icies, itself determined both by low habit resistance and high policy precision
Summary
Delusions are a puzzling phenomenon for all that encounter them, but for those who believe the brain performs inference according to Bayesian principles (or at least comes close to doing so), they must be especially mysterious. To change our immediate sur roundings, and mental or covert actions (Limanowski and Friston, 2018; Pezzulo, 2018); like the deployment of attention or selecting one hypothesis over another This licences the notion of attractor states in habit formation – the tendency to repeat actions that have previously been selected – and to model delusions as resulting from the acquisition of a ‘mental habit’. This is motivated by the idea that inferences about the world may be conditioned on (i.e., depend on) the actions we select (Friston et al, 2017a; Stachenfeld et al, 2017), and so habits may give rise to inferences that are confidently held and insensitive to sensory evidence, much in the same way as habits themselves. The ensuing active inference model accounts for related phenomena, such as the in fluence of choices on subsequent inferences (Brehm, 1956), an ‘opti mism bias’ in inferences about oneself (Sharot and Garrett, 2016), and, a computational mechanism of action of antipsychotic drugs
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