Reply: The Bayesian equation and psychosis

Sir, In a recent study (Chambon et al ., 2011), we investigated the ability of patients with schizophrenia to make accurate predictions about other people’s intentions. This ability has long been shown to be impaired in schizophrenia, and this impairment may be accounted for by an abnormal integration of two different sources of information: the sensory evidence conveyed by movement kinematics, and the observer’s expectations about how likely an intention is. In the task, these two types of information were systematically varied. Our results showed that patients with positive symptoms were prone to over-weight sensory evidence confirming their prior expectations and to disregard evidence that invalidated such priors. We hypothesized that this abnormal interplay of prior expectations and current sensory experiences—that normally guarantee accurate inferences—could result from a disturbance in prediction error signalling, possibly caused by alterations in the dopaminergic circuitry. We speculated that such aberrant prediction error signals might account for the formation, as well as for the update, of delusional beliefs as to how biological agents are most likely to behave.In their comment, Garrett and Singh suggest …