Prediction error and surprise

Prediction is one of the fundamental functions of the brain. Prediction allows the organism to prepare for events and direct attention to what is important: the unexpected, surprising and unknown. An event can only be identified as unexpected if there is an expectation or prediction to begin with, and if there is a large enough deviation from that prediction. Because there is random variation in events themselves, in the perception of events, and in their prediction, “large enough” can only be a statistical judgement. If either the criterion for what is surprising is inappropriate, or if the estimate of prediction error is systematically wrong, then the balance between type I and type II errors shifts. Excessive surprise caused by overestimation of prediction error has been proposed to be a cause of both psychosis and autism (Fletcher and Frith, 2009; Frith, 2005; van de Cruys et al., 2014). The question of whether the criterion for surprise might contribute has received little attention. In a simulation, we varied both the misestimation of prediction error and the criterion for surprise by the same factor, and calculated how often individuals with varying criteria and degrees of misestimation are surprised. We find that the criterion for surprise has a greater influence on the proportion of surprises than misestimation of prediction error. Evaluation of computational theories of psychosis and autism depends on developing experimental designs that can distinguish these factors.