Fig. 1.

Message passing in the predictive coding scheme. This schematic illustrates the key asymmetry between ascending prediction errors and descending predictions in predictive coding schemes. The descending predictions serve to explain away sensory or neural input in lower areas. This is achieved by suppressing prediction errors, which in turn are used to optimise expectations about the (hidden) causes of sensory inputs. The equations describe the optimisation scheme that is discussed in more detail in the main text.