A schematic representation of the predictive coding – using three levels of a hierarchy – is shown in Fig. 4(a).
Each level comprises two neuronal populations marked ‘P’ (prediction) and ‘E’ (error).
Prediction populations are located in deep cortical layers, while error populations are located superficially.
These populations are reciprocally connected within their levels and with the next hierarchical level, such that each prediction population updates its prediction based on prediction error from its own level and the level below.
Conversely, each error population encodes its prediction error based on predictions from its own level and the level above.
Evidence from electromagnetic recordings in humans (Arnal et al., 2011 and Iversen et al., 2009) suggest that top-down prediction and bottom-up prediction error may use different frequency bands; where descending predictions are transmitted in the beta range (blue arrows in the figure) and ascending prediction errors are conveyed predominantly in the gamma band (red arrows in the figure).
See (Bastos et al., 2012) for a fuller discussion.