From Whittington and Bogacz 2017 "The predictive coding framework describes a network architecture in which
such learning has a particularly simple neural implementation. The distinguishing
feature of the predictive coding models is that they include additional nodes encoding
the difference between the activity on a given level and that predicted by the higher
level, and that these prediction errors are propagated through the network (Rao and
Ballard, 1999; Friston, 2005)..."