An important question is what drives discrimination: what is the underlying mechanism? From the conditioning literature,
there seem to be two main perspectives. First, discrimination training may result in the formation of an excitatory
association to the S+ and an inhibitory association to the S−. These interact so that the conditioned response reflects the
net associative effect (excitation minus inhibition; Pearce, 1987, 1994; Spence, 1937). Because the inhibition from the S−
will also generalize over the generalization continuum, prior and online discrimination training is expected to sharpen the
gradient. This viewpoint is therefore compatible with the current set of results. It is unclear, however, to what extent these
basic associative mechanisms play a (decisive) role in human predictive learning (see Mitchel, De Houwer, & Lovibond,
2009).