Modified SOP model

An extension allowing association changes involving associatively activated, physically absent cues. In the common symmetric formulation, matched $A_1/A_1$ and $A_2/A_2$ states support excitation, while mismatched $A_1/A_2$ and $A_2/A_1$ states support inhibition. The extra rules allow forms of retrospective revaluation; neural reactivation alone does not verify their signs.