Fig. 9
Model of the stages of learning cued approach behavior and underlying NAc mechanisms. Top box: likelihood/vigor of approach responses prior to training (left box) and at different points of training (right box) in the presence (light blue) or absence (dark blue) of the S+. Bottom box: conceptual diagrams depicting, at different stages of learning, changes in the strength of S+-evoked excitatory responses in NAc neurons as well as NAc afferents that encode either the S− (left) or the S+ (right). Early in training (stage 1), small S+-evoked excitations appear and are unaffected by AP5, likely implicating plasticity elsewhere in the circuit in which the NAc is embedded (likely upstream of the NAc). Just before CP and after CP (stages 2 and 3), NMDAR-mediated plasticity within the NAc is required for the growth of NAc cue-evoked excitations. Between the trials before CP and the point at which animals are showing asymptotic performance, expression of S+-evoked excitations in the NAc requires NMDAR-mediated excitatory transmission, but it becomes independent from it with extended training (stage 4)
