 Although classical conditioning is not limited to the reward system, the enhancement of instrumental performance by stimuli (i.e., Pavlovian-instrumental transfer) requires the nucleus accumbens.Habitual and goal directed instrumental learning are dependent upon the lateral striatum and the medial striatum, respectively.During instrumental learning, opposing changes in the ratio of AMPA to NMDA receptors and phosphorylated ERK occurs in the D1-type and D2-type MSNs that constitute the direct and indirect pathways, respectively.