While projections from the different DA cell groups are not as topographically organized as inputs from the cortex, there is an important organization to this input that underlies a mechanism for modulating the different corticostriatal regions, especially across functional domains. This complex system includes both the DA-striatal pathway as well as the striatal input to the DA neurons. There is a general inverse dorsal-ventral topographic organization to the midbrain projection to the striatum. The dorsal DA cells project to the ventral parts of the striatum, while the ventrally placed cells project to the dorsal parts of the striatum.13,118 Thus, the dorsal tier projects to the VS, and the ventral tier projects centrally and throughout different regions of the striatum, with the most ventral part of this cell group projecting primarily to the dorsolateral (sensorimotor) striatum. The shell region of the VS receives the most limited input, primarily derived from the VTA. The rest of the VS receives input from the entire dorsal tier and from the most medial and dorsal part of the ventral tier. In contrast to the VS,
