to the midbrain extends beyond the tight ventral striatal/dorsal tier/ventral striatal circuit, terminating lateral and ventral to the dorsal tier. This area of terminal projection does not project back to the VS. Rather, cells in this region project more dorsally, into the striatal area that receives input from the dPFC. Through this connection, the same cortical information that influences the dorsal tier through the VS also modulates parts of the ventral tier region that projects to the central striatum. This central striatal region is reciprocally connected to the central part of the ventral tier. But it also projects to its more ventral parts. Thus, projections from the dPFC, via the striatum, are in a position to influence cells that project to motor control areas of the striatum. The dorsolateral striatum is reciprocally connected to the ventral tier. The confined distribution of efferent dorsolateral striatal fibers limits the influence of the motor striatum to a relatively small region involving the primarily dopaminergic cells embedded within the SNr. Taken together, the interface between different striatal regions via the midbrain DA cells is organized in an ascending spiral interconnecting different functional regions of the striatum (Figure 6). Through this spiral of inputs and
