Figure 1.

Parallel cortico-basal ganglia pathways in songbirds. A, Parallel circuits are formed by the axonal connections of core (gray) and shell (red) regions of LMAN. These parallel projections form recurrent loops through both the basal ganglia (Area X, a nucleus containing both striatal and pallidal neurons) and through other cortical regions: RA (robust nucleus of arcopallium) and AI_d (dorsal intermediate arcopallium), which are located in the analog of mammalian motor cortex. AI_d was referred to as Ad (dorsal arcopallium) in previous papers from our laboratory, but we have changed the terminology here to conform to the nomenclature suggested by Reiner et al. (2004). Core and shell regions of LMAN receive input from separate subgroups of neurons in the thalamic nucleus DLM (dorsolateral medial thalamus; VM, ventromedial; DL, dorsolateral). B, Anterograde label in coronal sections of RA and AI_d after iontophoretic injections of biotinylated dextran amine into LMAN_core. Individual core neurons send axons exclusively into RA in adult birds but send numerous collateral branches into AI_d in juvenile (35 dph) birds (Miller-Sims and Bottjer, 2012) (dorsal is up, medial is left; scale bar, 0.5 mm). Thus, LMAN_core neurons make a robust transient projection into the shell pathway during sensorimotor integration that is completely gone by adulthood.