Vocal control pathways through the anterior forebrain of a parrot (Melopsittacus undulatus).

Abstract

A feature of the telencephalic vocal control system in the budgerigar (Melopsittacus undulatus) that has been hypothesized to represent a profound difference in organization from the oscine vocal system is its reported lack of an inherent circuit through the anterior forebrain. The present study reports anatomical connections that indicate the existence of an anterior forebrain circuit comparable in important ways to the "recursive" pathway of oscine songbirds. Results from anterograde and retrograde tracing experiments with biocytin and fluorescently labeled dextran amines indicate that the central nucleus of the anterior archistriatum (AAc) is the source of ascending projections upon the oval nuclei of the anterior neostriatum and ventral hyperstriatum (NAo and HVo, respectively). Efferent projections from the latter nuclei terminate in the lateral neostriatum afferent to AAc, thereby forming a short recurrent pathway through the pallium. Previously reported projections from HVo and NAo upon the magnocellular nucleus of the lobus parolfactorius (LPOm), and after LPOm onto the magnocellular nucleus of the dorsal thalamus (DMm; G.F. Striedter [1994] J. Comp. Neurol. 343:35-56), are confirmed. A specific projection from DMm onto NAom is also demonstrated; therefore, a recurrent pathway through the basal forebrain also exists in the budgerigar vocal system that is similar to the anterior forebrain circuit of oscine songbirds. Parallels between these circuits and mammalian basal ganglia-thalamo-cortical circuits are discussed. It is hypothesized that vocal control nuclei of the avian anterior neostriatum may perform a function similar to the primate supplemental motor area.