PARALLEL PATHWAYS AND CONVERGENCE ONTO HVC AND ADJACENT NEOSTRIATUM OF ADULT ZEBRA FINCHES (TAENIOPYGIA-GUTTATA)

The structure and connectivity of the forebrain nucleus HVc, a site of sensorimotor integration in the song control system of oscine birds, were investigated in adult zebra finches. HVc in males comprises three cytoarchitectonic subdivisions: the commonly recognized central regio n with large and medium-sized darkly staining cells, a ventral caudome dial region with densely packed small and medium-sized cells, and a do rsolateral region with oblong cells and rows of cells. All three subdi visions project to area X and the robust nucleus of the archistriatum, with more complexity in the classes and distribution of cells than pr eviously reported. In females, HVc is very small and has a cytoarchite cture distinct from that of the three male subdivisions. The structure of HVc in females treated with estradiol at 15 days of age is similar to male HVc. Tracer studies in males with fluorescent and biotinylate d dextrans demonstrate nontopographic projections onto HVc that may ca rry auditory information, including type 1 and type 2 neurons in subdi visions L1 and L3 of the field L complex, a class of neurons in nucleu s interface, nucleus uvaeformis, the caudal neostriatum ventral to HVc , and intrinsic HVc connections. These data are interpreted in terms o f HVc's functional properties. Additionally, the neostriatum immediate ly ventral to HVc receives projections from field L, ventral hyperstri atum, and caudal neostriatum, and projects to a region surrounding RA and near to or into area X. The similarity of the connectivity of HVc and adjacent neostriatum suggests the possibility that they share a co mmon origin. (C) 1995 Wiley-Liss, Inc.