REAFFERENT THALAMOCORTICAL LOOPS IN THE SONG SYSTEM OF OSCINE SONGBIRDS

Songbirds have a complex vocal repertoire, much of which is learned by imitation. The vocal motor system of songbirds includes a set of tele ncephalic pathways dedicated to the acquisition and production of lear ned song. The main vocal motor pathway goes from the high vocal center (HVC) to the robust nucleus of the archistriatum (RA), which in turn innervates mesencephalic and medullary nuclei involved in vocalization . We used neural tract tracers (biotinylated dextran amines, fluoresce in- and rhodamine-linked dextran amines, and Fluorogold) to show that RA of adult male canaries (Serinus canaria) and zebra finches (Taeniop ygia guttata) sends an ipsilateral projection to the posterior portion of the dorsomedial thalamic nucleus (DMP). DMP projects to the medial portion of the magnocellular nucleus of the anterior neostriatum (mMA N), which is known to project to HVC, forming a feedback circuit. We a lso observed that the projection from DMP to mMAN is bilateral. Extrac ellular multi-unit recordings from awake restrained subjects have demo nstrated that mMAN has auditory responses that are selective for the b ird's own song. These auditory responses are similar to responses reco rded simultaneously in HVC, but with a longer latency, suggesting that mMAN receives auditory information from HVC through the circuit we ha ve described. We also saw a weaker projection from RA to the medial pa rt of the dorsolateral nucleus of the thalamus (DLM), which is known t o project to the lateral portion of the magnocellular nucleus of the a nterior neostriatum (IMAN). IMAN is known to project to RA, completing yet another feedback circuit; IMAN is also part of the anterior foreb rain pathway, which plays an essential role in song learning. These th alamo-telencephalic circuits are similar to thalamo-cortical circuits found in mammalian motor systems, and we suggest that the signals carr ied by these loops may be important for song perception, song learning , song production, and/or the bilateral coordination of vocal motor co mmands. (C) 1997 Wiley-Liss, Inc.