FEEDBACK CIRCUITRY WITHIN A SONG-LEARNING PATHWAY

The song system of birds consists of several neural pathways. One of t hese, the anterior forebrain pathway, is necessary for the acquisition but not for the production of learned song in zebra finches. It has b een shown that the anterior forebrain pathway sequentially connects th e following nuclei: the high vocal center, area X of lobus parolfactor ius, the medial portion of the dorsolateral thalamic nucleus, the late ral magnocellular nucleus of anterior neostriatum (IMAN), and the robu st nucleus of the archistriatum (RA). We now show in zebra finches (Ta eniopygia guttata) that IMAN cells that project to RA also project to area X, forming a feedback loop within the anterior forebrain pathway, The axonal endings of the IMAN projection into area X form cohesive a nd distinct domains. Small injections of tracer in subregions of area X backfill a spatially restricted subset of cells in IMAN, that, in tu rn, send projections to RA that are arranged in horizontal layers, whi ch may correspond to the functional representation of vocal tract musc les demonstrated by others. We infer from our data that there is a myo topic representation throughout the anterior forebrain pathway. In add ition, we suggest that the parcellation of area X into smaller domains by the projection from IMAN highlights a functional architecture with in X, which might correspond to units of motor control, to the represe ntation of acoustic features of song, or both.