Title: Differential Contributions of Basal Ganglia and Thalamus to Song 1 Initiation, Tempo, and Structure 2 3

20 Basal ganglia-thalamocortical circuits are multi-stage loops critical to motor behavior, 21 but the contributions of individual components to overall circuit function remain unclear. 22 We addressed these issues in a songbird basal ganglia-thalamocortical circuit (the 23 anterior forebrain pathway, AFP) specialized for singing and critical for vocal plasticity. 24 The major known afferent to the AFP is the premotor cortical nucleus, HVC. 25 Surprisingly, previous studies found that lesions of HVC alter song but do not eliminate 26 the AFP’’s ability to drive song production. We therefore used this AFP-driven song to 27 investigate the role of basal ganglia and thalamus in vocal structure, tempo, and initiation. 28 We found that lesions of the striatopallidal component (Area X) slowed song and 29 simplified its acoustic structure. Elimination of the thalamic component (DLM) further 30 simplified the acoustic structure of song and regularized its rhythm, but also dramatically 31 reduced song production. The acoustic structure changes imply that sequential stages of 32 the AFP each add complexity to song, but the effects of DLM lesions on song initiation 33 suggest that thalamus is a locus of additional inputs important to initiation. Together, our 34 results highlight the cumulative contribution of stages of a basal ganglia-thalamocortical 35 circuit to motor output, along with distinct involvement of thalamus in song initiation or 36