Cortical inter‐hemispheric circuits for multimodal vocal learning in songbirds

Vocal learning in songbirds and humans is strongly influenced by social interactions based on sensory inputs from several modalities. Songbird vocal learning is mediated by cortico‐basal ganglia circuits that include the SHELL region of lateral magnocellular nucleus of the anterior nidopallium (LMAN), but little is known concerning neural pathways that could integrate multimodal sensory information with SHELL circuitry. In addition, cortical pathways that mediate the precise coordination between hemispheres required for song production have been little studied. In order to identify candidate mechanisms for multimodal sensory integration and bilateral coordination for vocal learning in zebra finches, we investigated the anatomical organization of two regions that receive input from SHELL: the dorsal caudolateral nidopallium (dNCL_SHELL) and a region within the ventral arcopallium (Av). Anterograde and retrograde tracing experiments revealed a topographically organized inter‐hemispheric circuit: SHELL and dNCL_SHELL, as well as adjacent nidopallial areas, send axonal projections to ipsilateral Av; Av in turn projects to contralateral SHELL, dNCL_SHELL, and regions of nidopallium adjacent to each. Av on each side also projects directly to contralateral Av. dNCL_SHELL and Av each integrate inputs from ipsilateral SHELL with inputs from sensory regions in surrounding nidopallium, suggesting that they function to integrate multimodal sensory information with song‐related responses within LMAN‐SHELL during vocal learning. Av projections share this integrated information from the ipsilateral hemisphere with contralateral sensory and song‐learning regions. Our results suggest that the inter‐hemispheric pathway through Av may function to integrate multimodal sensory feedback with vocal‐learning circuitry and coordinate bilateral vocal behavior.