HOESE, W.J.*; MOONEY, R.; NOWICKI, S.: Neural Encoding of Multiple Song Types in the Swamp Sparrow HVc

Many songbirds, including swamp sparrows (Melospiza georgiana), sing multiple song types as adults. In Songbirds that sing only a single song type, such as zebra finches and white-crowned sparrows, neurons in the song nucleus HVc are song-selective, firing more strongly to forward than reverse playback of the bird’s own song (BOS), and little or not at all to other conspecific songs. The manner in which a swamp sparrow’s multiple song types are represented by its auditory HVc neurons is unknown, but could help further our understanding of the genesis and function of these highly selective neurons in song learning and perception. One idea is that all song types are equally effective at driving auditory responses in HVc neurons. A second possibility is that HVc neurons form a mosaic, with certain cells firing preferentially in response to playback of one song or another. We addressed this issue by making in vivo intracellular recordings from identified HVc neurons in urethane-anesthetized adult male swamp sparrows, using auditory playback to present the individual’s different song types. We found that multiple song types were encoded in HVc in a song and cell type-specific manner, and that there were marked differences in subthreshold versus firing rate responses. Almost all neurons fired more strongly (and often exclusively) to a single song and, within single birds, different neurons could prefer different songs. In contrast, inhibitory and/or excitatory subthreshold responses could be evoked by each of the bird’s song types, but these more broadly tuned subthreshold responses were still selective for forward over reverse playback. These results suggest a local hierarchy within HVc, where broadly selective inputs are refined to generate highly selective, song-type specific firing in certain neurons.