Rhythmic Continuous-Time Coding in the Songbird Analog of Vocal Motor Cortex

Songbirds learn and produce complex sequences of vocal gestures. Adult birdsong requires premotor nucleus HVC, in which projection neurons (PNs) burst sparsely at stereotyped times in the song. It has been hypothesized that PN bursts, as a population, form a continuous sequence, while a different model of HVC function proposes that both HVC PN and interneuron activity is tightly organized around motor gestures. Using a large dataset of PNs and interneurons recorded in singing birds, we test several predictions of these models. We find that PN bursts in adult birds are continuously and nearly uniformly distributed throughout song. However, we also find that PN and interneuron firing rates exhibit significant 10-Hz rhythmicity locked to song syllables, peaking prior to syllable onsets and suppressed prior to offsets—a pattern that predominates PN and interneuron activity in HVC during early stages of vocal learning. •The population of HVC projection neurons is continuously active during singing•Analysis shows that HVC activity is not tightly organized around motor gestures•However, projection neurons and interneurons are rhythmically modulated by the song•Coherence between projection neurons and song is stronger in juveniles than adults Lynch et al. show that HVC is not active only around motor gestures, as previously hypothesized. Instead, the population of HVC projection neurons is continuously active during song, with rhythmic modulations locked to song. Juvenile HVC exhibits stronger song-related modulation.