ELSC Seminar Series

In natural and flexible behaviors like speech and birdsong, discrete syllables are combined into sequences by following syntax rules. These rules define the likelihood of transitioning from one element to the next depending on behavioral context across multiple timescales. Despite extensive research, the neural mechanisms that implement and modify transitions remain unclear.

Canary song provides a powerful model for studying sequence generating circuits. Songs are composed of syllable repeats called phrases, whose transitions depend on both short and long-range song context. Leveraging large-scale behavioral datasets, we show that canary phrase transitions evolve across days, and that these changes cannot be fully explained by acoustic changes alone – suggesting that transition rules evolve independently from the constituent syllables.

To study neural activity that underlies such syntax rule changes, we used miniscopes to record projection neurons (PNs) in the premotor nucleus HVC of singing canaries. We found HVC PNs that fire sparsely and time locked to song syllables for up to 28 days, constituting robust premotor neural states underlying syllable production.

We set out to test if phrase transition probabilities change across days because of changes in these neural states, or because of changes in a transition mechanism that selects between states. Across neurons and neural ensembles, pre-transition activity reliably carries information about phrase transition probabilities. However, while neural activity predicts overall transition probabilities, it fails to account for a substantial fraction of day-to-day changes in phrase transition probabilities.

These results indicate that changes in syntax rules cannot be explained solely by changes in pre-transition neural states but instead require plasticity in the mechanism that transitions between syllable-driving premotor states. By dissociating the phrase transition process into these two components, we show that each is plastic on the timescale of days, which opens the door to studying them separately.