  Behavioral sequences of animals are often structured and can be described by
probabilistic rules (or "action syntax"). The patterns of vocal elements in
birdsong are a prime example. The encoding of such rules in neural circuits is
poorly understood. Here we locate the site of song syntax in the Bengalese
finch by rapidly and reversibly manipulating the temperature in the song
production pathway. Changing the temperature in the premotor nucleus HVC
(proper name) alters the transition probabilities between syllables. Most
prominently, cooling reduces the number of repetitions of long repeated
syllables, while heating increases repetition. In contrast, changing the
temperature of the downstream motor area RA (robust nucleus of the
acropallium), which is critical for singing, does not affect the song syntax.
Computational modeling suggests that temperature can alter the transition
probabilities by affecting the efficacy of the synapses in HVC that carry
auditory feedback to the motor circuits. The model is supported by a real-time
distorted auditory feedback experiment, which shows that perturbing auditory
feedback shortens syllable repeats similar to cooling HVC. Taken together,
these findings implicate HVC as a key player in determining birdsong syntax.
