The control of sequenced behaviors, including human speech, requires that the brain coordinate the production of discrete motor elements with their concatenation into complex patterns. In birdsong, another sequential vocal behavior, the acoustic structure (phonology) of individual song elements, or "syllables," must be coordinated with the sequencing of syllables into a song. However, it is unknown whether syllable phonology is independent of the sequence in which a syllable is produced. We quantified interactions between phonology and sequence in Bengalese finch song by examining both convergent syllables, which can be preceded by at least two different syllables and divergent syllables, which can be followed by at least two different syllables. Phonology differed significantly based on the identity of the preceding syllable for 97% of convergent syllables and differed significantly with the identity of the upcoming syllable for 92% of divergent syllables. Furthermore, sequence-dependent phonological differences extended at least two syllables away from the convergent or divergent syllable. To determine whether these phenomena reflect differences in central control, we analyzed premotor neural activity in the robust nucleus of the arcopallium (RA). Activity associated with a syllable varied significantly depending on the sequence in which the syllable was produced, suggesting that sequence-dependent variations in premotor activity contribute to sequence-dependent differences in phonology. Moreover, these data indicate that RA activity could contribute to the sequencing of syllables. Together, these results suggest that, rather than being controlled independently, the sequence and phonology of birdsong are intimately related, as is the case for human speech.
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