The role of HVC in coordinating anatomically asymmetric sound sources with a bilaterally controlled respiratory system

Abstract

Bengalese finch (Lonchura striata domestica) song is a learned behavior that is produced with remarkable asymmetry in spectral and amplitude control of sound production. Louder, higher frequency (2.2 kHz) notes are generated by the left side of the syrinx (avian vocal organ) whereas softer, lower frequency notes are right-side generated (Urbano, 2013). We compared changes in song features and rate of recovery following left or right HVC microlesions or large (full) HVC lesions. Song was recorded from birds prior to, and for the first week following unilateral damage to HVC. Left and right HVC lesions lowered peak frequency and amplitude and increased the intersyllable interval (ISI), compared to sham lesions. The song deterioration induced by the microlesion was transient and birds largely recovered within one week. We continued to record full lesion groups for five months. We found evidence that Bengalese finches rely on left HVC for retention of higher frequency syllables.^We also found evidence that left HVC damage leads to slower rates of short-term (within the first week) and long-term (over five months) recovery. Symmetric evidence of amplitude and temporal changes led us to investigate the effects of HVC lesion and inactivation on the underlying song respiratory pattern. Across several experiments, we provide direct evidence for how forebrain signals are translated into sound. In experiment 1, we presented data illustrating the relationship between unilateral HVC lesion and resultant reduced song amplitude. Experiment 2 illustrated that amplitude will gradually recover months after large HVC lesion. Experiment 3 shed light on why songs are sung more quietly: the underlying reduction in EP amplitude and slope that, interestingly, does not recover months after surgery.^Experiments 4 and 5 looked at potential peripheral mechanisms for the altered respiratory pattern and demonstrated that reducing HVC input (via lesion or inactivation) does two things: directly attenuates muscle activation and decouples the onset of dTB and EXP muscles. Studies 1 and 2 point establish the Bengalese finch as a potential animal model for studying the evolution of lateralized motor behavior. Studies 3-5 extend scientific understanding of phonation and the importance of coordinating vocal motor and respiratory motor systems for producing audible vocalizations.