Peripheral motor dynamics of song production in the zebra finch.

Singing behavior in songbirds is a model system for motor control of learned behavior. The target organs of its central motor programs are the various muscle systems involved in sound generation. Investigation of these peripheral motor mechanisms of song production is the first step toward an understanding of how different motor systems are coordinated. Here we review physiological studies of all major motor systems that are involved in song production and modification in the zebra finch (Taeniopygia guttata). Acoustic syllables of zebra finch song are produced by a characteristic air sac pressure pattern. Electromyographic (EMG) and sonomicrometric recording of expiratory muscle activity reveals that respiratory motor control is tightly coordinated with syringeal gating of airflow. Recordings of bronchial airflow demonstrate that most of the song syllables are composed of simultaneous independent contributions from the two sides of the syrinx. Sounds generated in the syrinx can be modified by the resonance properties of the upper vocal tract. Tracheal length affects resonance, but dynamic changes of tracheal length are unlikely to make a substantial contribution to sound modification. However, beak movements during song contribute to sound modification and, possibly, affect the vibratory behavior of the labia. Rapid beak aperture changes were associated with nonlinear transitions in the acoustic structure of individual syllables. The synergy between respiratory and syringeal motor systems, and the unique bilateral, simultaneous, and independent sound production, combined with dynamic modification of the acoustic structure of song, make the zebra finch an excellent model system for exploring mechanisms of sensorimotor integration underlying a complex learned behavior.