Functional neuroanatomy of the sensorimotor control of singing

Reviews of the songbird vocal control system frequently begin by describing the forebrain nuclei and pathways that form anterior and posterior circuits involved in song learning and song production, respectively. They then describe extratelencephalic projections upon the brainstem respiratory-vocal system in a manner suggesting, quite erroneously, that this system is itself well understood. One aim of this chapter is to demonstrate how limited is our understanding of that system. I begin with an overview of the neural network for the motor control of song production, with a particular emphasis on brainstem structures, including the tracheosyringeal motor nucleus (XIIts), which innervates the syrinx, and nucleus retroambigualis (RAm), which projects upon XIIts and upon spinal motor neurons innervating expiratory muscles. I describe the sources of afferent projections to XIIts and RAm and discuss their probable role in coordinating the bilateral activity of respiratory and syringeal muscles during singing. I then consider the routes by which sensory feedback, which could arise from numerous structures involved in singing, might access the song system to guide song learning, maintain accurate song production, and inform the song system of the requirements for air. I describe possible routes of access of auditory feedback, which is known to be necessary for song learning and maintenance, and identify potential sites of interaction with somatosensory and visceral feedback that could arise from the syrinx, the lungs and air sacs, and the upper vocal tract, including the jaw. I conclude that the incorporation of brainstem-based respiratory-vocal variables is likely to be a necessary next step in the construction of more sophisticated models of the control of vocalization.