The auditory-vocal-respiratory axis in birds.

A series of studies is described which in general aim to identify two sets of neural linkages in the brain and spinal cord of songbirds and non-songbirds, these avian types differing along a dimension of 'complexity of vocal communication'. One set of linkages is postulated to link the vocal system with the respiratory system, since birds, like humans, require controlled expiration in order to vocalize normally. The other set is thought to link the auditory system with the vocal system, at least in songbirds, because they are dependent upon auditory feedback for vocal learning. The systems and their linkages can be regarded as forming an 'auditory-vocal-respiratory axis', around which the animal's communication system evolves and revolves. The experimental strategy used was one which began at the periphery (the abdominal expiratory muscles), then progressively identified more central neural structures using retrograde transport methods in partial combination with recordings of single cell activity. The projections delineated by these methods were then defined in detail by anterograde tracing methods. The results of the studies confirmed the expectation that the vocal and respiratory systems have many neural elements in common. They also suggested that songbirds and non-songbirds possess similar neural pathways in the brainstem and spinal cord for the control of both vocalization and respiration but indicated that there may be significant differences between the two types of birds in the degree to which the telencephalon is able to modulate respiratory-vocal activity downstream. Thus, whereas there is a cascade of descending projections terminating upon syringeal and laryngeal motoneurons and expiratory premotor neurons in both songbirds and non-songbirds, the most rostral origin of this cascade is the telencephalic nucleus robustus archistriatalis in (male) songbirds but, apparently, the dorsomedial nucleus of the intercollicular complex of the midbrain (DM) in pigeons. Connectional studies of the auditory system in pigeons delineated a series of projections which originate in Field L2, the primary telencephalic auditory area, and leave the telencephalon via the nucleus archistriatum intermedium, pars medialis (Aivm), after traversing a minimum of three synapses within the telencephalon.(ABSTRACT TRUNCATED AT 400 WORDS)