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 'compl exity of vocal communication'. One set of linkages is postulated to li nk the vocal system with the respiratory system, since birds, like hum ans, 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 feed back for vocal learning. The systems and their linkages can be regarde d as forming an 'auditory-vocal-respiratory axis', around which the an imal's communication system evolves and revolves. The experimental str ategy used was one which began at the periphery (the abdominal expirat ory muscles), then progressively identified more central neural struct ures using retrograde transport methods in partial combination with re cordings of single cell activity. The projections delineated by these methods were then defined in detail by anterograde tracing methods. Th e results of the studies confirmed the expectation that the vocal and respiratory systems have many neural elements in common. They also sug gested that songbirds and non-songbirds possess similar neural pathway s in the brainstem and spinal cord for the control of both vocalizatio n and respiration but indicated that there may be significant differen ces between the two types of birds in the degree to which the telencep halon 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 i n both songbirds and non-songbirds, the most rostral origin of this ca scade is the telencephalic nucleus robustus archistriatalis in (male) songbirds but, apparently, the dorsomedial nucleus of the intercollicu lar complex of the midbrain (DM) in pigeons. Connectional studies of t he auditory system in pigeons delineated a series of projections which originate in Field L2, the primary telencephalic auditory area, and l eave the telencephalon via the nucleus archistriatum intermedium, pars medialis (Aivm), after traversing a minimum of three synapses within the telencephalon. The extratelencephalic projections of Aivm resemble those of deep layers of mammalian auditory neocortex, having terminat ions in close proximity to thalamic and midbrain auditory nuclei, but a projection upon DM is conspicuous by its absence. The way in which a uditory input gains access to vocal control nuclei in non-songbirds, s uch as pigeons, thus remains to be determined.