Bilateral organization of the vocal control pathway in the budgerigar, Melopsittacus undulatus

Abstract

Anatomical and electrophysiological methods were used to map the vocal control nuclei of the budgerigar, Melopsittacus undulatus. Beginning with the motor nucleus of the syrinx, nuclei were located using antidromic stimulation and then injected with horseradish peroxidase (HRP). Retrogradely transported HRP labeled afferents to the injected nucleus. This procedure was repeated at successively higher levels along the vocal pathway. Connections found using this strategy then were confirmed using anterograde transport of HRP and/or tritiated proline and orthodromic electrical stimulation. We found that the primary vocal control pathway consisted of the motor nucleus innervating the trachea and syrinx, nXIIts; an archistriatal nucleus, RA; and a neostriatal nucleus, 'HVc.' These nuclei correspond to similar, possibly homologous, nuclei in the vocal control pathway of the canary but, because of differences in gross brain morphology, are displaced considerably in absolute position. Furthermore, the projection from RA to the motor nucleus is bilateral in the budgerigar, whereas the same connection is strictly ipsilateral in the canary. The projection of the motor nucleus to muscles of the vocal organ is also bilateral in the budgerigar but ipsilateral in the canary. The possible significance of these species differences for lateralization of motor control is discussed.