Synaptic connections of thalamo-cerebral vocal nuclei of the canary.

The canary vocal nuclei include two systems: hyperstriatum ventrale, pars caudale (HVc), nucleus robustus archistriatalis (RA) and nucleus hypoglossus, pars tracheosyringealis (nXIIts) compose a motor driving system for vocalization. The other group of nuclei including HVc, nucleus X of the lobus parolfactorius (Area X), nucleus dorsointermedius posterior thalami (DIP) and nucleus magnocellularis of anterior neostriatum (MAN) is a modulator of the driving system. The HVc neurons receive mono- or polysynaptic innervation from the MAN and send their fibers to Area X. Axons of Area X terminated in the thalamic nucleus, the DIP, from which neurons extended axons back to the cerebral nucleus, the MAN. Accordingly, the HVc, Area X, DIP and MAN are in a closed loop. Auditory inflow may converge on HVc neurons, partly from either the MAN or Area X during feedback control of the song. Thalamic neurons in the DIP responded to MAN stimulation, and to tonal stimuli, with relatively long latency. The interconnections between the HVc and MAN neurons are presumably central in voco-auditory integration during song-learning.