Bilateral interactions within the vocal control pathway of Birds: Two evolutionary alternatives

Bilateral interactions between the right and left sides of the vocal control pathway of three avian species, the budgerigar (Melopsittacus undulatus), the zebra finch (Poephila guttata), and the canary (Serinus can‐arius), were compared by using microstimulation. Stimulation of the neo‐striatal nucleus, hyperstriatum ventrale pars caudale (HVc), caused a constant‐latency increase in neural activity of the tracheosyringeal (ts) nerve. This nerve supplies the avian vocal organ, the syrinx. Unilateral stimulation of HVc in budgerigars caused equal responses in both right and left ts nerves, whereas in canaries and zebra finches, much larger responses with lower thresholds and shorter latencies were evoked in the ipsilateral ts nerve than in the contralateral ts nerve. Furthermore, contralateral responses in the oscines were seen only at high rates of stimulation, whereas ipsilateral responses could be evoked by a single stimulus pulse. Simultaneous bilateral stimulation of HVc in budgerigars evoked a greater than linear summation of responses on each ts nerve, evidence for convergence of ipsi‐ and contralateral inputs onto the same set of motor neurons. No such summation was detected in zebra finches or canaries. The bilateral distribution of excitation within the song control pathway of budgerigars should lead to coactivation of bilaterally paired muscles on each side of the syrinx. This type of interaction should ensure that the two sides of the parrot syrinx operate as parts of a unitary, or single‐voiced, syrinx. In contrast, the two sides of the song control pathway in canaries and zebra finches are almost completely isolated from each other, and are thus free to act independently. This type of organization lends itself to the separate control of simultaneously produced, harmonically unrelated sound elements by the two independent sound sources present in the songbird's two‐voiced syrinx.