The relationship between rates of HVc neuron addition and vocal plasticity in adult songbirds

In adulthood, songbird species vary considerably in the extent to which they rely on auditory feedback to maintain a stable song structure. The continued recruitment of new neurons into vocal motor circuitry may contribute to this lack of resiliency in song behavior insofar as new neurons that are not privy to auditory instruction could eventually corrupt established neural function. In a first step to explore this possibility, we used a comparative approach to determine if species differences in the rate of vocal change after deafening in adulthood correlate positively with the extent of HVc neuron addition. We confirmed previous reports that deafening in adulthood changes syllable phonology much more rapidly in bengalese finches than in zebra finches. Using [(3)H]thymidine autoradiography to identify neurons generated in adulthood, we found that the proportion of new neurons in the HVc one month after labeling was nearly twice as great in bengalese than in zebra finches. Moreover, among the subset of HVc vocal motor neurons that project to the robust nucleus of the archistriatum, the incidence of [(3)H]thymidine-labeled neurons was nearly three times as great in bengalese than in zebra finches. This correlation between the proportion of newly added neurons and the rate of song deterioration supports the hypothesis that HVc neuron addition may disrupt stable adult song production if new neurons cannot be "trained" via auditory feedback.