| Adult neuronal replacement is often linked to learning, but the details of this link remain unclear. In zebra finches, new neurons are recruited to the song nucleus HVC at the highest levels during the sensitive period for song imitation, but once learning is over new neuron recruitment declines throughout life. Song imitation can be blocked by deafening, interfering with song system nucleus LMAN, or by removing the innervation of the syringeal muscles. The sensitive period in which zebra finches imitate song can also be extended beyond the normal 90 day period by isolating juveniles from male tutors. We looked at new neuron numbers in zebra finches that were (A) blocked from imitating song by any of the three manipulations above and (B) isolated to extend the sensitive period. We labeled new neurons in experimental and control birds with BrdU, a cell-birth marker, at multiple time points throughout development and perfused them 30 days after BrdU injection. In the youngest birds, in the midst of the sensitive phase for song learning (post-hatching days 30–65), there were no differences between experimental birds and controls in terms of BrdU-labeled neuron numbers. As the sensitive period came to a close, differences in new neuron recruitment appeared in isolates (90–120 or 120–150) and in birds that received unilateral tracheosyringeal denervation (d61–91). No differences were seen in birds that were blocked from imitating song by deafening, LMAN lesion, or bilateral syringeal denervation (d61–91). Thus, we only measured differences in BrdU-labeled neuron counts in birds that were impeded, but not blocked, from imitating a model. These birds maintained more new neurons in HVC thirty days after labeling when compared to controls, but had the same total number of new neurons in HVC. We inferred that these birds had a higher rate of neuronal turnover. Perhaps in the absence of a ‘correct’ learning context (access to a model and bilateral control of the syrinx), the brain withholds commitment to patterns it has learned by maintaining a higher rate of neuronal turnover. When satisfactory learning does occur, then commitment to a learned skill dampens neuronal replacement. |
