Experience affects recruitment of new neurons but not adult neuron number.

It is not known whether the addition of new neurons to the high vocal center (HVC) of juvenile zebra finches permits vocal learning or is the consequence of it. To tease apart these two, we performed surgery on 26-d-old juveniles. The operations were removal of both cochleae and unilateral or bilateral denervation of the syrinx. Ability to imitate a tutor song was little affected by unilateral syringeal denervation but was severely hindered by bilateral denervation or deafening. Recruitment of new HVC neurons was studied by injecting BrdU, a cell birth marker, on post-hatching days 61-65 and killing the animals 30 d later. Deafening or bilateral denervation did not alter the number of BrdU-labeled neurons in HVC, but unilateral denervation nearly doubled this number in the intact side. This doubling was transient, was blocked by deafening, and was not seen in birds that received BrdU injections earlier or later in vocal ontogeny. The adult number of HVC neurons was not affected by any of our surgical procedures. Apparently experience does not affect the total number of neurons in adult HVC, but some kinds of experience can, during narrowly defined times, influence the recruitment of new HVC neurons.