An investigation into the rules governing neuron addition in the adult zebra finch: Potential roles of hearing-dependent social interaction, age, and nest of origin

The birth of new neurons in the adult brain is a widespread phenomenon that occurs in all vertebrates examined to date. While many hypotheses have been put forth to explain the functions of adult neurogenesis, none are satisfactory. Songbirds offer a unique system to discover the contributions that new neurons make to behavior. One of the main areas for controlling the production of song, the High Vocal Center (HVC), receives new neurons in adulthood. As a crucial step in understanding why neurogenesis occurs, investigations of factors that might regulate neuron addition were undertaken. There are conflicting reports on the role of auditory experience in the regulation of HVC neuron addition. I hypothesized that the effect of deafening on new neuron addition depends upon social housing conditions. To test this hypothesis, neuron addition was studied in hearing and deaf birds in different housing conditions. [ 3H]-thymidine was used for cell birthdating because in direct comparisons with BrdU, it was found to be more sensitive. When groups of hearing and deaf birds were housed separately neuron addition did not differ between treatments. However, when hearing and deaf birds were housed together, in mixed housing conditions, deaf birds had an increase in neuron addition compared to hearing birds. Moreover, deaf birds showed an even greater increase in neuron addition when they were housed with hearing birds that were unfamiliar to them. Deafening did not affect HVC volume or total neuron number across the housing conditions. Previous work has shown a positive correlation between neuron addition and singing. In the present work, deafening influenced neuron addition in housing conditions known to maximize singing. We propose that under these conditions there is the greatest disparity between hearing and deaf birds in singing rate or song-related auditory feedback. Furthermore, HVC neuron addition was highly correlated between nest mates, and this correlation persisted regardless of whether nest mates were hearing or deaf. Together these findings suggest that variability in adult neuron incorporation can be accounted for both by experiences in adulthood as well as experiential or genetic factors associated with nest of origin.