| Recall and recognition of previously heard sounds is a well known but largely unexplained ability of the auditory system of humans and other animals that communicate with sounds. In species with unique, personally identifiable vocalizations, the ability to recognize and identify an individual by its vocalizations is behaviorally important. For zebra finches, previously published field studies have shown this behavioral ability (Zann, 1996), and laboratory studies have identified the auditory processing pathways. While the primary auditory cortex homolog known as field L shows specificity for species-specific sounds, it shows little or no selectivity between different conspecific songs (Grace et al., 2003), making downstream secondary cortical areas a likely candidate for further investigation. In this dissertation, I have investigated higher-order effects in the secondary auditory forebrain and found evidence of more specialized selectivity. In the first set of experiments, I adapted surgical and implant design techniques to perform anesthetized extracellular neural recordings in the caudal mesopallium. I recorded a number of conspecific stimuli of varying familiarity to the individuals being studied, and found neurons that responded selectively to familiar songs. I further quantified this selectivity via metrics incorporating spike timing. I then performed identical experiments in field L, finding no such selectivity to familiarity. Further experiments also showed that, while both areas were selective to songs of the natural direction over reversed-playback songs, the selectivity in CM was more pronounced. Seeing these results under anesthetized conditions, we sought to compare these effects to those seen in unanesthetized restrained animals. Effects of both stimulus familiarity and direction were even more pronounced in awake animals than in anesthetized ones, suggesting possible feedback effects from higher-order areas, which merit future study. Here for the first time in the zebra finch, I have shown a neural correlate for stimulus familiarity. |
