Isochronous rhythmic organization of learned animal vocalizations

Abstract

The evolutionary path that led to music as we know it today is difficult to trace. Cross-species comparative research can help us uncover the biological substrates that enabled humans to develop this peculiar behavior. Rhythm, the organization of events in time, is a central component in the structure of all forms of music. Oftentimes musical rhythm gives rise to a perceptionally isochronous beat, or pulse. Learned vocalizations of non-human animals, such as birdsong and the songs of certain bat species, show striking parallels to vocal music (i.e. human song). This thesis investigates these vocalizations for the presence of an isochronous rhythmic structure that could allow a conspecific listener to perceive such a beat. To this end, I have developed a generate-and-test (GAT) method to extract an isochronous pulse from a temporal sequence of events, such as the onsets of notes. This method is compared to a variety of existing analytic techniques for analyzing different aspects of rhythms in vocalizations, movements and other behaviors developing over time. The suitability of the different methods for addressing particular questions is illustrated through various examples. The application of the GAT approach to different types of vocalizations of the greater sac-winged bat (Saccopteryx bilineata) revealed a common temporal regularity that might point towards an interesting relationship between physiologically determined rhythm and the rhythm of learned social vocalizations. In the songs of zebra finches (Taeniopygia guttata) we discovered a hierarchical isochronous structure that is reminiscent of the metrical structure of many types of music. We then report the effect of genetic manipulations on the song learning success of zebra finches. The expression of FoxP2, a gene involved in speech acquisition and birdsong learning, as well as of two related genes, FoxP1 and FoxP4, was experimentally reduced in juvenile birds during their learning period. Among other effects, the adult birds produced song with an impaired isochronous structure. Surprisingly, control animals whose FoxP levels were not reduced, showed a similar effect in this regard. I discuss possible interpretations of this result in the light of current knowledge about neural mechanisms and behavioral processes of song learning and production.