A Fractal Model of Musical Complexity Biological and Behavioral Support for the Social Bonding Theory of Music

Abstract

Throughout history, humans have gathered to create, produce, or listen to music. The ubiquity of behavior suggests music provides a social bonding mechanism, a concept however, that remains theoretically controversial. This dissertation uses four studies to examine social bonding theory, by testing the hypothesis that music structure and social context interact in the brain to produce pro-social behaviors, such as music preference similarity. To begin, we quantify and validate musical structure by employing a fractal model (1/f βof pitch interval complexity, and measure the effects on higher order systems such as perception (i.e., complexity, melodicity), emotion (i.e., mood, preference), and cognition (i.e., memory) in both a novel and repeated exposure paradigm. Results show that when complexity reflects an optimal ratio of predictability to unpredictability, random tone sequences evoke the perception of music, positive mood, and near perfect memory recognition. In addition, optimal levels are unaffected by repeated exposure, but responses to higher and lower levels become more music-like as exposure increases, providing the first evidence of a categorical response to different levels of musical complexity. Neurally, we show that optimal levels of complexity engage the primary sensory cortex (i.e., bilateral A1) and the sub-cortical reward system, specifically the nucleus accumbens (NAcc), a structure known to process both pleasant music and social rewards. We finish by showing that for an adolescent population, social context (i.e., knowledge of peer ratings) interacts with the level of complexity. When sequences are initially rated as musical, knowledge of positive peer ratings increases the magnitude of ratings. In contrast, when highly complex sequences are initially rated as ambiguous (i.e., neither musical nor not musical), negative peer ratings result in subsequent strong non-musical ratings. Together, results show that social context causes an additive effect that segregates the perception of what is rated as musical and what is not, ultimately driving preference similarity. In addition, results may account for why some preferences are universal and others are highly specific to a group or culture. Future directions are discussed in light of potential neural assessment tools and sound-based therapies to facilitate social bonding