0.7 Music, biological evolution, and the brain  (Page 12/30)

This question is particularly salient since BPS is distinct in a number of ways from other examples of animal entrainment in nature, e.g. the synchronous chirping of katydids (Greenfield and Schul, 2008). For example, human synchronization to music is very flexible in terms of tempo, is a response to complex sound sequences (not just pulse trains), and is truly cross-modal since it often involves silent rhythmic movement in response to sound. No other species combines these features in their natural entrainment behavior (Patel et al., 2009a). Furthermore, familiar domestic animals such as dogs and cats show no tendency for spontaneous rhythmic movement to music, even though they have lived with humans and their music for thousands of years. Indeed, BPS has been proposed to be a uniquely human ability (Bispham, 2006), reflecting natural selection for musical behavior in our species, perhaps in the service of promoting group cohesion (cf. Dunbar, in press and section 2.1 above).

Yet neurobiology suggests that BPS may have hidden connections to brain systems with other “day jobs.” Specifically, BPS may build on the brain circuitry for complex vocal learning, a trait shared by humans and only a few other groups of mammals and birds. Vocal learning is associated with specific evolutionary modifications to the brain (Jarvis, 2009) and, like BPS, involves a high degree of neural integration between the auditory and motor systems (Patel et al., 2005). The “vocal learning and rhythmic synchronization hypothesis” (Patel, 2006) posits that vocal learning provides a neurobiological foundation for BPS. One prediction of this hypothesis is that non-vocal-learning species, which includes all non-human primates, are incapable of BPS. While direct tests of this prediction are still needed via training studies involving movement to music, some support is provided by a recent study that attempted to teach rhesus macaques to synchronize their finger movements to a metronome (Zarco et al., 2009). Despite more than a year of concerted training (six days/week, four hours/day), the monkeys were unable to learn to align their taps in time with the metronome signal—a task that is easy for humans, even young children with no musical training (McAuley et al., 2006).

Additional support for the vocal learning hypothesis has recently been provided by the discovery of entrainment to human music in several parrot species (Patel et al., 2009b, Schachner et al., 2009). Tempo flexibility was demonstrated in an experiment with a sulphur-crested cockatoo ( Cacatua galerita eleonora ), in which the tempo of a song was manipulated to create different versions ranging from twenty percent slower to twenty percent faster than the original song. The animal was able to synchronize its head bobs to the beat of the music at several different tempi. Synchronization occurred in “bouts,” or periods of sustained entrainment, interspersed in longer episodes of dancing not synchronized to the beat. Interestingly, the non-synchronized dancing was dominated by a preferred tempo of rhythmic movement, and synchronization was best when the musical tempo was near this preferred tempo (Patel et al., 2009c), patterns that have also been observed in how human children move to music (Eerola et al., 2006). Thus, it appears that parrots may resemble human children (vs. adults) in terms of how they move to rhythmic music, though further research is needed to test this idea.