Abstract
The human capacity for speech and vocal music depends on vocal imitation. Songbirds, in contrast to non-human primates, share this vocal production learning with humans. The process through which birds and humans learn many of their vocalizations as well as the underlying neural system exhibit a number of striking parallels and have been widely researched. In contrast, rhythm, a key feature of language, and music, has received surprisingly little attention in songbirds. Investigating temporal periodicity in bird song has the potential to inform the relationship between neural mechanisms and behavioral output and can also provide insight into the biology and evolution of musicality. Here we present a method to analyze birdsong for an underlying rhythmic regularity. Using the intervals from one note onset to the next as input, we found for each bird an isochronous sequence of time stamps, a “signal-derived pulse,” or pulseS, of which a subset aligned with all note onsets of the bird's song. Fourier analysis corroborated these results. To determine whether this finding was just a byproduct of the duration of notes and intervals typical for zebra finches but not dependent on the individual duration of elements and the sequence in which they are sung, we compared natural songs to models of artificial songs. Note onsets of natural song deviated from the pulseS significantly less than those of artificial songs with randomized note and gap durations. Thus, male zebra finch song has the regularity required for a listener to extract a perceived pulse (pulseP), as yet untested. Strikingly, in our study, pulsesS that best fit note onsets often also coincided with the transitions between sub-note elements within complex notes, corresponding to neuromuscular gestures. Gesture durations often equaled one or more pulseS periods. This suggests that gesture duration constitutes the basic element of the temporal hierarchy of zebra finch song rhythm, an interesting parallel to the hierarchically structured components of regular rhythms in human music.
Highlights
Rhythm is a key element in the structure of music and can be defined as the “systematic patterning of sound in terms of timing, accent and grouping” (Patel, 2008, p. 96)
For each of the 15 analyzed adult male zebra finches we found an isochronous pulseS that coincided with all note onsets, using two independent analysis methods
Using a generate-and-test approach (GAT; see Section Materials and Methods) we identified for each chunk of a bird’s recording a pulseS that fitted best to the note onsets, i.e., had the lowest frequency-normalized root-mean-square deviation (FRMSD; Figure 1)
Summary
Rhythm is a key element in the structure of music and can be defined as the “systematic patterning of sound in terms of timing, accent and grouping” (Patel, 2008, p. 96). 211) traditions, the timing of sonic events, mostly note onsets, is structured by a perceptually isochronous pulse (Nettl, 2001). This pulse is a cognitive construct that is usually implicit rather than being materialized in the acoustic signal itself For example, the pulse is perceptually divided into groups of three, of which the first one—the socalled downbeat—is perceived as more strongly accented than the following two (“one, two, three, one, two, three”) In this example, pulses on the lower level of the metrical hierarchy, i.e., every pulse, happen at three times the tempo of the higher level, consisting of only the strong pulses. The process of finding the pulse and frequently the subsequent attribution of meter allow us to infer the beat of a piece of music
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