“Bird Song Metronomics”: Isochronous Organization of Zebra Finch Song Rhythm

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.

Transformation of temporal sequences in the zebra finch auditory system

“…In effect, we chose click patterns that, in the primary auditory area, elicited a temporal response that loosely overlapped with the natural song response. We note that the selected inter-click intervals are also similar to intervals between sub-syllabic acoustic transitions found in zebra finch song (Amador et al, 2013; Norton and Scharff, 2016). For comparison, Figure 2 also shows the L2a PSTH inter-peak interval histogram for click sequences slowed by a factor of two.…”

Rhythm in speech and animal vocalizations: a cross‐species perspective

“…In addition, zebra finches’ songs exhibit a form of isochronous regularity: syllable onsets coincide, more often than not, with regular “beats” of an idealized isochronous grid (Fig. ) . Evidence for the interplay between plasticity and regularity makes intuitive sense: an underlying isochronous grid can provide anchor points in time from which songs can be learned, structured, and flexibly varied.…”

“…) to acquire and produce simple and slightly more complex temporally structured sequences, vocal learning alone does not suffice to acquire hierarchical temporal structures found in human speech and music . For example, zebra finches produce temporally structured syllable sequences and can perceptually group auditory input . Rhesus monkeys can produce single intervals and synchronize to a metronome, while macaques display auditory grouping .…”

“…2). 22 Evidence for the interplay between plasticity and regularity makes intuitive sense: an underlying isochronous grid can provide anchor points in time from which songs can be learned, structured, and flexibly varied.…”

Birds have swing: Multifractal analysis reveals expressive rhythm in birdsong

“…Rhythm, as the timing of individual notes within the sequence has rarely been investigated. An exception is a recent study showing that note timing in zebra finches is structured by an underlying fast, isochronous pulse that is very stable within individuals 38 , but relative timing of notes to each other has not been systematically investigated here. This also applies to a study by Saar et al 39 , who developed a method to visualize how zebra finch motif length increases across development by adding more notes.…”