Rules, rhythm and grouping: auditory pattern perception by birds

The origins of musicality in the motion of primates

Animals communicate acoustically to report location, identity, and emotive state to conspecifics. Acoustic signals can also function as displays to potential mates and as territorial advertisement. Music and song are terms often reserved only for humans and birds, but elements of both forms of acoustic display are also found in non-human primates. While culture, bonding, and side-effects all factor into the emergence of musicality, biophysical insights into what might be signaled by specific acoustic features are less well understood.

OBJECTIVES

Here we probe the origins of musicality by evaluating the links between musical features (structural complexity, rhythm, interval, and tone) and a variety of potential ecological drivers of its evolution across primate species. Alongside other hypothesized causes (e.g. territoriality, sexual selection), we evaluated the hypothesis that perilous arboreal locomotion might favor musical calling in primates as a signal of capacities underlying spatio-temporal precision in motor tasks.

MATERIALS AND METHODS

We used musical features found in spectrographs of vocalizations of 58 primate species and corresponding measures of locomotion, diet, ranging, and mating. Leveraging phylogenetic information helped us impute missing data and control for relatedness of species while selecting among candidate multivariate regression models.

RESULTS

Results indicated that rapid inter-substrate arboreal locomotion is highly correlated with several metrics of music-like signaling. Diet, alongside mate-choice and range size, emerged as factors that also correlated with complex calling patterns.

DISCUSSION

These results support the hypothesis that musical calling may function as a signal, to neighbors or potential mates, of accuracy in landing on relatively narrow targets.

On reappearance and complexity in musical calling

Music is especially valued in human societies, but music-like behavior in the form of song also occurs in a variety of other animal groups including primates. The calling of our primate ancestors may well have evolved into the music of modern humans via multiple selective scenarios. But efforts to uncover these influences have been hindered by the challenge of precisely defining musical behavior in a way that could be more generally applied across species. We propose an acoustic focused reconsideration of "musicality" that could help enable independent inquiry into potential ecological pressures on the evolutionary emergence of such behavior. Using published spectrographic images (n = 832 vocalizations) from the primate vocalization literature, we developed a quantitative formulation that could be used to help recognize signatures of human-like musicality in the acoustic displays of other species. We visually scored each spectrogram along six structural features from human music-tone, interval, transposition, repetition, rhythm, and syllabic variation-and reduced this multivariate assessment into a concise measure of musical patterning, as informed by principal components analysis. The resulting acoustic reappearance diversity index (ARDI) estimates the number of different reappearing syllables within a call type. ARDI is in concordance with traditional measures of bird song complexity yet more readily identifies shorter, more subtly melodic primate vocalizations. We demonstrate the potential utility of this index by using it to corroborate several origins scenarios. When comparing ARDI scores with ecological features, our data suggest that vocalizations with diversely reappearing elements have a pronounced association with both social and environmental factors. Musical calls were moderately associated with wooded habitats and arboreal foraging, providing partial support for the acoustic adaptation hypothesis. But musical calling was most strongly associated with social monogamy, suggestive of selection for constituents of small family-sized groups by neighboring conspecifics. In sum, ARDI helps construe musical behavior along a continuum, accommodates non-human musicality, and enables gradualistic co-evolutionary paths between primate taxa-ranging from the more inhibited locational calls of archaic primates to the more exhibitional displays of modern apes.

Vocal learning and flexible rhythm pattern perception are linked: Evidence from songbirds

We can recognize the cadence of a friend’s voice or the rhythm of a familiar song across a wide range of tempi. This shows that our perception of temporal patterns relies strongly on the relative timing of events rather than on specific absolute durations. This tendency is foundational to speech and music perception, but to what extent is it shared by other species? We hypothesize that animals that learn their vocalizations are more likely to share this tendency. Here, we show that a vocal learning songbird robustly recognizes a basic rhythmic pattern independent of rate. Our findings pave the way for neurobiological studies to identify how the brain represents and perceives the temporal structure of auditory sequences.

Rhythm perception is fundamental to speech and music. Humans readily recognize a rhythmic pattern, such as that of a familiar song, independently of the tempo at which it occurs. This shows that our perception of auditory rhythms is flexible, relying on global relational patterns more than on the absolute durations of specific time intervals. Given that auditory rhythm perception in humans engages a complex auditory–motor cortical network even in the absence of movement and that the evolution of vocal learning is accompanied by strengthening of forebrain auditory–motor pathways, we hypothesize that vocal learning species share our perceptual facility for relational rhythm processing. We test this by asking whether the best-studied animal model for vocal learning, the zebra finch, can recognize a fundamental rhythmic pattern—equal timing between event onsets (isochrony)—based on temporal relations between intervals rather than on absolute durations. Prior work suggests that vocal nonlearners (pigeons and rats) are quite limited in this regard and are biased to attend to absolute durations when listening to rhythmic sequences. In contrast, using naturalistic sounds at multiple stimulus rates, we show that male zebra finches robustly recognize isochrony independent of absolute time intervals, even at rates distant from those used in training. Our findings highlight the importance of comparative studies of rhythmic processing and suggest that vocal learning species are promising animal models for key aspects of human rhythm perception. Such models are needed to understand the neural mechanisms behind the positive effect of rhythm on certain speech and movement disorders.

Production of regular rhythm induced by external stimuli in rats

Rhythmic ability is important for locomotion, communication, and coordination between group members during the daily life of animals. We aimed to examine the rhythm perception and production abilities in rats within the range of a subsecond to a few seconds. We trained rats to respond to audio-visual stimuli presented in regular, isochronous rhythms at six time-intervals (0.5-2 s). Five out of six rats successfully learned to respond to the sequential stimuli. All subjects showed periodic actions. The actions to regular stimuli were faster than randomly presented stimuli in the medium-tempo conditions. In slower and faster tempo conditions, the actions of some subjects were not periodic or phase-matched to the stimuli. The asynchrony regarding the stimulus onset became larger or smaller when the last stimulus of the sequence was presented at deviated timings. Thus, the actions of the rats were tempo matched to the regular rhythm, but not completely anticipative. We also compared the extent of phase-matching and variability of rhythm production among the interval conditions. In interval conditions longer than 1.5 s, variability tended to be larger. In conclusion, rats showed a tempo matching ability to regular rhythms to a certain degree, but maintenance of a constant tempo to slower rhythm conditions was difficult. Our findings suggest that non-vocal learning mammals have the potential to produce flexible rhythms in subsecond timing.

The rhythm of attention: Perceptual modulation via rhythmic entrainment is lowpass and attention mediated

Modulation patterns are known to carry critical predictive cues to signal detection in complex acoustic environments. The current study investigated the persistence of masker modulation effects on postmodulation detection of probe signals. Hickok, Farahbod, and Saberi (Psychological Science, 26, 1006-1013, 2015) demonstrated that thresholds for a tone pulse in stationary noise follow a predictable periodic pattern when preceded by a 3-Hz amplitude modulated masker. They found entrainment of detection patterns to the modulation envelope lasting for approximately two cycles after termination of modulation. The current study extends these results to a wide range of modulation rates by mapping the temporal modulation transfer function for persistent modulatory effects. We found significant entrainment to modulation rates of 2 and 3 Hz, a weaker effect at 5 Hz, and no entrainment at higher rates (8 to 32 Hz). The effect seems critically dependent on attentional mechanisms, requiring temporal and level uncertainty of the probe signal. Our findings suggest that the persistence of modulatory effects on signal detection is lowpass in nature and attention based.

Sound sequences in birdsong: how much do birds really care?

The complex and melodic nature of many birds' songs has raised interest in potential parallels between avian vocal sequences and human speech. The similarities between birdsong and speech in production and learning are well established, but surprisingly little is known about how birds perceive song sequences. One popular laboratory songbird, the zebra finch (Taeniopygia guttata), has recently attracted attention as an avian model for human speech, in part because the male learns to produce the individual elements in its song motif in a fixed sequence. But psychoacoustic evidence shows that adult zebra finches are relatively insensitive to the sequential features of song syllables. Instead, zebra finches and other birds seem to be exquisitely sensitive to the acoustic details of individual syllables to a degree that is beyond human hearing capacity. Based on these findings, we present a finite-state model of zebra finch perception of song syllable sequences and discuss the rich informational capacity of their vocal system. Furthermore, we highlight the abilities of budgerigars (Melopsittacus undulatus), a parrot species, to hear sequential features better than zebra finches and suggest that neurophysiological investigations comparing these species could prove fruitful for uncovering neural mechanisms for auditory sequence perception in human speech. This article is part of the theme issue 'What can animal communication teach us about human language?'

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

Why does human speech have rhythm? As we cannot travel back in time to witness how speech developed its rhythmic properties and why humans have the cognitive skills to process them, we rely on alternative methods to find out. One powerful tool is the comparative approach: studying the presence or absence of cognitive/behavioral traits in other species to determine which traits are shared between species and which are recent human inventions. Vocalizations of many species exhibit temporal structure, but little is known about how these rhythmic structures evolved, are perceived and produced, their biological and developmental bases, and communicative functions. We review the literature on rhythm in speech and animal vocalizations as a first step toward understanding similarities and differences across species. We extend this review to quantitative techniques that are useful for computing rhythmic structure in acoustic sequences and hence facilitate cross-species research. We report links between vocal perception and motor coordination and the differentiation of rhythm based on hierarchical temporal structure. While still far from a complete cross-species perspective of speech rhythm, our review puts some pieces of the puzzle together.