Researchers from the University of Amsterdam and Leiden University have discovered that humans and songbirds use markedly different strategies to recognize sequences of complex sounds. The findings shed new light on how biology and experience shape the way we perceive complex sounds, with implications for music and speech.
When people hear a familiar melody, they generally follow its pitch pattern (how high or low a sound is), even when it is played on another instrument. But do songbirds recognize sound patterns in the same way?
A new study by researchers from the University of Amsterdam and Leiden University suggests that they do not. Using a listening task in which humans and zebra finches were exposed to the same artificial sounds, the researchers found that humans attend predominantly to pitch, whereas zebra finches combine pitch with changes in the spectral structure—or formants—of these sounds.
‘These findings show that vocal-learning species (such as humans and zebra finches) do not necessarily organize the same sounds in the same way’, states corresponding author Professor Henkjan Honing, director of the Music Cognition Group at the University of Amsterdam. ‘How we recognize complex sounds seems to depend on two things working together: species-specific auditory predispositions and what experience has taught us to pay attention to.’
A direct comparison between species
The researchers trained humans and zebra finches to distinguish sequences of five harmonic tones. They then presented modified versions of these sequences in which the pitch contour, formant contour, or both had been selectively changed.
The results revealed a clear difference. Zebra finches treated pitch and formant trajectories as complementary components of the sound pattern. Either dimension could support recognition when the other was made less informative.
Human listeners, by contrast, based their judgements primarily on pitch whenever pitch information was available. Changes to the formant pattern had relatively little influence on how they classified the sequences.
'Our findings show that humans and zebra finches can both use information about changes in pitch and spectral structure', says co-author Dr Zhi-Yuan Ning (at the time of this study working at the Institute of Biology Leiden of Leiden University). 'The key difference lies not simply in which acoustic features they can detect, but in how they use and combine those features when recognizing a sound sequence. Humans strongly prioritize pitch, whereas zebra finches combine pitch with changes in spectral structure, represented in the experiment by shifting formant patterns.'
Musical experience is linked to more flexible listening
The study also found that musical experience influenced human performance. When reliable pitch information was removed, listeners with more musical experience were better able to recognize the relationships within the sequences by using corresponding changes in the spectral structure. The response pattern suggests that experienced listeners were able to map relationships learned in pitch onto changes in another acoustic dimension.
'Musical training appears to make auditory processing more flexible', says Honing. 'Pitch remains the dominant dimension for human listeners, but people with more musical experience seem better able to recruit another acoustic dimension when pitch is no longer reliable.'
What comparisons with birds can tell us about evolution
The contrast between humans and songbirds challenges the assumption that pitch-based listening is a universal consequence of vocal learning. 'It shows that animals and humans do not necessarily process the same sounds in the same way', says Professor Carel ten Cate of Leiden University. 'Both zebra finches and humans can perceive pitch and spectral information, but they assign different weights to these features. Comparisons of this kind help us distinguish aspects of auditory processing that may be deeply rooted in evolution from those shaped by individual experience and human culture.'
Rather than demonstrating a simple biological divide between humans and birds, the findings indicate that listening strategies develop through an interplay between inherited auditory predispositions and experience. One possibility is that zebra finches are predisposed to integrate several properties of complex sounds, whereas extensive human experience with pitch-rich music and speech reinforces pitch as a default dimension for auditory pattern recognition.
Beyond music
The findings are relevant not only to music. Pitch and formant information also play important roles in speech, animal communication and the organization of complex auditory scenes.
Understanding how different species organize these dimensions can therefore contribute to broader theories of auditory cognition, vocal learning and the biological foundations of language and musicality.