The people of a tribe called the Tsimane’, who have been isolated from Western music, perceive music differently from Western listeners, raising questions about whether musical preference is innate or cultural. See Letter p.547
People have been making music for at least 35,000 years1. The remarkable diversity of musical expression across cultures raises the question of how the brain perceives musical sounds. Deep in the Bolivian Amazon lives a group of people, the Tsimane’, whose music is largely isolated from Western influences. On page 547, McDermott et al.2 report that the Tsimane’ perceive harmony — the relationship between multiple simultaneous tones — differently from Western listeners.
Concepts of 'consonance' and 'dissonance' in Western music are based on frequency relationships between simultaneous tones. Periodically vibrating sources such as the human vocal cords give rise to sound at a fundamental frequency, produced by oscillation of the whole source, and at integer multiples of that frequency, corresponding to vibrations of half the length of the source, one-third its length and so on. This physical principle has long been thought to be a determinant of how listeners perceive harmony. For instance, tones whose frequencies are in simple integer ratios to one another, such as 3:2 (called the perfect fifth) or 5:4 (the major third), are considered consonant and more pleasant than dissonant ones such as 16:15 (the minor second). If this hypothesis is correct, then all listeners, regardless of their culture, should prefer similar tone combinations.
However, the authors found that the Tsimane’ express no preference for consonance over dissonance. This result was replicated using several different stimuli, even dissonant versions of Tsimane’ songs. In a clever control, the experimenters showed that the Tsimane’ do prefer pleasant vocalizations, such as laughter, to unpleasant gasps, indicating that they understood what was being asked of them.
On the basis of their findings, McDermott and colleagues conclude that preference for harmonically related tones is not an innate feature of the human auditory nervous system, but is instead determined by exposure to music that contains harmonic relationships. Such a hypothesis provides much food for thought, but can be interpreted in several ways. For instance, one remarkable — and unusual — feature of Tsimane’ music is that only one line is played or sung at a time (Fig. 1). Despite repeated attempts to encourage Tsimane’ musicians to play or sing together, they demurred. It therefore seems likely that the lack of preference among the Tsimane’ for harmonically related tones is linked to the fact that simultaneous tones are irrelevant in their musical culture.
What is perhaps most surprising is the general lack of preference among the Tsimane’ for any one combination of tones over any other. Although music that contains simultaneous dissonant lines is relatively rare across cultures3, dissonance is often used to produce sonic effects or textures4, to create tension followed by resolution, or sometimes just for its own sake (for example, in contemporary jazz). Thus, rather than mapping onto a simple pleasantness dimension, dissonance and consonance may be better thought of as ways to manipulate sound expressively, and thereby engender emotions or moods. Again, the absence of multiple lines in Tsimane’ music probably explains why the Tsimane’ do not express a preference either way.
Speech may provide a useful parallel. When certain sound contrasts are absent from a given language (for example, the distinction between 'r' and 'l' sounds in Japanese), speakers lose the ability to distinguish between these sounds after a certain age5. The phenomenon represents tuning of the auditory system to the needs of the environment. Could something similar be happening here? We do not know how well the Tsimane’ can distinguish consonant from dissonant tone combinations. The authors did show that the Tsimane’ could determine (albeit with some difficulty) whether one or two tones were present on the basis of mistuning of the harmonics; but that does not quite answer the question. Lack of exposure to simultaneous tones may shape the auditory system such that the interactions between harmonics that influence preference in Western listeners are not processed in the same way, or perhaps simply not attended to.
Research into brain plasticity shows that neuronal responses to frequency and other features depend on the sounds that an individual is exposed to early in life6. Sensitivity to particular features may therefore change depending on the musical environment. Plasticity is also relevant in current models of musical pleasure, in which cultural influences and experiences shape the brain's cortical circuitry, which in turn influences the brain's reward system7.
This explanation is consistent with a strong role for environmental effects, in line with the authors' interpretation of their data. But does this idea rule out the possibility of innate factors? Not necessarily, because, despite their lack of preference for harmonically related tones, the Tsimane’ did display a similar dislike to Western listeners for roughness — the sensation of sound that is elicited by tones that are close together in frequency, especially minor and major seconds. This finding suggests that there are probably some innate biological constraints on which environmental input operates. Indeed, auditory cortical neurons in macaques show differential responses to minor and major seconds compared with other intervals8; it would be hard to argue that this effect is mediated by the monkeys' musical culture.
The findings also raise the question of how or whether harmonic preferences arise when a culture develops the use of simultaneous sounds. Although the Western harmonic system is definitely not the only way that multiple sounds can be organized, its principles are not arbitrary, and allow for the complex relationships known as tonality. Moreover, melodic (sequential) frequency relationships can also be based in part on harmonic relationships, perhaps owing to exposure to the harmonics of voiced speech9. Tsimane’ songs do use musical scales, as do most cultures3. McDermott et al. also mention that the Tsimane’ use major seconds, minor thirds and perfect fourths, all of which are based on simple integer ratios, and which are also prevalent in Western music. So the influence of harmonic relations might be expressed in melody, if not in harmony. Simple integer ratios are also prevalent in rhythm across cultures3 — it would be interesting to see whether this feature is preserved in Tsimane’ music.
Although many questions remain, this work represents an important contribution to our understanding of how the diversity of human cultural expression can influence perception. More generally, it provides clues to how the environment interacts with the nervous system to produce all manner of complex behaviours, feelings and thoughts.Footnote 1
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