Tuesday, November 13, 2012

Why dissonance strikes a wrong chord in the brain

Here’s the pre-edited version of my latest news story for Nature. There is a lot more one might say about this, in terms of what it does or doesn’t say about our preferences for consonance/dissonance. At face value, the work could be interpreted as implying that there is something ‘natural’ about a preference for consonance. But the authors say that the issue of innateness simply isn’t addressed here, and they suspect learning plays a big role. After all, it seems that children don’t express preferences for consonant chords until the ages of 8-9 (earlier if they have musical training). The experiments which report such preferences in babies remain controversial.

Besides, one would need to test such things in non-Western contexts. McDermott agrees with Trehub’s comments below, saying “It is true that intervals that are consonant to Western ears are prevalent in some other cultures, but there are also instances where conventionally dissonant intervals common (e.g. in some Eastern European folk music; moreoever, major seconds are fairly common in harmony all over the world). So I think the jury is out as of now. There really is a need for more cross-cultural work.”

And the other big question is how much these preferences are modified when the intervals are encountered in a real musical context. McDermott says this: “We measured responses to chords in isolation, but that is obviously not the whole story. Context can clearly shape the way a chord is evaluated, and whether that can be linked to acoustic phenomena remains to be seen. That is a really interesting issue to look at in the future.” Trehub says that context “makes a HUGE difference. The so-called dissonant intervals don't sound dissonant in musical contexts. They generate a sense of motion or tension, creating expectations that something else will follow, and it invariably does. Musical pieces that are considered consonant have their share of dissonant intervals, which create interest, excitement, expectations, and more.”


A common aversion to clashing harmonies may not stem from their grating ‘roughness’

Many people dislike the clashing dissonances of modernist composers such as Arnold Schoenberg. But what’s our problem with dissonance? It’s long been thought that dissonant musical chords contain acoustic frequencies that interfere with one another to set our nerves on edge. A new study proposes that in fact we prefer consonant chords for a different reason, connected to the mathematical relationship between the many different frequencies that make up the sound.

Cognitive neuroscientists Josh McDermott of New York University and Marion Cousineau and Isabelle Peretz of the University of Montreal have evaluated these explanations for preferences about consonance and dissonance by comparing the responses of a normal-hearing control group to those of people who suffer from amusia, an inability to distinguish between different musical tones.

In a paper in the Proceedings of the National Academy of Sciences USA [1] they report that, while both groups had an aversion to the ‘roughness’ – a kind of grating sound – that is created by interference of two acoustic tones differing only slightly in frequency, the amusic subjects had no consistent preferences for any interval (two notes played together a certain distance apart on the keyboard) over any other.

Consonant chords are, roughly speaking, made up of notes that ‘sound good’ together, for example like middle C and the G above it (an interval called a fifth). Dissonant chords are combinations that sound jarring, like middle C and the C sharp above (a minor second). The reason why we should like one but not the other has long vexed both musicians and cognitive scientists.

Consonance and dissonance in music have always excited passions, in more ways than one. For one thing, composers use dissonant chords to introduce tension, which may then be relieved by consonant chords, eliciting emotional responses from music.

It has often been suggested that humans (and perhaps some other animals) have innate preferences for consonance over dissonance, so that music in which dissonance features prominently is violating a natural law and bound to sound bad. Others, including Schoenberg himself, have argued that dissonance is merely a matter of convention, and that we can learn to love it.

The question of whether an aversion to dissonance is innate or learnt has been extensively studied, but remains unanswered. Some have claimed that very young infants prefer consonance, but even then learning can’t be ruled out given that babies can hear in the womb.

However, there has long been thought to be a physiological reason why at least some kinds of dissonance sound jarring. Two tones close in frequency interfere to produce a phenomenon called beating: what we hear is just a single tone rising and falling in loudness. The greater the frequency difference, the faster the beating, and within a certain difference range it becomes a kind of rattle, called acoustic roughness, which sounds unpleasant.

Evaluating the role of roughness in musical dissonance is complicated by the fact that real tones made by instruments or voices contain many overtones – frequencies that are whole-number multiples of the basic frequency – so that there are many frequency relationships to take into account. All the same, an aversion to beating has seemed consistent with the common dislike of intervals such as minor seconds.

Yet when McDermott and colleagues asked amusic subjects to rate the pleasantness of a whole series of intervals, their responses varied enormously both from person to person and from test to test, such that on average they showed no distinctions between any of the intervals. In contrast, normal-hearing control subjects rated small intervals (minor seconds and major seconds, such as C-D) and large but sub-octave intervals (minor sevenths C-B flat and major sevenths C-B) much lower than the others.

That wasn’t so unexpected – although the near-equal preferences of the control group for mid-span intervals seems odd to Sandra Trehub, an auditory psychologist at the University of Toronto at Mississauga. “The findings from controls don't replicate the usual pattern of preferences”, she says – where, for example, there tends to be a strong preference for octaves and fifths, and an aversion to the tritone (6 semitones, such as C-F sharp). “Hearing impairment, resulting from the need to have age-matched controls, could have influenced the control ratings somewhat”, McDermott admits.

Then the researchers tested how both groups felt about roughness. They found that the amusics could hear this and disliked it about as much as the control groups. So apparently something else was causing the latter to dislike the dissonant intervals.

These preferences seem instead to stem from the so-called harmonicity of consonant intervals. The relationship between overtone frequencies in these intervals is similar to that between the overtones in a single note: they are whole-number multiples. In contrast, the overtones for dissonant intervals don’t have that relationship, but looks more like the overtones for sounds that are ‘inharmonic’, such as the notes made by striking metal.

The control group preferred consonant intervals with these harmonic relationships over artificial ones in which the overtones were subtly shifted to be inharmonic even while the basic tones remained the same. The amusics, meanwhile, registered no difference between the two cases: they seem insensitive to harmonicity.

McDermott and his coworkers have reported previously that harmonicity seems more important than roughness for dissonance aversion in normal hearers [2]. They argue that the lack of sensitivity both to harmonicity and dissonance in amusics now adds to that case.

But Trehub is not so sure. “Most amusics don't like, or are indifferent to, music”, she says, “so it strikes me as odd to examine this population as a way of understanding the basis of consonance and dissonance.”

Peretz, however, points out that amusia doesn’t necessarily rule out musical appreciation. “A few amusics listen a lot to music”, she says.

Diana Deutsch, a music psychologist at the University of California at San Diego, says that the work is “of potential interest for the study of amusia”, but questions whether it adds much to our understanding of normal hearing. In particular she wonders if many of the findings will survive in the context of everyday music listening, where people seem to display contrary preferences. “Rock bands often deliberately introduce roughness and dissonance into their sounds, much to the delight of their audiences”, she says. “And many composers of contemporary Western art music would disagree strongly with the statement that consonant intervals and harmonic tone complexes are more pleasing in general than are dissonant intervals and inharmonic tones.”

Trehub agrees, saying that there are plenty of musical traditions in which both roughness and dissonance are appreciated. “Indonesian gamelan instruments are designed to generate roughness when played together, and that quality is considered appealing. Some folk-singing in rural Croatia and Bosnia-Herzegovina involves two people singing the same melodic line one semitone apart. Then there's jazz, with lots of dissonance. It's hard to imagine a folk tradition based on something that’s inherently negative,” she says.

But McDermott says the results do not necessarily imply that there is anything innate about a preference for harmonicity, and indeed he suspect that learning plays a role. “The amusic subjects likely had less exposure to music than did the control subjects, and this could in principle contribute to some of their deficits”, he says. “So other approaches will be needed to address the innateness issue,” he says.

1. Cousineau, M., McDermott, J. H. & Peretz, I. Proc. Natl Acad. Sci. USA doi:10.1073/pnas.1207989109 (2012).
2. McDermott, J. H., Lehr, A. J. & Oxenham, A. J. Curr. Biol. 20, 1035-1041 (2010).

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