Why rhythm is heard best in the bass

Then there’s this, also for Nature news. It shows, I think, that those boring bass solos beloved of jazz and rock bands really are superfluous after all. As for that image: never mind the bollocks, Bootsy is still the Man.

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There may be a physiological reason why low-pitched instruments keep the musical beat

How come the lead guitarist gets to play the flashy solos while the bass player gets only to plod to the beat? Bassists might bemoan the injustice, but in fact their rhythmic role seems to have been determined by the physiology of hearing. According to new research, people’s perception of timing in music is more acute for lower-pitched notes.

Psychologist Laurel Trainor of McMaster University in Hamilton, Canada, and her colleagues say that their findings, published today in the Proceedings of the National Academy of Sciences USA [1], explain why in the music of many cultures the rhythm is carried by the low-pitch instruments while the melody tends to be taken by the highest pitched. This is as true for the low-pitched percussive rhythms of Indian classical music and Indonesian gamelan as it is for the walking double bass of a jazz ensemble or the left-hand part of a Mozart piano sonata.

Earlier studies have shown that people have better pitch discrimination for higher-pitched notes – one reason why saxophonists and lead guitarists often solo at a squealing register [2]. It now seems that rhythm works best at the other end of the scale.

Trainor and colleagues used the technique of electroencephalography (EEG) – electrical sensors placed on the scalp – to monitor the electrical brain signals of people listening to trains of two simultaneous piano notes, one high-pitched and the other low-pitched, equally spaced in time. Occasionally one of the two notes was played slightly earlier – by just 50 milliseconds. The researchers looked for a sign in the EEG recordings that the listeners had noticed.

This showed up as a characteristic “spike” of electrical activity produced by the auditory cortex about 120-250 milliseconds after the deviant sound reaches the ear, called a mismatch negativity (MMN). It’s a known indication that the brain senses something wrong – a kind of “huh?” response. Trainor and other coworkers used the same approach previously to detect listeners’ responses to “errors” in pitch [2].

The MMN doesn’t depend on conscious recognition of the timing error – in fact, participants were told to watch a silent movie during the tests and to pay no attention to the sounds they heard. And although Trainor says that “the timing differences are quite noticeable”, the MMN response precedes any conscious awareness of them.

The researchers found that the MMN signals were consistently larger for mis-timing of the lower note than for the higher note. They also measured the participants’ ability to adjust their finger-tapping to deviant timings of the notes, and found that it was significantly better for the lower notes.

When the researchers used a computer model to figure out how the ear responds to their test sounds, they found that the signal from the auditory nerve connected to the cochlea showed more ambiguity about the timing of an early high-pitch note than a low note. They think this suggests that the differences arise at a fairly “low-level” stage of cognitive processing.

Cognitive scientist Tecumseh Fitch of the University of Vienna says that the study “provides a very plausible hypothesis for why bass parts plays such a crucial role in rhythm perception.”

“The fact that we are more sensitive to the timing of a low note compared to high note is surprising”, says cognitive musicologist Henkjan Honing of the University of Amsterdam. Nevertheless, he adds that “there are plenty of alternative interpretations” of the results.

For instance, he says, “the use of a piano tone could be contributing to the difference observed – different timbres should be used to prove it's really the pitch that causes the effect.”

And for louder, deeper bass notes than were used in these tests, says Fitch, we might also feel the resonance in our bodies, not just hear it in our ears. “I’ve heard that when deaf people dance they turn up the bass and play it very loud and can literally ‘feel the beat’ via torso-based resonance”, he says.

References
1. Hove, M. J., Marie. C., Bruce, I. C. & Trainor, L. J. Proc. Natl Acad. Sci. USA doi:10.1073/pnas.1402039111 (2014).
2. Fujioka, T., Trainor, L. J., Ross, B., Kakigi, R. & Pantev, C. J. Cogn. Neurosci. 17, 1578-1592 (2005).