Bass lays down the beat

Try tapping your finger to a song - any song. Are you focusing on the melodious vocals, or the strong pulsating beat of the bass?

Whether it's the walking bass lines in jazz, the left-hand rhythms in ragtime piano, or the electronic oompah-oompah in dance, across musical genres low-pitched bass (usually) provides the rhythmic drive. Are humans hard-wired to seek out musical beats in lower tones?

Using EEG, researchers recorded the brain activities of volunteers as they listened to a simple piano piece with two streams: one high in pitch, the other low. Although both tones were generally played together in a predictable beat, occasionally one tone would jump the gun, occurring 50 milliseconds ahead of its partner.

When the unpredictable element was low in pitch, parts of the volunteers' brains – including frontal and occipital regions – shot up in activity to signal the deviant beat. Conversely, the brain's mismatch response was much lower for the high-pitched tone, suggesting that the auditory cortex is less sensitive to the rhythmic information encoded in these sounds.

In a separate task, the volunteers were asked to strum their fingers to the rhythm of another two-toned musical piece. They adapted much faster to the out-of-synch beat when it was carried by the low-pitched, rather than high-pitched, tone.

Together, these experiments suggest that lower notes have a greater influence on rhythm processing than higher ones – something the authors dubbed the "low-voice superiority effect". Why this happens is still a mystery, although computer modelling suggests that the cochlea in the inner ear is involved.

The researchers hypothesize that the wide-spread convention of using bass-ranged instruments to carry rhythm stems from this basic property of our auditory system. It's a cool theory, but doesn't hold across cultures and history. Most traditional Chinese folk music, for example, uses both high and low pitches to lay down rhythm; the ska stroke is an important rhythmic base in reggae, yet it's not necessarily high in pitch.

If you want to know more about how we predict musical beats, check out this article.

Hove MJ, Marie C, Bruce IC, & Trainor LJ (2014). Superior time perception for lower musical pitch explains why bass-ranged instruments lay down musical rhythms. Proceedings of the National Academy of Sciences of the United States of America PMID: 24982142