Swing Jazz and Neural Oscillations
If you’ve ever listened to swing music, you may have noticed that the notes aren’t evenly spaced — there’s usually a little bit of extra time between eighth-notes that results in the characteristic “swinging” feel of the music. Whether you like swing or not, you can’t deny that for many people, this unevenness has a certain aesthetic appeal. But why is this noticeable? Why don’t we just attune to the adjusted beat?
A recent study shed some light on this very topic in a fascinating use of neuroimaging. The focus of the article was non-metrical rhythms; “metrical” meaning evenly spaced and on an integer ratio. For example, 4/4 time is metrical, as is a 1:3 ratio (which signifies a note being played at the beginning of a measure, 33% of the way through a the measure, and 67% of the way through the measure). A non-metrical rhythm, then, is something like 1:2.89.
The researchers used an EEG component — the omission elicited potential (OEP) — to see when their subjects predicted a note while listening to music. The OEP is a positive deflection seen when the brain is expecting a stimulus, but doesn’t perceive it. By analyzing the time at which the OEP occurs, it can be determined when the stimulus was expected. The researchers played beats for participants that included a 33% subdivision (1:3), a 43% subdivision, and a 53% subdivision.
What they found was that rhythms are regularized by the brain, meaning that participants expected the notes to be closer to an integer ratio than they actually were. Specifically, when the 33% stimulus was omitted from a trial, an OEP occurred around the 33% mark, or very near to when that note would have been played had it not been omitted. However, when the 43% note was omitted, the OEP occurred later than 43% — closer to the 1:2 mark. The same happened when the 53% stimulus was omitted, only this time the OEP came before the note would have been played, also moving closer to the 50% subdivision.
Although the brain doesn’t adjust its expectations all the way to a metrical rhythm (i.e., the OEP did not occur exactly at the 50% mark), it does seem to regularize non-metrical patterns to a certain degree, creating violations of our expectations with deviations from the expected pattern. Interestingly, expectations are not adjusted to the nearest metrical possibility, but to the nearest major one. For example, a 2/5 time would place a note closer to the 43% subdivision, but this is a strange and uncommon rhythm, so it was not the target of regularization.
The authors explain the creation of these expectations as effects of neural oscillatory mechanisms, one of the methods posited to perform temporal measurement in the brain. These oscillations coordinate and help us keep time, and the form the expectations that, when violated, result in the OEP.
Is it the violations of these assumptions that creates the appeal of swing music and other genres that use unusual timing and time signatures, such as other forms of jazz, indie, and metalcore? It’s hard to say. But it seems that our brains — at least for those of us who have grown up with Western music, which generally uses a specific timing system, do have a specific reaction to this musical property. Exactly what this finding means for neuroscience and for music is yet to be seen.
Motz BA, Erickson MA, & Hetrick WP (2013). To the beat of your own drum: cortical regularization of non-integer ratio rhythms toward metrical patterns. Brain and cognition, 81 (3), 329-36 PMID: 23434916