How do we perceive musical beats?
Have you ever found yourself subconsciously nodding your head, tapping a finger or matching your footsteps to the beat of a tune? You’re not alone. Across cultures, humans are uniquely attuned to identifying timing-related regularities in musical rhythms in a seemingly effortless way. For many, the beat simply “pops out”.
Yet under the hood many brain regions are hard at work. In particular, the putamen – a nugget-shaped area deep within the brain– activates in the presence of a beat structure. To some this may come as a surprise. As part of the basal ganglia, a highly interconnected network of neuron clusters, the putamen is perhaps best known for its role in the timing and control of automatic movements. Why is it responding to musical beats? What exactly is it doing?
One theory suggests that the putamen “scans” musical rhythms in search of a beat; its main role is to pick out regularity buried in cacophony. Another theory proposes that it only activates after a beat is found; here the idea is that the putamen helps generate and maintain an internal model of an existing beat, something dubbed “beat continuation”. Teasing the two apart would not only add to our understanding of the putamen’s role in music processing, but also reveal how we perceive temporal patterns in general.
Researchers used functional magnetic resonance imagining (fMRI) to monitor real-time changes in brain activity as 24 volunteers – some with musical training, others without –listened to short clips of musical rhythms of various types. Nonbeat clips contained no identifiable beats for the entire duration; new beat clips started with a stretch of irregular sequences but shifted to an obvious beat midway. The other clips contained an existing beat at the beginning and either continued the beat with the same or different rhythmic pattern (think different drum solos with the same beat), or adjusted it to slow down or speed up. As they listened, volunteers rated how much the rhythm contained a beat on a scale of 1-4.
Researchers found strikingly different results between rhythmic ratings and putamen activation. Volunteers deemed nonbeat clips the least rhythmic and new and continued beat clips the most. Yet the putamen remained stoically quiet in both nonbeat and new conditions; in other words, it didn’t seem to attend to an emerging beat. Instead, it strongly activated during clips with a continued beat and, to a lesser extent, in clips with adjusted rhythms. A deeper look into the data revealed that the right putamen activated more when the beat sped up as opposed to slowed down. A sudden speeding of temporal rhythm, in which a beat occurs slightly earlier than predicted, is processed as a larger change – and thus less continuity- than slowing down the beat.
Thus it seems that the putamen is involved in beat continuation rather than beat seeking, acting as an internal metronome to predict when the next beat will land. Like an eager child, the putamen shouts out its predictions when it is steadily accurate, such as in cases with beat continuation, but quiets down when the beat becomes less predictable and mistakes occur.
But who listens to the putamen? Whole-brain analysis revealed two motor-related cortical areas – the supplementary motor area and premotor cortex – that are highly connected to the putamen and activate in tandem during beat-containing rhythms. Researchers think that these areas may compute different aspects of rhythmic processing. For example, the putamen may spontaneously maintain an internal beat via its “predictive powers” even when you are not actively focusing on the external musical beat. In contrast, the two cortices may handle attention-demanding, movement-related features of rhythmic processing, such as the planning of movements coupled to the rhythm.
Surprisingly, in these experiments musical training did not influence putamen activity, although musicians did display brain activation patterns associated with less effort (or more efficiency) during the task. Once again, it seems that most people perceive musical beats spontaneously without any effort. A tragic case in which this ability breaks down is in people with Parkinson’s disease: as neurons in their midbrain degenerate, they lose control over both motor and musical rhythmic processing. They no longer feel changes in the beat.
Taken together, the putamen, as part of the basal ganglia, appears to play the role of a generic rhythmic predictor –but not detector - in speech, music and motor control. Along with its interconnected brain areas, the putamen maintains internal predications based in part on previously identified temporal patterns. This quirk in neural wiring may contribute to our intimate relationship with music: seemingly unique amongst the animal kingdom, we are endowed with the ability to feel the rhythm and groove to the beat.
Grahn JA, & Rowe JB (2013). Finding and feeling the musical beat: striatal dissociations between detection and prediction of regularity. Cerebral cortex (New York, N.Y. : 1991), 23 (4), 913-21 PMID: 22499797
