From Sound to Pulse: Neural Coding of Rhythmic Features
Have you ever unconsciously found yourself tapping your foot to music? This instinctive reaction occurs due to the brain's unique ability to detect rhythm, especially one that is predictable and has a regular beat. Rhythm can be described as the pattern of long or short sounds and silences in time. The beat is a time unit and the steady pulse you can tap your feet to. The speed of the music (how fast or slow the beat occurs) is called tempo, and it influences how the rhythm is played. An easy way to visualize this is by visualizing music as walking; where the rhythm is the pattern of your steps, the beat is your steady footsteps and the tempo is how quickly you walk. Brain areas such as the supplementary motor area (SMA - involved in planning movement) and the basal ganglia areas are part of larger brain networks that help control rhythm and time in general. Both areas play vital roles in perceiving musical beats and are shown to be more active when we hear regular rhythms (strong beats). A song with a strong, easy-to-follow beat is Uptown Funk by Bruno Mars, whereas Pyramid Song by Radiohead has a weaker, less obvious beat that takes a more mental effort to track. So, what happens in the brain when the rhythm is irregular or doesn’t have a strong beat? Is the brain responding to the beat itself or can it track details such as tempo or length between sounds?
To dig deeper into how the brain processes both beat and rhythm, Dr. Hoddinnott and Dr. Grahn from Western University utilized functional magnetic resonance imaging (fMRI) and a technique called representational similarity analysis (RSA). This approach looks not just at how much a brain region is active, but how different the activity patterns are when we hear different rhythms.
In this study, participants listened to 12 unique rhythms: four with a strong beat, four with weak beat, and four with no beat at all. The researchers found that activity patterns in the SMA and the putamen (part of the basal ganglia and part of the brain’s reward and motor system) were different when participants heard strong beats versus when they heard rhythms with no beat. The greater the beat strength difference between the two rhythms, the more different the brain patterns were. This means these areas are susceptible to beat strength.
On the other hand, the auditory cortex coded rhythm without clear beat specificity, meaning it did not show distinct neural patterns for rhythms with different levels of beat strength. This study also looked at the cerebellum, which had its activity pattern altered the most by tempo. The cerebellum is involved in rhythm coding; however, it is not sensitive to the beat. This could potentially mean that different parts of the brain may specialize for different components of rhythm: some for timing or rate and others for sound.
Although other aspects of rhythm such as tempo and number of onsets (number of sounds or note beginnings in a rhythm) were tested, they did not explain the brain activity and were not considered a factor in driving the brain’s responses. Interestingly, several brain areas outside the main rhythm and motor centers also seemed to play a role. The inferior frontal gyrus (involved in language and decision-making) and the inferior parietal lobe (important for attention and integrating sensory information) appeared to respond to both the steady beat and the overall rhythm pattern. The superior frontal gyrus showed a partial representation of beat strength but was not strong or specific enough to be labeled a primary beat-sensitive region. These areas are known as association areas, parts of the brain that help us think and make sense of what we see, hear, and feel. They don’t directly control movement, but they connect different types of information, helping us do things like understand language or remember a song. Motor areas (parts of the brain that help us move) like the right primary motor cortex (located in the precentral gyrus) also showed some sensitivity to beat strength. That means these regions may help blend beat perception with a broader understanding of rhythm.
These findings suggest that our brains may utilize an organized network for rhythm perception, where beat strength is represented in the brain primarily by the SMA and putamen which show different patterns of activity depending on the strength of the beat. The auditory cortices handle raw features, and motor/association areas encode higher-level constructs like beat strength. These findings offer strong support for theories that our brain’s motor and auditory cortices work together in rhythm and beat perception and provide a neural map of how we process rhythm in the brain.
Because fMRI captures brain activity slowly (low temporal resolution), future studies could look into procedures involving higher temporal resolution to show exactly when different areas respond. This could address when beat representations emerge and how they evolve, particularly in the SMA and putamen. Another question that may be addressed could be how familiar songs may affect results. Observing a group over an extended period of time could dissociate initial beat-driven responses from learned rhythmic predictability. This may help clarify how repetition and learning modulate SMA and basal ganglia activity. The SMA and putamen may play a role in predicting the beat and the brain's ability to track time, so they may gradually change how they represent a rhythm as it becomes more predictable.
Understanding how the brain processes beat and rhythm has several promising applications. In rehabilitation, rhythmic cues are already used in therapies for movement disorders such as Parkinson’s disease; these findings could help refine those techniques by targeting beat-related brain regions like the SMA and putamen. In education, this research may inform music-based learning strategies, especially for children with language or timing-related challenges.
The findings indicate that motor and association areas of the brain are involved in how we perceive sound over time and show a link between hearing and movement - especially involving music and rhythm. Overall, this study shows that the way we hear and feel rhythm relies on a strong connection between our hearing and movement systems. It's part of what makes us tap our feet or nod along when we hear a good beat, indicating that listening to songs is not only an auditory experience but a whole-body one.
Original Article: Hoddinott, J. D., & Grahn, J. A. (2024). Neural representations of beat and rhythm in motor and association regions. Cerebral Cortex, 34(10). https://doi.org/10.1093/cercor/bhae406