Wednesday, 9 April 2014

Beat box: How the brain processes rhythm

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People have little trouble recognising and following the beat in a piece of music. We can even continue to play the beat in our minds once a song has finished. However, despite the ease with which we carry out such a task, the brain activity which underpins it remains a topic of investigation.

Finding the beat – why does it matter?
We’re all familiar with the niggling irritation of a song that’s stuck in our heads. However, few of us are aware that our knack for holding a beat in our thoughts actually makes life easier. The capacity to identify patterns in streams of sound supports many forms of human behaviour, including moving, speaking and listening. 

If the ability to generate this internal rhythm is disrupted, such as in Parkinson’s disease, problems begin to arise. People with Parkinson’s disease have difficulty with psychological tasks, such as holding a beat in their minds, as well as with practical tasks, such as walking. The more we know about the regions of the brain that are causing these difficulties, the more effective we will be in designing treatments to combat them. Previous research suggests that a set of brain structures known as the basal ganglia are involved in identifying and following a beat.

What are the basal ganglia?
The basal ganglia are a network of brain regions that are involved in movement and action. Many of the regions within the basal ganglia appear to play a role in the processing of rhythm. One such region is the putamen, a round structure near the centre of the brain. Studies measuring levels of brain activity have found that the putamen is active when a person is listening to a beat. However, it is not clear whether the putamen is merely identifying the presence of the beat, or whether it is actually helping us to recreate that beat in our minds.

How can we tell what the putamen is doing?
Jessica Grahn and James Rowe designed a study which allowed them to distinguish between these two possibilities. They began their work by creating small snippets of sound. Some of the sound-bites contained a beat, while others did not. These ‘non beat’ sound-bites involved sounds which were not arranged in any rhythmic order. The researchers then combined the order of the sound-bites to create different sequences. These sequences fell into one of four key groups: (1) no beat, (sequences with no beat present), (2) new beat, (sequences in which participants heard a non beat followed by a beat), (3) beat continuation, (sequences with two versions of the same beat), and
(4) beat adjustment, (sequences where the original beat got faster or slower).

The researchers asked the participants to listen to the different sequences whilst a scanner measured how their brain responded. In order to measure this brain activity, they used functional magnetic resonance imaging, often know as fMRI. This is a form of brain imaging that allows us to see which regions of the brain are involved in which tasks. It does this by measuring the amount of oxygen that a specific brain region is using relative to other regions. If a region is using a lot of oxygen, it suggests that the region is ‘active’, that is, it’s involved in carrying out the task.
Image Source: Shutterstock Copyright: GrandeDuc
What did the researchers find?
They found that the putamen responded differently to different beat sequences. When there was no beat, the putamen wasn’t active. Similarly, when participants heard a new beat, the putamen didn’t respond. By contrast, when participants heard the same beat twice, the putamen was highly active. It was also active, but to a lesser extent, when the sequence involved the same beat played at different speeds.

These results suggest that the putamen was not responding to the presence of a beat per se, but was processing the continuation of the beat across the sequence. This supports the theory that the putamen is involved in our ability to recreate a beat in our minds.

Why is this important?
Prior to this study, researchers knew that the putamen was related to beat processing, but they didn’t know what its specific role was. This study showed that the putamen is important for the mental generation of a beat.

In addition to advancing our knowledge of the putamen’s role in beat processing, these findings have notable clinical implications. People with Parkinson’s disease are capable of identifying a beat in a piece of music, but have difficulty when it comes to reproducing the beat in their own minds. The research shows that this pattern of symptoms could be caused by damage to the putamen. Consequently, it highlights the necessity of focusing on the putamen as a target for future treatment of Parkinson’s disease.

This summary by Elizabeth Kirkham was shortlisted for Access to Understanding 2014 and was awarded first place. It describes research published in the following article, selected for inclusion in the competition by the Medical Research Council:

PMCID: PMC3593578
J.A. Grahn & J.B. Rowe.
Cerebral Cortex (2013) 23(4), 913-921.

Access to Understanding entrants are asked to write a plain English summary of a research article. For Access to Understanding 2014 there were 10 articles to choose from, selected by the Europe PMC fundersThe articles are all available from Europe PMC, are free to read and download, and were supported by one or more of the Europe PMC funders.

Look out here and on Twitter @EuropePMC_news for further competition news and other Europe PMC announcements.   

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