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How does our brain respond when we perceive a beat?
Stimuli
These stimuli were used in a rhythm reproduction task and an fMRI discrimination task. (Grahn and Brett, 2007)
The metric simple and complex rhythms are composed of intervals related by integer-ratios (1:2:3:4), whereas nonmetric use noninteger-ratios (1:1.4:3.5:4.5).
The metric simple rhythms also have a regular grouping which induces a stronger perception of a beat than in the metric complex rhythms. As the behavioural data show below, integer-ratios are not enough to lead to better rhythm performance--regular grouping is also required.
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Task and rhythm reproduction results
For the reproduction task, participants heard each rhythm 3 times, then tapped back what they heard on a computer keyboard.
As you can see from the graph on the left, the metric simple rhythms are remembered correctly significantly more often than the other two types.
This pattern has been replicated with rhythms made with brief percussion noises (rather than longer sine tones).
For the fMRI study, participants performed a rhythm discrimination task. They listened to 3 presentations of a rhythm, and had to decide if the third presentation was the same as or different from the first 2 presentations of that rhythm.
A sample trial (decide if the third presentation is different from the first two)
We measured brain activity when participants were hearing the first 2 presentations (we only measured activity related to listening and trying to remember rhythms, not activity related to discriminating the change).
Neuroimaging results
This picture shows greater activity in the basal ganglia to metric simple rhythms compared to the other two types. This suggests the basal ganglia play a role in beat perception.
This graph shows relative activity to the three rhythm types. Significantly greater activity is observed for the metric simple condition than the other two conditions in the basal ganglia, supplementary motor area, and superior temporal gyri (auditory cortex).
Several other areas of the brain (including the cerebellum and premotor cortex) did not show the same pattern. Instead, they responded similarly to all three rhythm types.