One of the unanswered questions in Krista Hyde and Isabelle Peretz’s research on amusia (“tone-deafness”) is why amusics frequently say they are unable to clap to the rhythm of a song, or to dance well. In Hyde and Peretz’s study, amusics could detect rhythm changes as well as normal individuals, even while being unable to detect changes in musical pitch.
Hyde and Peretz speculated that amusics might not be able to detect rhythm changes when the pitch of the notes change — that their problems with rhythm might be directly related to the fact that amusics can’t detect many changes in pitch. But there is another possibility. Perhaps Hyde and Peretz’s rhythm test was too simple: it involved playing five identical notes at a constant rate, then changing the timing of one note in the sequence. So it’s also possible that amusics have difficulty only with more complex rhythmic structures.
A team led by Jessica Foxton has developed an elegant way to test these two hypotheses. They created four types of rhythmic sequences, each just five notes long. The first, most basic sequence was just five identical notes, played at a constant rate. The second sequence made the rhythm slightly more complex by alternating between tones repeated rapidly and slowly. The final two sequences had the same rhythm as the first two, but instead of playing the same tone, each note was randomly varied.
Volunteers listened to the sequences in pairs, and their task was to determine if the rhythmic pattern of the second sequence was the same as the first. Here’s an example of the complex rhythm:
Most listeners should be able to detect that the rhythm of the second sequence is different from the first. Foxton’s team systematically varied how different the two rhythms were (the difference was always in the fourth tone), testing both amusics and people with normal pitch peception. They repeated the task with a new sequence that randomly varied the tones played. The rhythm was the same, as were the differences between the second sequence and the first. Here’s an example of this type of sequence:
Listen for a difference in rhythm between the first set of five tones and the second set — ignore differences in pitch. Could you discern a difference?
The Foxton team’s task was a little more complicated than this, because they adapted the sequences they played based on the accuracy of responses, honing in on the exact threshold where each individual could detect differences between each type of sequence. Here are their results:
Foxton et al. did indeed find a difference in rhythm perception between people with amusia and those with normal pitch perception. While there was no difference between the variable-pitch and the constant-pitch conditions for normal individuals, for amusics, performance was significantly impaired when they listened to rhythm changes while pitch varied. Interestingly, these pitch changes were large enough that they were probably detectable even by amusics.
So while people with tone-deafness can hear both large pitch changes and small rhythm changes in isolation, when small rhythm changes are combined with large pitch changes, the ability to keep rhythm deteriorates dramatically.
And what about the samples I provided? I’ll put an update with the correct response to the poll question in the comments later today.
Foxton, J.M., Nandy, R.K., & Griffiths, T.D. (2006). Rhythm deficits in ‘tone deafness.’ Brain and Cognition, 62, 24-29.