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Thursday, December 27, 2012

Monkeys can’t hear the beat

Only a few animals have been shown to have ‘beat induction’, that is the ability to perceive a regular beat or pulse. Interestingly, this list has included birds, but not non-human primates. However, this deficit could merely represent a lack of response to rhythms rather than an inability to perceive those rhythms. In other words, just because an animal doesn’t bob its head or tap its feet doesn’t mean that it hasn’t noticed the beat.

Researchers from the University of Amsterdam and from the Universidad Nacional Autonoma de México decided to bypass physical manifestations of beat induction and study actual brain activity in two rhesus monkeys (Macaca mulatta). They played a drum pattern either in its entirety or with specific notes omitted. Human adults and even newborn babies will notice those missing elements. Was that true for the monkeys?

To find out, the researchers looked for mismatch negativity in the monkeys’ brains. This occurs when something unexpected occurs, such as when the incoming stimuli don’t match expectations. For example, if you’re listening to a sound pattern and the music skips a beat, this will activate mismatch negativity in your brain. It turns out that while monkeys can detect rhythms based on the duration of sounds, they can’t pick out the regularity, or beat, of those sounds the way humans and some birds can.

The authors speculate that beat induction emerged by convergent evolution in creatures that are able to mimic sounds. If so, the ability to copy not only sounds but specific rhythms may have been one of the factors involved in the acquisition of human language.

Honing H, Merchant H, Háden GP, Prado L, & Bartolo R (2012). Rhesus Monkeys (Macaca mulatta) Detect Rhythmic Groups in Music, but Not the Beat. PloS one, 7 (12) PMID: 23251509.