Although monkeys seem to
notice regularity in rhythmic sounds, they are not able to detect the actual
beat. This is the finding of a new study by researchers from the University of
Amsterdam (UvA) and the National Autonomous University of Mexico (UNAM). The
study, published on 16 July 2018 in the journal Frontiers of Neuroscience, lends further evidence to the hypothesis
that beat perception is omnipresent in humans but only gradually developed in
primates.
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| The gradual audiomotor evolution (GAE) hypothesis. The GAE hypothesis
suggests connections between medial premotor cortex (MPC), inferior
parietal lobe (IPL), and primary auditory area (A1) to be stronger in
humans as compared to other primates (marked with red lines), suggesting
beat-based timing to have gradually evolved. Line thickness indicates
the hypothesized connection strength. |
Even
a cursory glance at the animal kingdom shows that most animals exhibit some
sort of rhythmic behavior, like walking, flying, crawling or swimming. Based
on this behavior, it wouldn’t be outlandish to think that the perception and
enjoyment of rhythm might be shared by most animals, and not only humans. While
recent experimental research is finding some support for this view, studies
also show that there are certain aspects of rhythm cognition that are indeed
species-specific, such as the capacity to perceive a regular pulse (beat) in a
varying rhythm and consequently being able to synchronize or dance to it.
A rhythmic sequence
Building
on their earlier research, the researchers investigated whether rhesus monkeys
(Macaca mulatta) are able to perceive
beat through a s0-called auditory oddball paradigm, an experiment in which
sequences of repetitive sounds are infrequently interrupted by a deviant sound.
‘Most existing animal studies on beat-based timing and rhythmic entrainment
have used behavioural methods to probe the presence of beat perception, such as
tapping tasks or measuring head bobs’, says Henkjan Honing, professor of Music
Cognition at the UvA and lead author. ‘However, even if certain species do not
show a physical ability to synchronise their movements to a regular beat, this
doesn’t automatically mean they are incapable of perceiving it.’
For
their study, the researchers instead used electroencephalography (EEG) to
measure neural correlates of rhythm cognition, including beat perception. The researchers
presented two rhesus monkeys with a rhythmic sequence in two versions: an
isochronous version that was acoustically accented in such a way that it could
induce a duple metre (like a march), and a jittered version using the same
acoustically accented sequence but presented in a randomly timed fashion so as
to disable beat induction.
No evidence of beat perception
The results showed
that monkeys are sensitive to the isochrony of the stimulus, but not its
metrical structure. This so-called mismatch negativity (MMN) was influenced by
the isochrony of the stimulus, resulting in a larger MMN in the isochronous as
opposed to the jittered condition. However, the MMN for both monkeys revealed
no interaction between metrical position and isochrony. Honing: ‘Even though
the monkey brain appears to be sensitive to the isochrony of the stimulus, we
couldn’t find any evidence in support of beat perception.’
The findings
further strengthen the Gradual Audiomotor Evolution (GAE) hypothesis (Merchant& Honing 2014), which suggests ‘beat perception’ to be gradually developed
in primates, peaking in humans but present only with limited properties in
other non-human primates. The GAE is an alternative to the well-known ‘vocal
learning hypothesis’, which suggests that only species who can mimic sounds
share the ability for beat induction.
Publication details
Henkjan Honing, Fleur L. Bouwer, Luis Prado and Hugo Merchant, ‘Rhesus
monkeys (Macaca mulatta) sense isochrony in rhythm, but not the beat:
additional support for the gradual audiomotor evolution hypothesis’ in Frontiers in Neuroscience, 16 July, 2018. Doi: 10.3389/fnins.2018.00475
