'Vocal mimicry hypothesis' falsified? [Part 2]

Figure (a) Ai tapped C4, (b) Ai tapped C5, (c) Time sequence of a test trial.

A few entries ago I uploaded a fragment from a study (Hattori et al., 2013) that discusses an intriguing experiment with three chimpanzees (Pan troglodytes) which were trained to tap regularly on a piano keyboard.

While the video below is convincing, the study reports that only one of the three chimps participating in the experiment was able to do the task: a chimp named Ai (See video).  Furthermore, Ai was only able to synchronize with stimuli at a rate of 600 ms (and not at rates of 400 or 500 ms). In addition, Ai did this in reaction (positive asynchrony) and not in anticipation of the beat (negative asynchrony).

This is similar to what has been found in studies with macaques (Zarco et al., 2009; Konoike et al., 2012) that also seem to opt for a strategy of to react instead of anticipating to a regular beat. All this in contrast with humans that can intentionally synchronize their tapping to various rates (ranging roughly from 200 ms to 1800 ms) of a varying rhythmic stimulus (and not simply a metronome) while showing a negative synchronization error, i.e. in anticipation of the beat.

Another point of a more methodological nature is that the experimentators used, next to sound, what they called 'light navigation' (see diagram above), a visual cue for the chimps to 'remind them' of which key to press. While the authors write "it was unlikely that the visual stimuli affected tapping rhythm by chimpanzees" we can not be sure this is evidence for rhythmic entrainment in the auditory domain.

Nevertheless, with behavioral methods that rely on overt motoric responses it is difficult to separate between the contribution of perception and action (beat perception vs beat production). This makes electrophysiological measures (such as event-related potentials) a more direct and hence attractive alternative. The latter method has been shown a worthwhile, non-invasive alternative in studying cognitive and neural processing in primates (see, e.g., Ueno et al., 2009) and it was used recently in a study probing beat perception in macaques (Honing, Merchant et al., 2012).*

And lastly, these and earlier observations have lead to the auditory timing dissociation hypothesis (Honing, Merchant et al., 2012). This hypothesis accommodates the fact that nonhuman primates performance is comparable to humans in single interval tasks (such as interval reproduction, categorization and interception), but differs substantively in multiple interval tasks (such as rhythmic entrainment, synchronization and continuation).

* N.B. We are eager to collaborate with a primate lab that is willing to do such a relatively simple listening experiment using EEG with chimpanzees; Would be great to compare the results we now have for human adults, newborns, and macaques with the perception of Great Apes ! Feel free to email me :-)

Hattori, Y., Tomonaga, M., & Matsuzawa, T. (2013). Spontaneous synchronized tapping to an auditory rhythm in a chimpanzee. Scientific Reports, 3 DOI: 10.1038/srep01566.

Hasegawa, A., Okanoya, K., Hasegawa, T., & Seki, Y. (2011). Rhythmic synchronization tapping to an audio–visual metronome in budgerigars Scientific Reports, 1 DOI: 10.1038/srep00120

Honing, H., Merchant, H., Háden, G., Prado, L., & Bartolo, R. (2012). Rhesus Monkeys (Macaca mulatta) detect rhythmic groups in music, but not the beat. PLoS ONE, 7 (12) DOI: 10.1371/journal.pone.0051369