Since it was Zak’s birthday yesterday, I let him choose the topic of the blog for this month, so he chose one of the essays that I wrote for my final exams in Cambridge, which looks at whether entrainment is a uniquely human capacity and if this tells us anything about our cognitive processes. It might be a bit heavier and more complex than some of my other blogs but if you like science, animals or rhythms it’s still worth a read so please enjoy!
Entrainment occurs when the behaviours (and attentional foci) of two or more people become periodically aligned in time (Clayton, Sage and Will, 2005). The fact that this definition specifically says “people” is but one reason for assuming that it is unique to humans. Entrainment is a complex process involving continuous reciprocal adaptation of periods and phases of sounds and actions produced, with each participant continually switching between leading and following each other (Cross, 2014). As such, it may be easy to presume that this process would be too complicated for non-humans, since humans are significantly more cognitively developed than animals. Another reason may be that entraining together can affect memory for and attitudes towards each other (Cross, 2014) and therefore have powerful effects on social bonding in humans (McNeill, 1995). This evidence could also encourage assumptions that non-humans are incapable of experiencing this and therefore unable to demonstrate all aspects of entrainment.
However, while this may be the initial assumption, empirical evidence has shown that some animals are also capable of entrainment (Patel, 2006; Patel et al., 2009; Schachner et al., 2009; Schachner, 2010; Hattori et al., 2013; Merchant & Honing, 2014 and others). It is generally accepted that most animals do not entrain (Wilson & Cook, 2016), but as these studies have proved, some do. It is this distinction between which animals can and cannot entrain that aids us in our understanding of entrainment’s place in our cognitive processes. As such, this essay focuses on theories surrounding why certain animals can and cannot entrain, particularly the evidence for and against vocal mimicry as a necessary requirement for entrainment, before discussing what this can tell us about entrainment in the cognitive processes of humans. The majority of research on entrainment in humans is based on rhythmic entrainment (entraining to rhythmic stimuli), so this is the main focus, but mutual entrainment (e.g. group locomotive behaviour and chorusing) is also mentioned as it is very important to animals and may relate to rhythmic entrainment.
First, it is important to look at initial research on entrainment in animals, which supports the theory that only vocal mimicking animals can entrain. Vocal mimicry occurs when an individual learns a sound from another species that inhabits the environment. Humans, cockatoos and budgerigars are among the examples of vocal mimics, suggesting it is a relatively recent evolutionary process. Patel (2006) claimed that entrainment to a musical beat relies on the neural circuitry for complex vocal learning, an ability that requires a tight link between auditory and motor circuits in the brain. He suggested this meant that only vocal learning species are capable of synchronising movements to a musical beat. Patel et al. (2009) later supported this vocal mimicry theory with evidence that a sulphur-crested cockatoo spontaneously adjusts the tempo of its rhythmic movements to stay synchronised with the beat of a musical excerpt.
Schachner et al. (2009) also supported this theory. They analysed a global video database for evidence of entrainment in hundreds of species both capable and incapable of vocal mimicry and found that only vocal mimics showed evidence of entrainment. These findings support the hypothesis that entrainment evolved as a by-product of selection for vocal mimicry. However, Schachner’s next study (2010) brought this into question. The results of this study indicated that not every vocal mimicking animal shows evidence for entrainment, which she explained by suggesting there are additional factors needed for entrainment in conjunction with vocal mimicry. However, it could also suggest that vocal mimicry is not a necessary requirement for entrainment.
The theory that vocal mimicry is a requirement for entrainment was disproven by Cook et al. (2013), which found that a less vocally flexible animal, a sea lion, can entrain head bobbing to an auditory rhythm. The sea lion met three criteria deemed necessary for entrainment by the experimenters: a behavioural response that does not reproduce the stimulus, performance transfer to a range of novel tempos and entrainment to complex musical stimuli. As such, findings show that that the capacity for rhythmic entrainment does not depend on the capacity for vocal mimicry, so may be more widespread in the animal kingdom than previously hypothesised.
Cook & Wilson (2016) then went on to do a more comprehensive analysis of entrainment in animals. Though not rhythmic entrainment, many animals have the capacity for mutual entrainment, which is the dominant and most ancient form of entrainment in the animal world (Chauvigne, Gitau & Brown, 2014), and may be a precursor to rhythmic entrainment. Fireflies, fish, marine crustaceans, insects, frogs, and crabs can all entrain, proving that the mechanism for automatic entrainment is not an isolated evolutionary occurrence but has emerged through convergent evolution (Cook & Wilson, 2016). Synchrony has also been noted in dolphins and orcas, for example in schooling behaviour and surfacing (Cook & Wilson, 2016). Cook & Wilson also mention further studies on rhythmic entrainment that oppose the vocal mimicry theory. They note that several primates can also synchronise, with particularly high precision and synchrony in gelada monkeys and bonobos. This demonstrates vocal synchrony in species that are not robust vocal learners, indicating the ability to entrain the timing of a given behaviour need not depend on a great deal of voluntary control over the content of that behaviour (Cook & Wilson, 2016). Hattori et al. (2013) also support this by proving that chimpanzees, bonobos and Japanese macaques are able to synchronise a non-vocal behaviour to an ambient rhythmic stimulus.
Earlier it was mentioned that the social aspect of entrainment may mean that parts of the process are uniquely human. However, this is also not the case as it has been found that social motivation to entrain occurs not only in humans, but also in birds, who are sensitive to social reward (Cook & Wilson, 2016), and dolphins and orcas, who have special social motivation to synchronise (Abramson et al. 2013). This disproves the assumption that non-humans are incapable of experiencing the social bonding aspect of entrainment and may mean that entrainment can also promote meaningful social bonds in animals as it was shown to in humans by Hove & Risen (2009).
It is now important to consider how all this evidence relates to entrainment in human cognitive processes. From initial research supporting the vocal mimicry theory, it was thought that entrainment in humans and non-humans was a by-product of selection for vocal mimicry (Schachner et al., 2009) and required a tight link between auditory and motor circuits in the brain (Patel, 2006). The fact that vocal mimicry has been disproven to be necessary for entrainment and that entrainment is possible in non-vocal mimicking animals, suggests that the cognitive processes underlying entrainment are evolutionarily older than those which give rise to vocal mimicry. This enables identification of older cognitive processes, that are common to both humans and animals, that could be responsible for entrainment.
Fitch, 2013 suggests that Broca’s area in the brain could be involved. Importance has been assigned to Broca’s area by Friederici (2002), Hagoort (2005) and Friederici et al. (2006). This region and its right hemisphere homolog are preferentially activated by tasks involving harmonic syntax (Maess et al., 2001; Koelsch et al., 2002; Levitin & Menon, 2003), so Fitch proposes that it would not be surprising to find that parts of Broca’s area are also important in constructing hierarchical structures in rhythmic perception. Broca’s region is 6.6 times larger in humans than in chimpanzees, suggesting a recent expansion of the abilities this region supports, potentially including rhythmic or entraining abilities.
Other research (Wilson & Cook, 2016) suggests that neural oscillations (neurons in the brain firing in synchrony) could be involved, since they are a ubiquitous fundamental design feature in animal brains. This older, shared neurobiological architecture can cause entrainment of brain oscillations to sensory stimuli that are objectively rhythmic, as shown in humans, macaque monkeys and zebrafish (Cook & Wilson, 2016). Cook & Wilson also suggest that the basal ganglia, which are conserved in all vertebrates, could be involved. The two subcortical systems involved in timing in humans were found to be the cerebellum and basal ganglia (Chavigne, Gitau & Brown, 2014), and interval timing, a precursor to entrainment, is governed by the basal ganglia (Cook & Wilson, 2016). This suggests that the basal ganglia are part of a very old neurological framework within which entrainment could arise.
There are various conclusions that can be drawn from this research. Firstly, there is unequivocal evidence that rhythmic entrainment is not a uniquely human capacity. Early research on entrainment in non-humans supported the theory that only vocal mimicking animals could entrain, suggesting entrainment is a relatively recent evolutionary process. However, this was disproven as not all vocal mimicking animals have the capacity to entrain and some non-vocal mimicking animals do have the capacity to entrain. This suggests that the cognitive processes involved in entrainment are evolutionarily older than those responsible for vocal mimicry, explaining why entrainment is more widespread in the animal kingdom than initially thought. Therefore, this indicates that older cognitive regions or processes, common to both humans and a wide variety of animals, may be responsible for entrainment. These might involve Broca’s region, neural oscillations, or basal ganglia. However, none of these hypotheses have yet been proven, so more research on the exact processes involved in entrainment and how they evolve needs to be completed.
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