![]() ![]() Juice rewards were used after each trial to motivate learning. #Visual metronome series#The first study of macaque synchronization to a beat used both auditory and visual metronomes at several different tempi and trained the animals to tap in time with a series of 4 metronome events and then to continue with 3 self-timed taps at the same tempo 25. Macaque monkeys are ideal animals in this regard, because they are known vocal non-learners without close vocal-learning relatives and are capable of learning complex sensorimotor tasks including those requiring temporal prediction 15– 24. One way to distinguish between the vocal learning hypothesis and the intrinsic reward hypothesis is to test animals that are definitely vocal non-learners without close vocal-learning relatives. This idea can be referred to as the “intrinsic reward and rhythmic synchronization” hypothesis. ![]() This raises the possibility that the capacity for predictive and tempo-flexible synchronization is widespread in animal brains including vocal non-learners, but that only certain vocal learning species are intrinsically motivated to engage in this behaviour. ![]() However, a challenge to this “vocal learning and rhythmic synchronization” hypothesis is the finding that a putative vocal non-learning animal, the California sea lion, was able to acquire the ability through reward-based training 13, 14. Because humans and parrots share the rare ability to imitate complex sounds 9, the evolution of vocal learning might develop auditory-motor connections in the brain that are necessary for predictive and tempo-flexible synchronization 10– 12. To date, spontaneous predictive and tempo-flexible synchronization to an auditory beat has only been demonstrated in humans and parrots 4, 5, while predictive synchronization has also been reported in a chimpanzee 6, a bonobo 7, macaque monkeys 8, and Asian elephants 5 under limited conditions (e.g., for a specific tempo, in the presence of visual cues, or under social situations). “Tempo-flexible” means that this sort of predictive synchronization generalizes across a broad range of tempi 3. For example, when tapping with a metronome, humans align their taps very closely in time with metronome events, often tapping slightly before each event. “Predictive” means that movements anticipate the beat rather than react to it. Two key properties of these movements are that they are predictive and tempo-flexible. ![]() It is common for humans to spontaneously synchronize rhythmic movements to this beat, a response that is fundamental to dance in every known culture 2. When processing certain temporal patterns humans often perceive a “beat” or underlying metronome-like periodicity 1. These results suggest that even vocal non-learners may have the capacity for predictive and tempo-flexible synchronization to a beat, but that only certain vocal learning species are intrinsically motivated to do it. In contrast, when humans were asked to make a sequence of reactive saccades to a visual metronome, they often unintentionally generated predictive movements. Furthermore, monkeys could flexibly switch from predictive to reactive saccades when a reward was given for each reactive response. This behaviour generalized to novel tempi, and the monkeys could maintain the tempo internally. We found that monkeys could generate predictive saccades synchronized to periodic visual stimuli when an immediate reward was given for every predictive movement. Does this reflect the lack of capacity for predictive synchronization in monkeys, or lack of motivation to exhibit this behaviour? To discriminate these possibilities, we trained monkeys to make synchronized eye movements to a visual metronome. Prior research training macaques (vocal non-learners) to tap to an auditory or visual metronome found their movements to be largely reactive, not predictive. To date, only certain vocal learning species show this behaviour spontaneously. Predictive and tempo-flexible synchronization to an auditory beat is a fundamental component of human music. ![]()
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