Abstract
Predictive and tempo-flexible synchronization to an auditory beat is a fundamental component of human music. To date, only certain vocal learning species show this behaviour spontaneously. Prior research training macaques (vocal non-learners) to tap to an auditory or visual metronome found their movements to be largely reactive, not predictive. 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. We found that monkeys could generate predictive saccades synchronized to periodic visual stimuli when an immediate reward was given for every predictive movement. This behaviour generalized to novel tempi, and the monkeys could maintain the tempo internally. Furthermore, monkeys could flexibly switch from predictive to reactive saccades when a reward was given for each reactive response. In contrast, when humans were asked to make a sequence of reactive saccades to a visual metronome, they often unintentionally generated predictive movements. 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.
Highlights
When processing certain temporal patterns humans often perceive a “beat” or underlying metronome-like periodicity[1]
We found that macaque monkeys were capable of predictive synchronization of eye movements with metronomic visual stimuli when rewards were given for each predictive saccade
When human participants were asked to make a sequence of reactive saccades to a metronomic visual stimulus, they often unintentionally generated predictive eye movements, making their saccade latency shorter and more variable than monkeys
Summary
When processing certain temporal patterns humans often perceive a “beat” or underlying metronome-like periodicity[1]. 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] 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. Subsequent work in the same lab attempted to reduce the latency of monkey tapping by requiring smaller latencies for reward, the shortest tapping latency was ~100 ms, with taps always following metronome events rather than preceding them, on average[26] These results were inconsistent with the intrinsic reward hypothesis, which predicts that monkeys should be capable of synchronizing to metronome with near zero (or even negative) mean asynchronies even for untrained tempi. Once monkeys had been trained to synchronize at a specific set of metronome tempi, they were tested for generalization to new, untrained tempi
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