A two-stage model of concurrent interval timing in monkeys.

Accurate timing is critical for a wide range of cognitive processes and behaviors. In addition, complex environments frequently necessitate the simultaneous timing of multiple intervals, and behavioral performance in concurrent timing can constrain formal models of timing behavior and provide important insights into the corresponding neural mechanisms. However, the accuracy of such concurrent timing has not been rigorously examined. We developed a novel behavioral paradigm in which rhesus monkeys were incentivized to time two independent intervals. The onset asynchrony of two overlapping intervals varied randomly, thereby discouraging the animals from adopting any habitual responses. We found that only the first response for each interval was strongly indicative of the internal timing of that interval, consistent with previous findings and a two-stage model. In addition, the temporal precision of the first response was comparable in the single-interval and concurrent-interval conditions, although the first saccade to the second interval tended to occur sooner than in the single-interval condition. Finally, behavioral responses during concurrent timing could be well accounted for by a race between two independent stochastic processes resembling those in the single-interval condition. The fact that monkeys can simultaneously monitor and respond to multiple temporal intervals indicates that the neural mechanisms for interval timing must be sufficiently flexible for concurrent timing.