A behavioral integration mechanism underlies action timing.

Animals often display stereotyped behavioral patterns during timekeeping tasks. However, it remains unclear whether these behaviors are simply incidental-used by the animals to kill time-or whether they serve a functional role as part of the internal clock that enables animals to keep time. Here, using optogenetic stimulation of various brainstem, thalamic, and dopaminergic cell populations in mice trained on an action-timing task, we manipulated actions preceding the timing decision to causally test these hypotheses. Our findings show that bidirectional regulation of actions during the waiting period leads to corresponding temporal shifts in timing, with a consistent quantitative relationship between changes in the number of waiting-period actions and shifts in stop timing. Notably, nigrostriatal dopamine encodes actions, but not timing, directly, influencing timing only when waiting-period actions are modulated. These findings reveal a behavioral integration mechanism underlying action timing, in which actions performed during the waiting period function as a pacemaker for the internal clock.