Distinct motor preparatory signals for coarse vs fine temporal control of action

Making each action at the right time is crucial for motor control. In this study, we investigated how two prominent motor preparatory processes identified in human EEG — beta-band desynchronization and the readiness potential (RP) — might each contribute to determining the time of a simple voluntary action. In our paradigm, participants learned when to act in each block of trials, based on probabilistic trial-by-trial feedback. A reward schedule reinforced a specific optimal time to act in some blocks, but allowed greater variability around the same mean action time in other blocks. We found that contralateral motor beta-band power at the beginning of each trial increased with the mean waiting time in each block. Computationally, this pattern is consistent with a neural implementation of distance-to-bound for a noisy accumulation process, in which baseline motor activity starts off closer to an action-triggering bound when fast actions are required. In contrast, the RP was independent of the mean waiting time before action, but its slope increased when participants developed a precise motor plan. We suggest that beta desynchronization and the RP track distinct, complementary processes involved in the temporal control of action. While beta-power activity at trial onset is consistent with a neural implementation of urgency, the RP captures an internal decision variable that reflects a noisy accumulation process. Simulations suggest that acting at the right time may involve increasing the strength of the input to this accumulator, which can cause the decision variable to drift towards the decision bound.