Beta Oscillations as Internal Timers: Independent Contributions of Temporal Predictability to Motor Preparation

Anticipating when and where an event will occur can facilitate motor preparation, yet the independent role of temporal predictability-and its neural basis-remains unclear. This study tested the hypothesis that motor-related beta oscillations reflect an internal timing process sensitive to the regularity of stimulus timing. We recorded EEG while 28 participants performed a modified serial reaction time task in which spatial and temporal predictability were orthogonally manipulated. Spatial predictability was set at three levels (high, medium, low) by varying the regularity of target location sequences. Temporal predictability was manipulated via response-stimulus intervals (RSIs) that were either fixed at 700, 1200, or 1700 ms throughout a block or randomly selected from these three intervals on each trial. Behaviorally, temporal predictability significantly improved reaction times (RTs), even in the absence of spatial regularities. When both types of predictability were present, their facilitative effects were additive, especially at shorter intervals. Neurally, temporal predictability modulated beta-band (13-30 Hz) activity during the post-movement phase. In fixed-RSI conditions, beta power declined gradually and independently of upcoming stimulus onset during the movement-related beta desynchronization (MRBD) phase, suggesting reliance on internal timing. In contrast, under random-RSI conditions, beta power showed a sharper decline after stimulus onset, indicating increased reliance on external sensory input. These results support the view that beta oscillations serve as a flexible internal timer, adapting to the temporal structure of the environment to support anticipatory motor control.