Time-dependent facilitation of homologous actions.

Unimanual actions can interfere with or facilitate similar actions performed with the opposite hand, especially when in close temporal proximity. Across three sequential button-press experiments, we tested how effector homology – anatomical similarity between fingers – and temporal delays between actions shape these effects. Specifically, we examined whether a priming action altered the reaction time (RT) of a subsequent action. Compared with baseline unimanual RTs, we indexed slowing of the second action’s RT as interference, and quickening as facilitation. Priming with homologous actions (e.g., index finger-index finger) produced interference (RTs slower than baseline) for temporal delays ≤ 200 ms and facilitation (RTs faster than baseline) at temporal delays ≥ 400 ms. Priming with non-homologous actions (e.g., little finger-index finger) also produced interference at short temporal delays, but never resulted in facilitation. Critically, these patterns emerged whether the priming actions were performed with the opposite or the same hand, indicating that interference and facilitation do not depend on interhemispheric dynamics. Our results reveal a previously undocumented phenomenon whereby homologous actions separated by longer delays result in behavioural facilitation, challenging models that explain the interaction between parallel actions solely by competitive interhemispheric dynamics or central bottleneck processes. We propose that facilitation and interference flexibly coexist, and are shaped by effector homology and action timing. These findings extend current models of bimanual coordination and highlight new opportunities for enhancing motor performance and neurorehabilitation.