Different Factors Determining Motor Execution and Motor Imagery Performance in a Serial Reaction Time Task with Intrinsic Variability

<b>Background/Objectives:</b> Motor imagery (MI) is the mental practice of motor actions with temporal dynamics and neural features in common with motor execution (ME). Although MI can improve motor performance, it remains unclear how closely performance-related signatures of MI resemble those of ME during learning, particularly in tasks with intrinsic variability. This study investigated similarities and differences between MI and ME during a probabilistic sequence-learning task. <b>Methods:</b> Participants performed a finger-tapping serial reaction time task in either a motor execution (ME; <i>n</i> = 10) or motor imagery (MI; <i>n</i> = 10) condition. The task consisted of 750 auditory stimuli mapped to right-hand finger movements and generated by a probabilistic sequence with deterministic and variable events. Reaction times were analyzed using ANOVA designs to assess the effects of Group, Block, Event Type, and the Last Variable event. <b>Results:</b> The MI group showed a significant reduction in reaction times across blocks (<i>p</i> < 0.001), indicating learning-related performance improvement, whereas no block-wise improvement was observed in the ME group. Both groups were sensitive to the probabilistic structure of the sequence, with reaction times differing across event types. A significant Group × Event interaction (<i>p</i> < 0.01) indicated distinct performance signatures for MI and ME. In both groups, reaction times were modulated by the last variable event. <b>Conclusions:</b> Motor imagery supported learning in a probabilistic sequence task but was influenced by factors distinct from those governing motor execution, suggesting partially different underlying mechanisms.