Simultaneity of two effectors in synchronization with a periodic external signal

Several studies have shown that the control of simultaneous movements differ according to the execution context. For instance, when subjects raise simultaneously their index finger and heel as fast as possible after an auditory signal, the simultaneity is controlled by sending synchronously the motor commands to both effectors. On the other hand, when subjects self-pace their movements, the simultaneity is controlled by processing the delay between afferent signals from both movements at the central level (Paillard, 1948). It has been hypothesized that a mode of control similar to the self-paced condition is also used when subjects produce simultaneous and repetitive movements in synchronization with a metronome (Fraisse, 1980). We examined this hypothesis by asking subjects to move simultaneously the index finger and heel in synchronization with metronome sounds. Results showed that the events chronology (i.e., heel movement first, finger movement second and metronome sound third) was a function of the relative distance of the effectors and auditory organ from the central comparator. We deduced that the synchronization and simultaneity was evaluated by computing the time elapsed between the arrival of the sensory feedback of the movement and auditory signal. The second goal of the study was to assess whether, in such a task, each effector is synchronized separately to the metronome sound or together as an unit. A strong positive correlation was found between finger and heel synchronization errors. This supports the hypothesis that finger and heel movements are synchronized as an unit to the metronome rather than independently. In conclusion, simultaneity between effectors and synchronization between effectors and an external signal, although they share similar processes based on afferent information, are likely to be controlled separately.