Is there symmetry in motor imagery? Exploring different versions of the mental chronometry paradigm

Motor imagery and motor execution share similar processes. However, only some factors that affect motor execution affect motor imagery in the same way. We investigated whether bimanual coordination constraints (parallel movements are performed slower than symmetric movements) are observed in motor imagery and whether the way of implementing the mental chronometry paradigm, which is used to investigate motor imagery, influences the results. Participants imagined and executed repetitive symmetric and parallel bimanual movements in three different tasks. Participants performed a certain number of movement repetitions (number task), repeated movements for a fixed duration (duration task), and performed movements in synchrony with pacing sounds (synchronization task). In both, imagination and execution, inter-response intervals were longer with parallel movements than with symmetric movements (number task and duration task), and the percentage of correct movements was lower with parallel than with symmetric movements (synchronization task). Performance of imagined and executed movements was correlated in all tasks. However, imagination took longer or was rated as less accurate than execution, and in the synchronization task the coordination constraint affected accuracy more in execution than in imagination. Thus, motor imagery and overt execution involve shared and unique processes. The synchronization task offers a promising alternative to investigate motor imagery, because the speed-accuracy trade-off is taken into account, different tempi can be used, and psychometric functions can be calculated.