Probing the motor program: Effects of output competition during movement preparation

A probe reaction-time paradigm was used to investigate the capacity demands of planning rapid aiming movements. Subjects were required to respond either vocally or manually to an auditory probe presented during the reaction-time interval preceding a pointing response. When a vocal response was required probe reaction time increased systematically with the complexity of the pointing movements. Presumably this is because a more complex task requires more programming resources. When a manual response was required, reaction-time data for both the pointing task and the probe indicate that the structural constraints inherent in programming two similar movements may force subjects to engage in common response preparation. The methodological and theoretical implications of these findings are discussed.