Perception-action coupling and S-R compatibility.

How is an aiming movement toward a visual target amended when the target suddenly steps to a new position just before or after the movement has started? Three hypotheses are examined: (1) the initial movement needs to be actively terminated before the new movement can be planned and executed, (2) substitution of the initial target position code results, after a normal RT, in the simultaneous termination of the initial movement and initiation of the movement to the new target position, or (3) a second movement from the initial to the second target is initiated after a normal RT, and superimposed on the ongoing movement toward the initial target. The substitution hypothesis assumes a highly continuous and parallel mode of operation of the perceptual-motor system, whereas the other hypotheses assume a distinctly discrete mode of operation. Detailed analyses of double-step movement trajectories clearly favored the substitution hypothesis. These results are discussed with reference to current views on motor control, overlapping-task performance, and the discrete-continuous issue. It is argued that the nature of the perception-action interface depends on the ideomotor compatibility of the task. Perceptual and motor processes operate in a highly continuous and parallel fashion in ideomotor compatible tasks, whereas the interposition of a limited-capacity response selection mechanism results in a discrete and intermittent mode of communication between these processes in non-ideomotor compatible tasks.