Breaking the flow of an action.

The present study was aimed at investigating whether the execution of a sequential action changes when the temporal contiguity between the motor steps composing it is altered. Participants were requested to reach and grasp an object and pour its contents into a container under two conditions: a 'fluent pouring' condition in which participants were instructed to execute the action fluently and an 'interrupted pouring' condition in which participants were instructed to reach and grasp the object, wait for an acoustic signal and then complete the pouring action. A 'control' condition in which participants were requested to reach and grasp the object without performing any subsequent action was also administered. Results indicate that movement duration and hand kinematics varied depending on the temporal relationship between the reach-to-grasp and the lift-to-pour phases. When a delay at object contact was introduced, reach duration was longer and the thumb/index abduction angle was greater than when such a delay was not introduced. These results are interpreted in light of 'internal model' theories suggesting that a strict temporal contiguity between the motor steps composing an action is a prerequisite for a skilful movement to be planned and executed.