ABSTRACT
This article reports two experiments using a visuo-manual step-input tracking task. One of the two spots of light of an oscilloscope served as target for tracking. The second spot of light (cursor) was controlled by a slide connected to a potentiometer. Subject was instructed to keep the cursor on the target at all times, that is, to reposition the slide as fast and as accurately as possible when the target was displaced. In order to establish base line data, successive displacements of the target occurred with time intervals that allowed the subject to align the cursor with the target before the target moved again. On test trials the target moved into a new position before the onset of the overt motor response toward the initial target position. The question was raised whether on tests subject was able to amend the ongoing response programming in such a way as to make the tracking movement to the second (amended) target position. In the first experiment the amended response called for either lengthening or shortening of a highly prepared initial movement. Better reprogramming of extent was found in the lengthening than in the shortening case. In the second experiment, the amended response called for either response nullification, or direction reversal or extent modification. Amendment of the movement’s extent turned out to be easiest, whereas amendment of movement direction and of “intention to move” proved to be difficult, if not impossible. The results are discussed with regard to the response organization processes underlying movement direction and movement extent.
