How is performance limited: Testing the notion of central capacity

Two-dimensional pursuit tracking task was employed in three experiments designed to test three predictions of the central capacity model of performance limitations under time-sharing conditions. These are the predicted effects of change in task difficulty, task emphasis and their interaction. Each of simultaneously performed tracking dimensions (horizontal and vertical) was treated as a separate task and manipulated independently. Tracking difficulty on each dimension and their relative emphasis were jointly manipulated. When frequency or velocity of target movement served as difficulty parameters, and control complexity was relatively low, tradeoffs between dimensions in different priority conditions were small and task difficulty had no effect on the performance of the concurrent task, neither did it interact with task emphasis. When control complexity was increased and in addition was manipulated as the difficulty parameter, linear tradeoff was observed and difficulty seemed to interact with task emphasis. These results cannot be easily accommodated within a strict central capacity model. An alternative interpretation based on a multiple capacity approach is outlined.