Different difficulty manipulations interact differently with task emphasis: evidence for multiple resources.

To test the notion of multiple resources, a two-dimensional pursuit tracking task was paired with a letter-typing task, the difficulty of which was manipulated by varying cognitive (size of stimulus set) and motor (repetitiveness of finger chords) factors. In addition, task priority was manipulated. The latter factor had a large effect on the performance of the two tasks, which indicates that they compete for resources. Both types of typing difficulty manipulations affected typing performance, but only motor difficulty interacted with priorities. Since difficulty manipulations that tap resources common to both tasks are predicted to interact with priorities, the results are interpreted to indicate that in joint performance, typing and tracking compete mainly for motor-related concept, the letter-typing task is argued to require at least two kinds of resources.