Direction and extent uncertainty in step-input tracking.

In 3 experiments, Ss controlled a horizontal lever to track a step input where the degrees of choice of direction and extent of movement were varied systematically. The experiments differed in the way the trials were presented and in the conditions under which these 2 sources of response choice were compared. Increasing choice of direction of movement from 1 to 2 led to an increase in RT of between 34 and 40 msec. No significant increase in RT was found as choice of extent of movement was raised from 1 to 4. Considered with other evidence the results indicate the necessity to distinguish 2 types or levels of decisions associated with response factors. It is suggested that at one level the sequencing of muscle activity is established and at another the fine details of this activity are controlled.