When subjects used a manual joystick to track the motion of a visual target (in either zero-order or first-order control), hesitations in tracking often occurred when the other hand responded to an auditory stimulus. These hesitations are related to postponement in the psychological refractory period effect. Because few hesitations occurred when the auditory stimulus was the no-go case of a go-no-go paradigm, hesitations must arise primarily during "late" processing associated with the concurrent response rather than during "early" perceptual or decision-making processes. Other findings suggest that the single-channel processing limit is in programming (as opposed to selecting or generating) concurrent responses. Blanking of the target also produced hesitations through a different mechanism.

Subjects performed continuous, visually guided pursuit tracking with the right hand while giving simultaneous discrete left-hand responses, which were signaled by auditory tones appearing at the average rate of one tone per 30 s. This left-hand secondary task was frequently associated with tracking hesitations lasting 333 ms or longer. The rate of occurrence of these hesitations was about the same when the left-hand response involved a choice between competing responses as when the left hand responded in a previously specified direction. Hesitations occurred for three different mechanical tracking manipulanda using different controlling muscles and appeared to be due to active muscular freezing rather than to relaxation. The rate of hesitations declined with practice, and this improvement in right-hand performance was accompanied by an improvement in performance of the concurrent left-hand response.

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