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The effects of aural, visual, and combined displays on human tracking performance were evaluated by means of the critical tracking task which employs an unstable controlled element. Four visual displays and an aural display were used in this study. Values of Â¿c9, the critical root or maximum degree of instability, were developed for each display alone and also for the visual displays used in combination with the aural presentation. These experiments were motivated by the results of a previous study by the authors which indicated that combined displays might lead to improved performance. Tracking performance, as measured by Â¿c9, was consistently better for combined visual and aural presentations than for either type of display used alone. This trend was exhibited for each of the four visual displays and for both experienced and inexperienced trackers. These results suggested that combined displays supply greater net usable information to the human's central processor, allowing a significant reduction in estimation errors by the human operator. The effects of external disturbance input to the tracking loop were investigated. Tracking performance was degraded with increasing disturbance amplitude but no clear trends with bandwidth were noted.