This paper reports the results from three experimental studies of reaching behavior in a head-coupled stereo display system with a hand tracking sub-system for object selection. It is found that lag in the head tracking system is relatively unimportant in predicting performance, whereas lag in the hand tracking system is critical. The effect of hand lag can be modeled by means of a variation on Fitts' Law with the measured system lag introduced as a multiplicative variable to the Fitts' Law

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... x of difficulty. This means that relatively small lags can cause considerable degradation in performance if the targets are small. Another finding is that errors are higher for movement in and out of the screen, as compared to movements in the plane of the screen, and there is a small (10%) time penalty for movement in the Z direction in all three experiments. Low frame rates cause a degradation in performance, however, this can be attributed to the lag which is caused by low frame rates, particularly if double buffering is used combined with early sampling of the hand tracking device.