Virtual Reality (VR) systems enable users to interact with computer-generated objects in a natural manner. However, inevitable time lags occur when a Virtual Reality system responds to dynamic head or hand movements of a user. Many studies have reported the time lag problems with Virtual Reality systems but studies on the combined effects of head and hand movement-related lags are few. The purposes of this research are to determine the effects of (i) head movement-related lags, (ii) hand movement-related lags, and (iii) combined head and hand movement-related lags on manual task performance. Both discrete and continuous manual tasks have been investigated and the mechanisms responsible for the degraded performance with lags have also been studied and identified. In addition, quantitativ...[ Read more ]

Virtual Reality (VR) systems enable users to interact with computer-generated objects in a natural manner. However, inevitable time lags occur when a Virtual Reality system responds to dynamic head or hand movements of a user. Many studies have reported the time lag problems with Virtual Reality systems but studies on the combined effects of head and hand movement-related lags are few. The purposes of this research are to determine the effects of (i) head movement-related lags, (ii) hand movement-related lags, and (iii) combined head and hand movement-related lags on manual task performance. Both discrete and continuous manual tasks have been investigated and the mechanisms responsible for the degraded performance with lags have also been studied and identified. In addition, quantitative models to predict the task performance in the presence of hand movement-related lags and combined hand and head movement-related lags are presented.

Four experiments were conducted. With discrete manual tasks, both additional head movement-related lags (≥ 110ms) and hand movement-related lags ([is more than or equal to 55ms) significantly increased the task completion times. Effects of hand movement-related lags were found to be significantly greater than the effects of head movement-related lags of the same magnitudes. With continuous manual tracking tasks, tracking errors increased with increasing hand movement-related lags but not with increasing head movement-related lags (up to 440ms). It was found that the effects of hand movement-related lags had significant interactions with the effects of target width but not with target distance. This contradicts some previous findings. A model explaining this interaction phenomenon has been developed. With the presence of head movement-related lags, the opposite was found. That is, the effects of head movement-related lags had significant interactions with the effects of target distance but not with target width. The interaction results indicate that, in the presence of lags, the effect of target width (W) and distance (D) should not be analyzed as a single effect of 'Index-of-Difficulty, ID' (ID = log₂ 2D/W). This is a new finding. With a constant lag, discrete target-directed hand movement times were found to be consistent with Fitts' laws (R² [is more than] 0.8). Regression models were developed to predict the effects of hand movement-related lags (R² [is more than] 0.9) and the combined effects of hand and head movement-related lags (R² [is more than] 0.9). The implications of the effects of lags are discussed.