Recently, we reported that group subjective measures of presence as well as observers' postural responses are sensitive to increasing the realism of a display with motion content, by the addition of stereoscopic information, using a 20-inch stereoscopic screen with an effective horizontal field of view of 28 deg. (Freeman, Avons, Meddis, Pearson, & IJsselsteijn, 2000). The experiment presented here employed a large projection display with a 50 deg. horizontal field of view showing a rally car traversing a curved track at speed. The independent variables included image motion and stereoscopic presentation as within-subjects factors and screen size as a between-subjects factor. Dependent variables included subjective measures of presence, vection, involvement, and sickness, as well as observers' lateral postural responses, which served as a candidate objective corroborative measure of presence. Results demonstrated a noisy yet positive effect of stereoscopic presentation on the lateral postural responses. Post-test subjective ratings revealed a significant effect of stereoscopic presentation on the subjective judgments of presence, but not on those of vection, involvement, or sickness. Image motion had a large and significant effect on the subjective judgments of presence, vection, and involvement. The effect of image motion was considerably larger than that of stereoscopic viewing. By comparing results between experiments, a large effect of screen size on subjective presence ratings could be demonstrated, but only for the video stimulus that contained motion. The postural response measure did not differentiate between screen sizes, thus limiting its utility as an objective corroborative measure of presence, although further research is required to be able to be more firm in our conclusion regarding this issue.

We recently reported that direct subjective ratings of the sense of presence are potentially unstable and can be biased by previous judgments of the same stimuli (Freeman et al., 1999). Objective measures of the behavioral realism elicited by a display offer an alternative to subjective ratings. Behavioral measures and presence are linked by the premise that, when observers experience a mediated environment (VE or broadcast) that makes them feel present, they will respond to stimuli within the environment as they would to stimuli in the real world. The experiment presented here measured postural responses to a video sequence filmed from the hood of a car traversing a rally track, using stereoscopic and monoscopic presentation. Results demonstrated a positive effect of stereoscopic presentation on the magnitude of postural responses elicited. Posttest subjective ratings of presence, vection, and involvement were also higher for stereoscopically presented stimuli. The postural and subjective measures were not significantly correlated, indicating that nonproprioceptive postural responses are unlikely to provide accurate estimates of presence. Such postural responses may prove useful for the evaluation of displays for specific applications and in the corroboration of group subjective ratings of presence, but cannot be taken in place of subjective ratings.