Enabling veridical spatial perception in immersive virtual environments (IVEs) is an important yet elusive goal, as even the factors implicated in the often-reported phenomenon of apparent distance compression in HMD-based IVEs have yet to be satisfactorily elucidated. In recent experiments [e.g. 3], we have found that participants appear less prone to significantly underestimate egocentric distances in HMD-based IVEs, relative to in the real world, in the special case that they unambiguously know, through first-hand observation, that the presented virtual environment is a high fidelity 3D model of their concurrently occupied real environment. We had hypothesized that this increased veridicality might be due to participants having a stronger sensation of `presence' in the IVE under these conditions of co-location, which state of mind leads them to act on their visual input in the IVE similarly as they would in the real world (the presence hypothesis). However, alternative hypotheses are also possible. Primary among these is the visual calibration hypothesis: participants could be relying on metric information gleaned from their exposure to the real environment to calibrate their judgments of sizes and distances in the matched virtual environment. It is important to disambiguate between the presence and visual calibration hypotheses because they suggest different directions for efforts to facilitate veridical distance perception in general (non-co-located) IVEs. In this paper, we present the results of an experiment that seeks novel insight into this question.