  Virtual Reality headsets enable users to explore the environment by
performing self-induced movements. The retinal velocity produced by such motion
reduces the visual system's ability to resolve fine detail. We measured the
impact of self-induced head rotations on the ability to detect quality changes
of a realistic 3D model in an immersive virtual reality environment. We varied
the Level-of-Detail (LOD) as a function of rotational head velocity with
different degrees of severity. Using a psychophysical method, we asked 17
participants to identify which of the two presented intervals contained the
higher quality model under two different maximum velocity conditions. After
fitting psychometric functions to data relating the percentage of correct
responses to the aggressiveness of LOD manipulations, we identified the
threshold severity for which participants could reliably (75\%) detect the
lower LOD model. Participants accepted an approximately four-fold LOD reduction
even in the low maximum velocity condition without a significant impact on
perceived quality, which suggests that there is considerable potential for
optimisation when users are moving (increased range of perceptual uncertainty).
Moreover, LOD could be degraded significantly more in the maximum head velocity
condition, suggesting these effects are indeed speed dependent.
