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Abstract

INTRODUCTION: The prevailing notion of visually induced illusory self-motion perception
(vection) is that the illusion arises from bottom-up perceptual processes. Therefore, past research
has focused primarily on examining how physical parameters of the visual stimulus
(contrast, number of vertical edges etc.) affect vection. In this study, we examined the inuence
of a top-down process: Spatial presence in the simulated scene. Spatial presence was
manipulated by presenting either a photorealistic image of the T¨ubingen market place or modi
ed versions of the same stimulus. Modied stimuli were created by either slicing the original
image horizontally and randomly reassembling it or by scrambling image parts in a mosaic-like
manner. We expected scene modication to decrease spatial presence and thus impair vection.
METHODS: Ten naive observers viewed stimuli projected onto a curved projection screen
subtending a eld of view (FOV) of 54
~
x40.5
~
. We measured vection onset times and had
participants rate the convincingness of the self-motion illusion for each trial using a 0–100
scale. In addition, we assessed spatial presence using standard presence questionnaires.
RESULTS: As expected, scene modication led to both reduced presence scores and impaired
vection: Modied stimuli yielded longer vection onset times and lower convincingness
ratings than the intact market scene (t(9)=-2.36, p=.043 and t(9)=3.39, p=.008, resp.). It should
be pointed out that the scrambled conditions had additional high contrast edges (compared
to the sliced or intact stimulus). Previous research has shown that adding vertical high contrast
edges facilitate vection. Therefore, one would predict that the scrambled stimuli should
improve vection. The results show, however, a tendency towards reduced vection for the scrambled
vs. sliced or intact stimuli. This suggests that the low-level information (more contrast
edges in the scrambled stimulus) were dominated by high level information (consistent reference
frame for the intact market scene). Interestingly, the number of slices or mosaics (2, 8,
or 32 per 45
~
FOV) had no clear inuence on either perceived vection or presence; two slices
were already enough to impair scene presence.
CONCLUSIONS: These results suggest that there might be a direct relation between spatial
presence and self-motion perception. We posit that stimuli depicting naturalistic scenes
provide observers with a convincing reference frame for the simulated environment which enables
them to feel “spatially present” in that scene. This, in turn, facilitates the self-motion
illusion. This work not only can shed some light on ego-motion perception, but also has important
implications for motion simulator design and application.