Biomedical Engineering Reference
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whereas incoherent jitter impaired vection [
74
]. This was found even when the dis-
play was perceived as flat and did not contain any depth cues [
64
]. Overall, simulated
viewpoint jitter shows a larger vection-facilitating effect if it is orthogonal to the main
vection direction [
64
,
73
,
78
]. In VR, such findings could be used to enhance vection
by, for example, adding viewpoint oscillations induced by walking or head motions
[
4
,
19
] as is sometimes done in gaming. This should be carefully tested, however,
as adding image jitter or oscillations can increase not only vection, but also motion
sickness [
73
].
2.7.2 Cognitive and Top-Down Contributions to Vection
While earlier vection research focused predominately on perceptual and lower-level
factors, there is increasing evidence that vection can also be affected by what is
outside of the moving stimulus itself, by the way we move and look at a moving
stimulus, our pre-conceptions, intentions, and how we perceive and interpret the
stimuli, which is of particular importance in the context of VR. Vection might even
be directly or indirectly affected by cognitive/top-down processes [
3
,
57
,
61
,
96
].
Below we will discuss some of these examples. More comprehensive reviews are
provided by [
85
,
86
,
100
].
Viewing pattern and perceived foreground-background relationship.
Fixation
on a stationary foreground object or simply staring at the moving visual stimulus
has long been known to enhance visual vection, as compared to natural viewing
or smooth pursuit [
28
,
60
,
121
,
122
]. Suppressing the optokinetic reflex seems to
play a central role here, and this is facilitated when a fixation object is provided [
8
].
Potentially related to this, stationary foreground objects facilitate vection (especially
if centrally presented), whereas stationary background stimuli reduce vection, espe-
cially if presented peripherally [
17
,
42
,
62
]. Of particular importance seems to be the
perceived foreground-background or figure-ground relationship, in that vection tends
to be dominated by motion of the perceived background, even if the background is not
physically further away than the perceived foreground [
17
,
45
,
53
,
63
,
65
,
67
,
68
].
This “object and background hypothesis for vection” has been elaborated upon and
confirmed in an elegant set of experiments using perceptually bistable displays like
the Rubin's vase that can be perceived either as a vase or two faces [
103
].
In VR simulations, these findings could be used to systematically reduce or
enhance illusory self-motions depending on the overall simulation goal, e.g., by
modifying the availability of real or simulated foreground objects (e.g., dashboards),
changing peripheral visibility of the surrounding room (e.g., by controlling light-
ing conditions), or changing tasks/instructions (e.g., instructions to pay attention to
instruments which are typically stationary and in the foreground).
Naturalism, presence, and interpretation of the moving stimulus.
Naturalism
and ecological validity of the moving stimulus has also been suggested to affect
vection [
84
,
116
], potentially due to our inherent assumption of a stable environment
