Biomedical Engineering Reference
In-Depth Information
be distinguished from physical self-motion any more. When perceived depth is held
constant, vection strength linearly increases with increasing stimulus size, indepen-
dent of stimulus eccentricity [
63
]. This suggests that most affordable fishtank VR
(desktop-monitor-based) and HMDs are unsuitable for reliably inducing compelling
vection, as their field of view is typically not sufficiently large.
Eccentricity of moving stimulus.
Earlier studies argued that visual motion in the
periphery is more effective in inducing vection than central motion [
16
,
23
,
47
].
When display areas are equated, however, central and peripheral stimulus areas have
similar vection-inducing potential [
3
,
41
,
63
,
79
,
125
]. However, peripheral stimuli
need to be of lower spatial frequency to be maximally effective in inducing vection,
as our visual acuity systematically decreases in the periphery [
76
]. From an applied
perspective, this suggests that peripheral displays need not be of high resolution
unless users frequently need to focus there [
125
].
Stimulus velocity.
Increasing stimulus velocities generally tends to enhance both the
perceived velocity and intensity of vection, at least up to an optimal stimulus velocity
of, e.g., around 120
◦
/s for circular visual vection [
1
,
16
,
23
,
39
,
101
]. Note that these
maximum effective velocities are larger then the maximum stimulus velocities that
can easily be displayed in VR without noticeable and disturbing image artifacts (such
as motion blur or seeing multiple images) due to the limited update/refresh rate of
typically 60 Hz.
Density of moving contrasts.
The occurrence and strength of vection in general
increases with the number and density of moving objects and contrasts [
17
,
23
].
This suggests that VR simulations that are too sparse (e.g., driving in fog, or flight
simulations in clouds with low density of high-contrast objects) might not be able to
reliably induce vection without artificially increasing contrast and/or the density of
moving objects.
Viewpoint jitter.
A common explanation why vection does not occur instantaneously
is the inter-sensory conflict between those cues indicating stationarity (e.g., vestibular
cues) and those suggesting self-motion (e.g., moving visual cues or circular tread-
mill walking). This cue conflict account is corroborated by showing that bilaterally
labyrinthine defective participants perceive visual vection much earlier and more
intensely [
48
], and can perceive unambiguous roll or pitch vection through head-
over-heels orientations [
22
]. All the more surprisingly, however, there are situations
where increasing visuo-vestibular conflicts can enhance vection, as reviewed in [
72
]:
In a series of carefully designed experiments, Palmisano and colleagues demonstrated
that forward linear vection occurred earlier, lasted longer, and was more compelling
when coherent viewpoint jitter
1
was added to the expanding optic flow display [
77
],
1
Viewpoint jitter
refers to a specific optic flow pattern that simulates the visual “jittering” effects of
small head movements of the observer, similar to “camera shake”: For example, a constant, radially
expanding optic flow pattern that simulates forward linear motion would get an additional jittering
optic flow component on top if the visual effects of oscillating up-down head movements that occur
during normal walking is added to the expanding optical flow field.
