Biomedical Engineering Reference
In-Depth Information
induced vection are provided by, e.g., [
2
,
23
,
38
,
39
,
61
,
123
]. Vection with a specific
focus on VR, motion simulation, and undesirable side-effects has more recently been
reviewed in [
34
,
86
,
100
].
When stationary observers view a moving visual stimulus that covers a large part
of the field of view (FOV), they can experience a very compelling and embodied
illusion of self-motion in the direction opposite to the visual motion. Many of us
have experienced this illusion in real life: For example, when we are sitting in a
stationary train and watch a train pulling out from the neighboring track, we will
often (erroneously) perceive that the train we are sitting in is starting to move instead
of the train on the adjacent track [
33
]. This phenomenon of illusory self-motion has
been termed “vection” and has been investigated for well over a century [
33
,
60
,
114
,
122
,
127
]. Vection has been shown to occur for all motion directions and along all
motion axes: Linear vection can occur for forward-backward, up-down, or sideways
motion [
38
]. Circular vection can be induced for upright rotations around the vertical
(yaw) axis, and similarly for the roll axis (frontal axis along the line of sight, like in a
“tumbling room”), and also around the pitch axis (an imagined line passing through
the body from left to right). The latter two forms of circular vection are especially
nauseating, since they include a strong conflict between visual and gravitational cues
and in particular affect the perceived vertical [
11
].
2.2.1 Circular Vection
In a typical classic circular vection experiment, participants are seated inside an
upright rotating drum that is painted with black and white vertical stripes (see illus-
tration in Fig.
2.1
a), a device called optokinetic drum [
16
,
23
]. After the optokinetic
drum starts to rotate around the earth-vertical axis, the onset latency until the par-
ticipant reports perceiving self-motion is measured, which ranges from about 2-20 s
typically, depending on various stimulus and procedural parameters as discussed
below.
Note that vection typically does
not
occur instantly with the stimulus motion, and
takes some time to saturate, as sketched in Fig.
2.2
. The strength of the illusion
can be measured by a variety of introspective measures including the onset latency
and duration of the illusion, or by some indication of perceived speed, intensity,
or compellingness of self-rotation, e.g., by magnitude estimation or by letting the
participant press a button every time they think they have turned 90
◦
[
8
]. As Riecke
et al. point out, one of the challenges for utilizing self-motion illusions in VR is to
reduce the vection onset latency and increase the intensity and compellingness of the
illusion [
94
].
The most frequently investigated type of vection is circular vection around the
earth-vertical axis (see illustrations in Fig.
2.1
). In this special situation where the
observer perceives self-rotation around the earth-vertical axis, there is no interfering
effect of gravity, since the body orientation always remains aligned with gravity dur-
ing illusory self-rotation. Roll and pitch vection are consequently harder to induce and
