self-motion. There are two challenges in this process. First, the information must
be fused quickly so that it is up to date and relevant (e.g., the person must know
that and how she has tripped in time to regain balance and footing before hitting
the ground). Second, the total information across all the sensory channels is often
incomplete. One theory states that, at any given time, a person has a
model
or
hypothesis of how she and surrounding objects are moving through the world.
This model is based on assumptions (some of which are conscious and cognitive,
while others are innate or hardwired) and previous sensory information.
New incoming sensory cues are evaluated
in terms of this model
, rather than
new models being continuously constructed from scratch. The model is updated
when new information arrives.
Consider the familiar example of a person on a stopped train that is beside
another stopped train. When the train on the adjacent track starts to move, the
person might have a short sensation that
her
train has started moving instead.
This brief, visually induced illusion of self-motion (vection) is consistent with all
of her sensory information thus far. When she looks out the other side of her train
and notices the trees are stationary (relative to her train), she has a moment of
disorientation or confusion and then, in light of this new information, she revises
her motion model such that her train is now considered stationary. In short,
one
tends to perceive what one is expecting to perceive
. This is an explanation for
why so many visual illusions work (Gregory, 1970). An
illusion
is simply the
brain’s way of making sense of the sensory information; the brain builds a model
of the world based on assumptions and sensory information that happen to be
wrong (Berthoz, 2000).
For a visual locomotion interface to convincingly give users the illusion that
they are moving, it should reduce things that make the (false) belief (that they
are moving) inconsistent. For example, a small display screen gives a narrow field
of view of the virtual scene. The users see the edge of the screen and much of the
real world; the real world is stationary and thus inconsistent with the belief that
they are moving. A large screen, on the other hand, blocks out more of the real
world, and makes the illusion of self-motion more convincing.
Perception is an active process, inseparably linked with action (Berthoz,
2000). Because sensory information is incomplete, one’s motion model is con-
stantly tested and revised via interaction with and feedback from the world. The
interplay among cues provides additional self-motion information. For example,
if a person sees the scenery (e.g., she is standing on a dock, seeing the side of a
large ship only a few feet away) shift to the left, it could be because she herself
turned to her right or because the ship actually started moving to her left.
If she has concurrent proprioceptive cues that her neck and eyes are turning to
the right, she is more likely to conclude that the ship is still and that the motion
in her visual field is due to her actions. The active process of self-motion percep-
tion relies on prediction (of how the incoming sensory information will change
because of the person’s actions) and feedback. Locomotion interfaces must
maintain such a feedback loop with the user.
4.1 Nature of the Interface
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