Biomedical Engineering Reference
In-Depth Information
11.3.1.1 Perceptual Foundation
As people move, their view of their surroundings changes, and information about the
layout of the environment and the shape of surfaces, as well as their relative position
within the environment, is revealed.
The illusion of self-motion, known as
vection
, can be produced by visual stimulation
alone. For example vection can occur when a person is sitting in a stationary car and
the adjacent car starts to move forward, causing the person in the stationary car to
perceive the sensation of backwards motion.
Movement, essential for accurate perception of the environment, causes
optic flow
,
the changing pattern of light on the optic array caused by the relative motion of the
observer and environment. Optic flowpatterns contain information about self-motion,
the motion of objects, and the environment's three-dimensional (3D) structure. If an
observer is moving forward, the optic flow will radiate outward from the
center of
expansion
—the point toward which the person is moving; if a person is riding in a
train and looking out the window, the optic flow will move horizontally across the
observer's retina producing
lamellar flow
.
The results of a study by Warren led him to speculate that optical information could
be exploited to control locomotion [
38
]. An experiment by Konczak found that as
optic flow slowed, subjects' walking speed slightly increased; however increasing
the speed of optic flow appeared to have no effect on participants' real speed [
17
].
Konczak's results suggest that increasing the ratio between the users' virtual and real
walking speeds (i.e., increasing optical flow speed relative to walking speed) could
be employed to enable users to travel greater virtual distances in the same number
of steps.
11.3.1.2 Interfaces that Manipulate Speed
Real-walking locomotion techniques that alter the ratio between the user's real and
virtual speeds, thus altering optic flow, include Seven League Boots [
13
,
30
] and
Scaled Translational Gain [
42
]. Each of thesemethodsmaps the user's real translation
into increased virtual translation. For example, when the user takes one step in the
real world she is translated two or three steps in the virtual world.
Altering the ratio between the user's real and virtual speed enables the size of the
virtual scene to be scaled to a multiple of the size of the tracked space, based on
the ratio between real and virtual speeds. However problems can occur if the ratio
becomes very large. For example, if the user's motion is increased by a factor of 100,
then when the user takes one real step she travels 100 steps forward in the virtual
scene. This motion, although smooth and in the same direction as the user's motion,
may cause disorientation as it places the user far away from their starting location.
This rapid change in the user's location is similar to teleportation which is known to
disorient the user [
3
].
