Stepping-Driven Locomotion Interfaces - Human Walking in Virtual Environments - page 248

An additional problem with speed-scaling methods arises because people move their
heads side-to-side as well as forward-to-backward as they walk. When the ratio
between real and virtual motion is large, the side-to-side motions are also multiplied
and can cause the scene to appear unstable. To eliminate the side-to-side motion,
Interrante et al. computed the user's forward direction and scaled user motion only
in this predicted direction [ 13 ].
Another potential problem with altering user speed is that when the difference
between physical and virtual speeds is large, people will be able to notice the dis-
crepancy. A method introduced by Bruder et al. uses change blindness techniques
to effectively move the user forward in the VE while the user is unaware of it [ 5 ].
Change blindness theory posits that people are unaware of changes made in their
view when the changes occur during saccadic eye movements. Change blindness is
discussed further in Chap. 14 . As is common in change blindness techniques, Bruder
et al. display a blank screen that flashes in the HMD for 60-100ms. While the screen
is blanked, the virtual scene is translated in the user's direction, thus altering the ratio
between the user's real and virtual speed. Due to change blindness, the user is less
aware of the alterations that have occurred.
11.3.2 Manipulating Direction
Manipulating direction for real-walking techniques can be thought of as altering the
ratio between real world direction and virtual world directions of movement.
11.3.2.1 Perceptual Foundation
Altering the ratio between real and virtual directions is possible because vision guides
heading direction , the user's direction of motion. The egocentric direction hypothesis
and Gibson's theories about optic flow [ 10 ] provide theoretical support for locomo-
tion systems that guide user direction by manipulating the user's view of the virtual
scene as generated by the IVE system.
The egocentric direction hypothesis states that heading direction is determined by the
anterior-posterior axis of the body. This theory was explored by Rushton et al. after
observing a subject who suffers from unilateral visual neglect (UVN)—damage
to one side of the cerebral hemisphere and the inability to respond to stimuli on
the side opposite the lesion [ 31 ]. UVN is often associated with a misperception of
location. Rushton et al. observed the subject walking in curved paths to reach target
objects. To simulate the misperception of the target location for individuals without
UVN, Rushton et al. had participants wear prisms in front of their eyes and found
participants walked a curved path toward the target. The prism translates not only the
target object, but also the optic flow produced when the participant walked toward
the target (Fig. 11.8 ).