Biomedical Engineering Reference
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The authors suggest that this surprising finding might be caused by a decrease
of the relative weight of the visual cues when observers are walking as compared
to standing still. We propose that this effect might also be related to Wertheim and
Reymond's explanation of the freezing illusion (where an optic flow pattern suddenly
appears to freeze when vestibular stimulation is added) and the Pavard and Berthoz
effect, in that the perceived relative velocity of the visual motion might be reduced
by the biomechanical motion [
124
]. Additional factors might also have contributed:
Apart from affecting the occurrence and amount of vection, differences in the veloc-
ity of treadmill walking and visually presented motion can also induce changes in
perceived self-motion and stepping movements [
25
,
55
] as well as adaptation and
re-calibration (e.g., [
25
,
98
]).
While Kitazaki and colleagues observed an inhibition of vection when locomotor
cues matched the direction of visual motion, Seno et al. recently reported the oppo-
site effect [
104
]: Using visual motions that were 30 times faster than the treadmill
walking motions (58 km/h as compared to 2 km/h, respectively), they observed that
visually-induced forward vection was facilitated by consistent biomechanical cues,
whereas inconsistent walking cues impaired vection. In addition, they showed that
locomotion cues from walking on a linear treadmill could systematically bias the
strength and direction of vection perceived for up-down and left-right translational
visual motion. Comparing the results from Kitazaki et al. and Seno et al. suggests
that the differences between visual and walking speed might be critical, with vec-
tion facilitation occurring for larger visual motion speeds, and impairment found for
matching visual speeds.
A recent study confirmed that forward walking on a linear treadmill can indeed
impair visually induced vection when visual and treadmill velocities are matched
[
4
]. Similar impairments of visually-induced linear were observed when the visual
display depicted backward motion while participants walked forwards (exp. 2) or
when participants simply walked on the spot while viewing forward vection displays
(exp. 3). When the head motions that naturally occurred during treadmill walking
were tracked and used to update the visual stimulus according to the changed view-
point (thus mimicking real-world walking), vection strength increased [
4
]. However,
a similar facilitation of vection was observed in passive viewing conditions when
participants stood still and simulated viewpoint jitter was added to the visual display,
thus confirming earlier studies (see review by Palmisano et al. [
72
]). Thus, even
when head motions were tracked during treadmill walking, vection was still reduced
compared to standing still and passively viewing the jittered display.
In conclusion, it remains puzzling how adding velocity-matched treadmill walking
to a visual motion simulation can impair vection [
4
,
52
,
69
] while active head motions
and simulated viewpoint jitter clearly enhance vection [
72
]. More research is needed
to better understand under what conditions locomotion cues facilitate or impair linear
vection, and what role the artificiality of treadmill walking might play. Nevertheless,
the observation that self-motion perception can, at least under some circumstances,
be impaired if visual and biomechanical motion cues are matched seems paradoxical
(as it corresponds to natural eyes-open walking) and awaits further investigation.
These results do, however, suggest that adding a walking interface to a VR simulator
