Biomedical Engineering Reference
In-Depth Information
In fact, this strong coupling has lead Frissen et al. [
42
] to argue for a “mandatory inte-
gration” hypothesis which holds that during walking the brain has adopted a strategy
of always integrating the two signals. It also leads to substantial experimental dif-
ficulty when attempting to obtain independent measures from the individual senses
(see also [
24
]). Consequently, during the often used, “proprioceptive only” walking
in place condition, vestibular inputs are in fact concurrently present, yet specify a
stationary position. This thus creates a potential sensory conflict when the aim is to
obtain unbiased unisensory estimates. The reverse conflict occurs in the “vestibular
only” PM condition, where the proprioceptive input specifies a stationary position.
Although in this case, it should be noted, that there are numerous instances in which
vestibular excitation is experienced without contingent proprioceptive information
from the legs, including whenever we move our head, or when moving in a vehicle.
In other words, in the case of passive movements, the coupling may not be as tight.
Despite the fact that it is difficult to obtain unisensory proprioceptive and vestibu-
lar estimates, it is possible to create conditions in which the conflict between the
vestibular and proprioceptive cue are much reduced and, moreover, controllable.
This will enable us to determine the relative weighting of the individual cues. One
way is to use a rotating platform in combination with a handlebar that can be moved
independently. An early example of this was a platform used by Pick et al. [
79
],
which consisted of a small motorized turntable (radius 0.61m) with a horizontal
handle mounted on a motorized post extending vertically through the center. Using
this setup Bruggeman et al. [
14
] introduced conflicts between proprioceptive and
vestibular inputs while participants stepped around their earth-vertical body axis.
Participants always stepped at a rate of 10 rotations per minute (rpm) (constituting
the proprioceptive input), but because the platform rotated in the opposite direc-
tion, participants were moved through space at various different rates (constituting
the vestibular input). They found that when the proprioceptive and vestibular inputs
were of different magnitudes, the perceived velocity fell somewhere between the
two presented unisensory velocities, thus suggesting that the brain uses a weighted
average of vestibular and proprioceptive information as predicted by MLE (see also
[
5
]). However, a limitation of this type of relatively small setup is that it only allows
participants to perform rotations around the body axis. That is, it allows participants
to step in place, which is a very constrained and rather unnatural mode of locomotion
with biomechanics that are different from normal walking.
Such restrictions do not apply to the CTM (Fig.
6.2
) which allows for full stride
curvilinear walking. This unique setup also allows us to manipulate vestibular, pro-
prioceptive (and visual) inputs independently during walking. In one of our recent
studieswe assessedmultisensory integration during self-motion using a spatial updat-
ing paradigm that required participants to walk through space with and without
conflicting proprioceptive and vestibular cues [
42
]. The main condition was the
multisensory, “walking through space” condition during which both vestibular and
proprioceptive systems indicated self-motion. This condition consisted of both con-
gruent and incongruent trials. In the congruent trials, participants walked behind
the handlebar while the treadmill disk remained stationary. Thus, the vestibular
and proprioceptive inputs conveyed the same movement velocities; in other words,
