Biomedical Engineering Reference
In-Depth Information
the proprioceptive-vestibular gain was 1.0. In the incongruent trials, systematic con-
flicts were introduced between the vestibular and proprioceptive inputs. This was
achieved by having participants walk at one rate, while the disk was moved at a
different rate. Specifically, proprioceptive gains of 0.7 and 1.4 were applied to two
vestibular velocities (25
◦
/s and 40
◦
/s). To achieve a gain of 0.7, the disk moved in
the same direction as the handlebar but at 30% of its speed. To achieve a gain of
1.4, the disk moved at 40% of the handlebar speed but in the opposite direction.
We also tested two additional conditions. In the “walking in place” condition, par-
ticipants walked in place on the treadmill but did not move through space. Like in
previous studies, participants were instructed to use the proprioceptive information
from their legs to update their egocentric position as if they were moving through
space at the velocity specified by the CTM. In the “passive movement” condition,
participants stood still while they were passively moved by the CTM. Spatial updat-
ing was measured using a continuous pointing task similar to that introduced by
Campos et al. [
22
] and Siegle et al. [
92
], which expanded upon a paradigm originally
developed by Loomis and colleagues [
43
,
67
]. The task requires the participant to
continuously point at a previously viewed target during self-motion in the absence of
vision. A major advantage of this method is that it provides continuous information
about perceived target-relative location and thus about self-velocity during the entire
movement trajectory. The results were consistent with an MLE model in that par-
ticipants updated their position using a weighted combination of the vestibular and
proprioceptive cues, and that performance was less variable when both cues were
available.
Unfortunately the results did not allow us to determine the relative weighting of
the two cues (see [
42
]). We therefore conducted a new experiment which employed
a standard psychophysical 2-interval forced choice (2-IFC) paradigm (see [
45
], for
an introduction). Experimental details are provided in the caption of Fig.
6.4
.In
each trial participants walked two times and they indicated in which of the two
they had walked faster. In one interval (the standard) participants walked under
various conditions of conflicting vestibular and proprioceptive signals, while in a
second interval (the comparison) they walked through space without cue conflict. By
systematically changing the comparison (i.e., handlebar velocity) we can determine
the point at which the standard and comparison were perceptually equivalent (i.e.,
the point of subject equality, or PSE).
Figure
6.4
a shows the mean PSEs as a function of vestibular input. In the con-
ditions with conflicting inputs, the PSEs lie between the two extreme cases (solid
horizontal and diagonal line). Also, the PSEs are not on a straight line, indicating that
the relative weighting depends on the vestibular input. This is illustrated in Fig.
6.4
b
where the vestibular weights are plotted for the different conflict conditions. The
proprioceptive input is weighted higher in the two conditions where the vestibu-
lar input was smaller (20 or 30
◦
/s) than the proprioceptive input (40
◦
/s). However,
when the vestibular input was larger (50
◦
/s) than the proprioceptive input, their
respective weights were practically equal. This raises the question of whether, con-
trary to the instruction to judge their walking speed, participants were simply using
their perceived motion through space (i.e., the vestibular input) to perform the task.
