Biomedical Engineering Reference
In-Depth Information
This was particularly true when participants walked in a dense forest on a cloudy
day, with the sun hidden behind the clouds. Most participants also repeatedly crossed
their own path without any awareness of having done so. However, under conditions
in which directional references such as landmarks or the solar azimuth were present,
people were actually able to maintain a fairly straight path, even in an environment
riddled with obstacles, such as a forest. A popular explanation for walking in circles
is based on the assumption that people tend to be asymmetrical with respect to, for
instance, leg length or leg strength. If this were true, it would be hypothesized that a
particular individual would always turn in the same direction. However, this was not
the case. In fact, inconsistency in turning and veering direction was very common
across participants. Moreover, measured leg strength differences could not explain
the turning behavior, nor could leg length.
Interestingly, the recorded walking trajectories show exactly the kind of behavior
that would be expected if the subjective sense of straight ahead were to follow
a correlated random walk. With each step, a random error is added to the subjective
straight ahead, causing it to drift away from the true straight ahead. As long as the
deviation stays close to zero, people walk in randomly meandering paths. When the
deviation becomes large, it results in walking in circles. This implies that circles
are not necessarily an indication of a systematic bias in the walking direction but
can be caused by random fluctuations in the subjective straight ahead resulting from
accumulating noise in the sensorimotor system, in particular the vestibular and/or
motor system.
Another possible contribution to deviating from a straight path, not considered
in Souman et al. [
93
] study is the instantaneous orientation of the head with respect
to the trunk. It has been shown that eccentric eye orientation (e.g., [
82
]) and head
orientations tend to be related to the direction of veer from a straight course. The most
common finding is that people veer in the direction of eye/head orientation [
113
].
For instance, in a series of driving experiments, Readinger et al. [
82
] consistently
found that deviations in a driver's gaze can lead to significant deviations from a
straight course. Specifically, steering was biased in the direction of fixation. They
tested a large range of eye positions, between
45
◦
. Interestingly, the
largest effect was obtained with an eccentric eye position of as little as 10
◦
and
leveled off beyond that. Thus, even a small deviation of 5
◦
created a significant bias.
A very similar bias has been found during visually guided walking [
28
,
111
]. Jahn
et al. [
59
] asked participants to walk straight towards a previously seen target placed
10m away while they were blindfolded. Their results demonstrated, contrary to all
previous work, that with the head rotated to the left, participants' path deviated to the
right, and vice versa. The effect of eye position showed the same pattern, but was not
significant. The authors interpreted this as a compensation strategy for an apparent
deviation in the direction of gaze due to the lack of the appropriate visual feedback.
Intrigued by the counterintuitive results of Jahn et al. [
59
] study we conducted
a very similar experiment in an attempt to replicate these results. The results (see
Fig.
6.6
. and caption for details) suggest a bias in the direction of veering in the same
direction as the head turn. The bias was asymmetric in that it was larger when the
head was turned to the left than when the head was turned to the right. There was
45
◦
and
−
+
