Biomedical Engineering Reference
In-Depth Information
copy signals representing the commands issued to generate our movements. There
is also some suggestive evidence for a role of the auditory system (e.g., [
99
]) and
somatosensory system (e.g., [
33
]). Much work has been done to understand how
each of these sensory modalities contribute to self-motion individually, however,
researchers have only recently begun to evaluate how they are combined to form a
coherent percept of self-motion and the relative influences of each cue when more
than one is available.
6.3.1 Multisensory Nature of Walking
Since no single sense is capable of operating accurately under all circumstances,
the brain has evolved to exploit multiple sources of sensory information in order
to ensure both a reliable perception of our environment (see [
20
]) and appropriate
actions based on that perception [
37
]. A fundamental question in the cognitive neu-
rosciences asks what mechanisms are used by the central nervous system to merge
all of these sources of information to form a coherent and robust percept. It seems
that it employs two strategies to achieve robust perception. The first strategy, sensory
combination, describes interactions between sensory signals that are not redundant.
That is, information is specified in different coordinate systems or units. The second
strategy, sensory integration, reduces the variance of redundant sensory estimates,
thereby increasing their reliability [
37
].
Human locomotion is particularly interesting from the perspective of sensory
integration as it involves a highly dynamic system, meaning that the sensory inputs
are continuously changing as a function of our movements. For instance, with each
stride (i.e., from the heel strike of one foot to the next heel strike of the same foot)
the head moves up and down twice in a near sinusoidal fashion [
62
,
106
], thereby
generating continuously changing accelerations that are registered by the vestibular
system. Similarly, with each stride, the musculoskeletal system generates a set of
dynamically changing motor signals, the consequences of which are registered by
the proprioceptive system. Finally, the visual flow is likewise marked with periodic
vertical and horizontal components. Thus, the various pertinent sensory inputs are
in a systematic state of flux during walking. Moreover, findings that visual [
54
],
somatosensory [
116
], and vestibular [
6
] signals exhibit phase-dependent influences
on postural control during walking suggest the interesting possibility that the relia-
bilities of the sensory signals are also continuously changing and possibly in phase
with the different stages of the gait cycle.
A particularly influential group of models of multisensory integration have con-
sidered the problem from the point of view of efficiency. These efforts are often
referred to as the “Bayesian approach”, which was originally applied to visual per-
ception (e.g., [
15
,
17
,
64
]). It is acknowledged that neural processes are noisy [
38
]
and consequently, so are sensory estimates. The goal is then for the brain to come
up with the most reliable estimate, in which case the variance (i.e., noise) of the
final estimate should be reduced as much as possible. If the assumption is made
