Biomedical Engineering Reference
In-Depth Information
attempt to alter the pattern of variability exhibited by the individual's gait cycle
(e.g., [
47
]). VR can also be used to manipulate the locomotor trajectory of patients
during over-ground walking (e.g., [
12
]). In other words, VR can be used to modify
control parameters, thereby allowing researchers to test specific predictions about
the role of those parameters in clinical assessment. These behaviors are a result of
complex interactions at various control levels. Consequently, the examination of the
various control parameters must take place at multiple scales of observation to fully
understand the system dynamics. The remainder of this chapter will focus on several
novel VR applications for the assessment of functional mobility at the level of the
gait cycle and the level of the locomotor trajectory.
15.2.1 Dynamic Measures for Assessing Local Functional
Mobility Using VR
Synchronizing to a stimulus is an experimental method commonly used to influence
the timing properties of motor behavior. For example, much like the van Wegen et al.
[
60
] study in which visual cues were employed to influence the step frequency, and
consequently the mobility, of patients with Parkinson's disease, rhythmic auditory
stimulation with a metronome has been used to improve the mobility of these patients
as well [
30
,
35
,
56
,
77
]. The perceptual-motor differences between synchronizing
to an auditory versus a visual metronome have been described elsewhere [
23
], but
it remains unclear which is optimal for purposes of assessment and rehabilitation.
Recently, it has been demonstrated that more efficient adaptation to a perturbation
(i.e., visual or auditory disruption of the stimulus rhythm) occurs when elderly par-
ticipants synchronize to a visual stimulus [
2
]. This finding provides evidence for the
privileged role visual information seems to play in the modification of acute changes
to the gait cycle in healthy elderly adults. Given the biological nature of human
gait, however, synchronizing to a metronome with fixed time intervals may not be
effective in facilitating adaptive gait patterns and enhancing functional mobility.
Variability in the gait cycle, once thought to be a random by-product of bio-
logical noise, is now believed to reflect adaptive, functional gait (c.f., [
20
,
54
]).
Specifically, the variation in the stride-to-stride time intervals of healthy adults
exhibits scale invariant (fractal) temporal correlations, as indexed by detrended fluc-
tuation analysis (DFA
7
;[
21
]). Accordingly, asking a patient to synchronize to a
metronome having fixed time intervals may actually reduce this natural variation,
interfering with functional gait. Conversely, if humans can synchronize to a variable,
or “noisy”, visual metronome, this may enhance adaptive variation in their gait. A
noisymetronome produces irregular intervals—some are shorter and some are longer
7
DFA computes scaling exponents that relate a measure of variability, the detrended fluctuation
function, to the time scale over which the function was computed. It is used to identify the presence
or absence of persistence (i.e., a large value tends to follow a large value and a small value tends to
follow a small value) in a time series. For full details, see Peng et al. [
42
].
