Biomedical Engineering Reference
In-Depth Information
State-of-the-Art on Neurolocomotion
the biped was composed of two links, and each
oscillator controlled the movement of a single
link. Bipedal walking and hopping were simulated
by using the oscillators' output to determine the
angular positions of the respective links. Transi-
tions between out-of-phase and in-phase gaits
were generated by changing the nature of the
inter-oscillator coupling, e.g., the polarities of
the network interconnections were reversed to
produce the walk-to-hop transition.
This approach is, in principle, physiologically
reasonable. The notion that supraspinal centres
may call on functionally distinct sets of coordi-
nating interneurons to generate different gaits is
plausible but not yet experimentally established.
In addition, from a different but relevant perspec-
tive, it is shown that rhythmic neuronal networks
can be modulated, e.g., reconfigured, through the
actions of neuroamines and peptides, and thereby
enabled to produce several different motor pat-
terns (Pearson, 1993).
Legged animals usually adopt various gait pat-
terns in their terrestrial locomotion for various
reasons, e.g., avoiding dangers, adapting terrain,
or just obeying the willingness of changing gaits.
Although many biological experiments have
shown that generation of animal's gait patterns
is a result of interactions between CNS and feed-
back of external stimulations, which induces wide
admission of the existence of CPGs, the neural
mechanisms underlying CPGs are still not well
understood. One of the crucial questions unde-
termined is whether a unique CPG is sufficient
for governing switching among various gait pat-
terns or different CPGs are required to generate
different gaits in the real-life biological systems
(Collins, 1995). So far many models have been
suggested on CPGs mechanism of vertebrate
and invertebrate animals, for instance, the biped
(Bay & Hemami, 1987; Taga et al., 1991; Taga,
1995), quadruped (Schöner et al., 1990; Collins &
Richmond, 1994; Collins & Stewart, 1993a), and
hexapod gait models (Collins & Stewart, 1993b;
Beer, 1990). Most of them follow these two lines
and are based on the coupled, nonlinear oscillator
method for modelling.
A Synergetic Approach
Synergetics deals with cooperative phenomena.
In synergetics, the macroscopic behaviour of a
complex system is characterised by a small num-
ber of collective variables which in turn govern
the qualitative behaviour of the system elements
(Collins, 1995).
Schöner and colleagues used a synergetic
approach in a study of quadruped locomotion
(Schöner et al., 1990). They analysed a network
model that was made up of four coupled oscillators,
each representing a limb of a model quadruped.
The phase difference among limbs were used as
collective variables to characterise the interlimb
coordinative patterns of this discrete dynamical
system. Gait transitions were simply modelled as
phase transition, which could also be interpreted
as bifurcations in a dynamical system.
This approach is significant in that it relates
system parameter changes and stability issues to
A Neuromodulatory Approach
As a model for legged locomotion control, Grillner
proposed that each limb of an animal is governed
by a separate CPG (Grillner, 1975, 1985), and that
interlimb coordination is achieved through the
actions of interneurons which couple together
these CPGs. With this scheme, gait transitions
are produced by switching between different sets
of coordinating interneurons.
Grillner's strategy has been adopted, in spirit,
by some CPGs modelling studies. For instance,
Bay and Hemami (1987) used a CPG network of
four coupled van der Pol oscillators to control the
movements of a segmented biped. Each limb of
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