Towards a “Brain-Guided” Cognitive Architecture - Bioinspired Approaches for Human-Centric Technologies - page 203

mainly because the propagation delays in the feedback loop are a severe, potential
source of instability. In contrast, intrinsic muscle stiffness has two strong beneficial
effects: (1) it provides, locally (i.e., in a muscle-wise manner), an instantaneous
disturbance compensation action, and (2) it induces, globally (i.e., in a total body-
wise manner), a multidimensional force field with attractor dynamics. This allows
to achieve complex body postures “for free,” without a complex, high-dimensional
computational process, but simply by allowing the intrinsic dynamics of the
neuromuscular system to seek its equilibrium state.
In this framework, movement becomes the transition from an equilibrium state
to another, with the remarkable property of “equifinality” (Kelso and Holt 1980 ),
namely, the fact that movement endpoints should be scarcely affected either by
small, transient perturbations or by variations in the starting position of the body.
Such attractor properties of motor control were confirmed by several studies of
electrical stimulation of different parts of the nervous system, such as interneurons
in the spinal cord of the frog (Giszter et al. 1993 ) or pyramidal neurons in the
precentral cortex of the monkey (Graziano et al. 2002 ).
In reality, the picture is more complicated, in the sense that detailed experimen-
tal investigations show, for example, that muscles can only be approximated by
ideal springs and that equifinality can be somehow violated by small, impulsive
force disturbances (Popescu and Rymer 2000 ) or specific environmental conditions.
In spite of this, we believe that EPH can explain a lot of the overall rationale
underlying synergy formation, although it cannot cover the whole range of situa-
tions. Consider, for example, the stability of the upright standing body and the
coordination in whole-body aiming movements: in this case, muscle stiffness alone
is insufficient to achieve stability (Loram and Lakie 2002 ) and requires a parallel
intermittent control action (Asai et al. 2009 ); on the other hand, the appropriate
synchronization of ankle and hip strategies, which is essential for whole-body
aiming, is nicely explained by means of an extended force field-based coordination
model (Morasso et al. 2010 ), based on the Passive Motion Paradigm (see below).
Motor imagery is quite important, again, for framing the discourse in the right
perspective. Since in humans and other species in the high stages of phylogenetic
development, actions can be goal oriented, not necessarily stimulus oriented, and
can occur in anticipation of events/stimuli or in learned cycles, real/overt actions
can alternate with covert/mental actions in order to optimize the chance of success
in a game or during social interaction. Therefore, overt actions are just the tip of an
iceberg: under the surface it is hidden a vast territory of actions without movements
(covert actions) which are at the core of motor cognition . This has two main
consequences: (1) the format of spatiotemporal patterns of purposive actions,
namely, the organization of the synergy formation process, must be shared by
covert and overt actions; (2) this format cannot be strictly dependent upon the
physics of the body and the neuromuscular system, because in covert actions there
is no motion of body masses or contraction of the muscles. We may then derive the
hypothesis that the endogenous dynamics of cortical maps is basically the same in
overt movements, when it drives the formation of neuromuscular activation pat-
terns, and in covert movements, when it carries out mental simulations of the same