Towards a “Brain-Guided” Cognitive Architecture - Bioinspired Approaches for Human-Centric Technologies - page 199

internal simulations on an interconnected set of neuronal networks is, in our view,
the main function of what is known as body schema .
7.2.2 Synergies
Synergy is a compound noun of Greek origin that implies the interaction and
cooperation of two or more elements for carrying out some function or work
which is difficult or impossible to achieve with isolated elements. Bernstein
( 1935 ) was among the first ones to use this term for describing the complexity of
the motor system, recognizing that the central problem in the neural control of
movement is motor redundancy , namely the imbalance between (a small number
of) task-related variables and the (extremely large number of) muscles and mechan-
ical degrees of freedom (DoF). He suggested that the brain uses synergies to solve
this problem, giving this term a strongly cybernetic meaning, indeed years before
Norbert Wiener invented the term cybernetics: the idea, although not developed in a
mathematical model, was that synergies allow the brain to get rid of task-irrelevant
degrees of freedom, thus focusing on the simpler problem of mastering a smaller
number of task-relevant variables. In this sense, a synergy can be conceived as a
“dimensionality-reduction device,” and as such it has been criticized by some (e.g.,
Diedrichsen and Classen 2012 ) considering that deterministic constraints on the
evolution of DoFs would imply the inability to achieve large subsets of physically
possible postures, an inability which is contradicted by a number of experimental
findings in speech motor control, whole-body reaching, brain-machine interfaces,
etc. However, this criticism can be overcome by supposing that the computational
mechanism, responsible for constraining DoFs and muscle activation patterns in
such a way to allow a small number of command variables to coordinate them in a
purposive manner, is not hardwired but is sensitive to task requirements, imposing
task-related constraints in the preparation time of an action. In this view, biologi-
cally plausible synergy formation mechanisms must be multireferential, in the
sense of allowing task-modulated bidirectional dynamic interactions among differ-
ent spaces: end-effector space, joint and muscles space, and possibly spaces related
to the DoFs of manipulated tools. If such dynamic interactions are acquired by the
brain of a subject via training in the real world, they will incorporate implicitly
causality constraints, thus allowing a synergy formation mechanism to bind
together high-dimensionality and low-dimensionality computational processes.
This means that dimensionality reduction can coexist with full dimensionality
representation also in a deterministic framework, provided that suitable dynamic
processes link the different spaces. Later on we describe a mathematical model,
based on Passive Motion Paradigm (PMP), that can achieve this goal.
In recent years a lot of effort has been focused on muscle synergies (D'Avella
et al. 2003 ). It has been found that, for a wide variety of motor tasks, muscle
activation patterns evolve in low-dimensional manifolds and thus can be approxi-
mated by the linear composition of a small set of predefined/primitive patterns or