Towards a “Brain-Guided” Cognitive Architecture - Bioinspired Approaches for Human-Centric Technologies - page 201

practice for improving performance in athletic skills (Suinn 1972 ) and the fact that
stroke patients can use mental practice to regain motor function (Sharma
et al. 2006 ). We should also take into account that, in spite of similarities, there is
also evidence that motor imagery and neural processes during overt motor behavior
are not exactly the same (Coelho et al. 2012 ). Nevertheless, the same existence of
motor imagery indicates that muscle synergies are unlike as basic building blocks of
the synergy formation circuitry and suggests that what occurs in the brain, during
mental rehearsal or mental training, reflects an endogenous dynamics, not a dynam-
ics related to the neuromuscular system, as involved in overt movements. In other
words, “muscleless” motor synergies, occurring in covert movements, might be the
hidden building blocks which stand behind the recorded muscle synergies.
In any case, there is mounting evidence accumulated from different directions
such as brain imaging studies (Frey and Gerry 2006 ; Grafton 2009 ), mirror neuron
systems (Rizzolatti et al. 1996 ; Rizzolatti and Luppino 2001 ; Rizzolatti and
Sinigaglia 2010 ), and embodied cognition (Gallese and Sinigaglia 2011 ; Gallese
and Lakoff 2005 ) that generally supports the idea that action “generation, obser-
vation, imagination, and understanding” share similar underlying functional net-
works in the brain: distributed, multicenter neural activities occur not only during
imagination of movement but also during observation and imitation of other's
actions (Buccino et al. 2001 ; Anderson 2003 ; Frey and Gerry 2006 ; Grafton
2009 ; Iacoboni 2009 ) and comprehension of language, namely action-related
verbs and nouns (Pulverm¨ ller and Fadiga 2010 ; Glenberg and Gallese 2012 ).
Such neural activation patterns include premotor and motor areas as well as areas
of the cerebellum and the basal ganglia. During the observation of movements of
others, an entire network of cortical areas, called “action observation network,” is
activated in a highly reproducible fashion (Grafton 2009 ). The central hypothesis
that emerges out of these results is that motor imagery and motor execution draw on
a shared set of cortical and subcortical mechanisms underlying motor cognition.
On the other hand, single-cell recordings of motor cortical neurons have pro-
vided an apparently different picture, showing that those neurons are characterized
by rather broad tuning functions and suggesting the theory of population coding of
some kind of population parameter. However, after the early seminal study by
Georgopoulos et al. ( 1986 ), who proposed that movement direction might be the
coded parameter, alternative interpretations were proposed also on theoretical
ground (Mussa-Ivaldi 1988 ), by showing that the same experimental findings can
be correlated indeed with different movement-related parameters. Other experi-
mental studies have also shown that the activity of motor cortical neurons correlates
with a broad range of parameters of motor performance from spatial target location
to hand or joint motion, joint torque, muscle activation patterns, etc. In other words,
the correlation between an internal variable, such as the discharge frequency of a
motor neuron, and a specific aspect of an empirically measured movement is a very
weak form of explanation of the organization of the motor system.
This kind of indeterminacy is also found in a related area of motor control study:
the attempt to explain motor invariants, such as the speed-accuracy trade-off
(Woodworth 1899 ),

the bell-shaped speed profile of aiming movements