Biomedical Engineering Reference
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proposing a bottom-up design, named
Subsumption Architecture
, that is supposed
to achieve “intelligence without representation”: this architecture is organized in
layers, decomposing complicated intelligent behavior into many “simple” behav-
ioral modules, which in turn are organized into layers of simpler behaviors, down to
reflex-like mechanisms. Each layer implements a particular goal of the agent, and
higher layers are increasingly general and abstract. However, this kind of layered
bottom-up architecture scales up badly when one attempts to deal with complex
bodies and complex behaviors in a complex environment.
In contrast with the Sherringtonian view, Hugo Liepmann (
1905
) was the first
one to suggest that actions are generated from within, requiring the existence of an
internal state
where they would be encoded, stored, and ultimately performed
independently of the stimuli coming from the external environment. To account
for the implementation of action plans, he proposed that the elementary chunks of
action are assembled according to an internal representation: he called
movement
formula
the result of this process, i.e., an anticipatory hierarchical structure where
all the aspects of an action are represented, before it is enfolded in time. Liepmann's
legacy is still quite influential in motor neuroscience, although the term
movement
formula
was later replaced by several others, like
engram
,
schema
,or
internal
model
. In the same vein, Nikolai Bernstein (
1935
) had an interesting analogy for
explaining this mode of organization: he suggested that the representation of an
action must contain, “like an embryo in an egg or a track on a gramophone record,”
the entire scheme of the movement as it is expanded in time and it must also
guarantee the order and the rhythm of the realization of this scheme.
In the field of human motor cognition, only recently advanced brain imaging
techniques allowed to gain direct access to cognitive/mental states in the absence of
overt behavior, thus making clear that actions involve a covert stage. It is now
accepted that the covert stage is a representation of the future that includes
The goal of the action
The means/tools to reach it
The consequences on the body
The effects on the external world
Covert and overt stages thus represent a continuum, such that every overtly
executed action implies the existence of a covert stage, whereas a covert action does
not necessarily turns out into an overt action. Jeannerod (
2001
) provided a very
important contribution by formulating the Mental Simulation Theory, which posits
that cognitive motor processes such as motor imagery, movement observation,
action planning, and verbalization share the same representations with motor
execution. Jeannerod interpreted this brain activity as an internal simulation of a
detailed representation of action and used the term
S-state
for describing the
corresponding time-varying mental states. The crucial point is that since S-states
occurring during covert actions are, to a great extent, quite similar to the states
occurring during overt actions, then it is not unreasonable to posit that also real,
overt actions are the results of the same internal simulation process. Running such
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