A functional neurodynamics for the own body - III
6. Remodelling induced by Experience (2): The motor cortex
What direct electrical stimulations of the precentral cortex evoke are bodily
movements; what Penfield and the first mappers of the brain sketched out in the
form of the homunculus are parts of the body: the fingers of the hand contralateral
to the stimulated hemisphere which, in anatomical order, are represented
in the latero-medial plane. But movements are rarely evoked in one part of
the body without being evoked in the neighbouring parts. The mastery of the
independence of the hand with conductors, that of the fingers of pianists or
typists, requires a difficult learning process that most probably draws upon important
cerebral resources. This inconsistency has probably only been noticed
quite late on. A somatotopic organisation of the cortical representation of the
hand suggests the existence of a neuron (or several) for the index finger, that is,
of a neuronal group exclusively dedicated to the control of a particular finger,
alongside other neuronal groups devoted to the control of each of the other
fingers. However, nothing of the kind is found. The recording of neurons of
the motor cortex during the carrying out of flexional movements and of movements
extending different fingers with the monkey shows that the movement
of each finger mobilises neurons distributed throughout the entire area of the
hand and that the map of the cortical representations of the movements of the
fingers is not somatotopic (Schieber 1993). However, just as an unequivocal
correspondence between representations on a somatotopic map and the parts
of the body would exclude any possibility of reorganisation, in that way the activations
distributed throughout the totality of a neuronal network according
to a certain given configuration would lend itself to a functional reorganisation
due to the varying usages of the body.8
The methods of human cerebral imagery (measuring the cerebral blood
flow in PET) which proceed by averaging the results obtained with several
subjects and which identify regions of interest by subtraction of images are
disadvantaged for the examination of phenomena of plasticity linked to a motor
learning process. That is due to the fact that the procedure adopted to
arrive at the mastery of a new task is not necessarily uniform from one subject
to another and to the fact that the non super-imposable activation sites
are automatically erased from the resulting image. To get around this diffi-
culty a technique of individualised imagery has been developed which suggests
the existence in each subject of a relation which is not that of a simple correspondence
movement-cortical area, but that of a complex relation between
a particular schema of adaptation to the task and a type of change in the
schemas of cerebral activation distributed over varied regions. The task is to
carry through blindfolded, as fast as possible and without mistakes, a complex
series of movements involving an opposition between the thumb and each of
the other fingers of the right hand. Progress over one hour of training differs
largely according to the criterion employed: acceleration of the process or correction
of the mistakes. Despite an activation of the left primary sensori-motor
(and pre-motor) region in all subjects, the authors noted a considerable diversity
in the areas of activation from subject to subject, and this no matter the
areas in question were cortical (mesio-frontal, parietal, cingular, Broca) or subcortical.
This is a discovery that raises questions pertaining to the contribution
of each of these regions to the particular profile established by the performance
of the trained subject (Schlaug et al. 1994).
A longitudinal study of a similar learning task with a training of several
weeks adds complementary information resulting from an MRI examination
of the regional blood flow in the motor cortex. Starting from an equivalent activation
of M1, first with the sequence of learned movements and then with
a sequence composed of he same elementary movements in another order,
passing a paradoxical though transitory reduction of the area of motor activation
corresponding to the sequence of learnedmovements, one finishes with
a significant extension of this area in the fourth week, an extension which can
be maintained for several months. According to the authors, this durable expansion
of the representation of the ordered sequence of learned movements
would make of the primary motor cortex a memory of the know-how in the
adult (Karni et al. 1995; 1998).9
7. Pluralism in the models of neurobiological explanation
In spite of the fact that the interdisciplinary character of the neurosciences
makes it possible to hold to the belief in the equal rights of all participating disciplines
to their claim for being fundamental, the familiar practise of all these
disciplines is still far from being able to risk comparison to any science which
is genuinely fundamental, such as quantum mechanics. A fundamental science
seeks to develop the paradoxes hidden in its concepts without being afraid of
exposing itself to controversy, even on the contrary, seeking controversy. It does
not attempt to clothe these concepts with the garb of consensual unanimity, or
even to surround the emerging divergences which might menace its dogmas.
Those dogmas, moreover, pushed to the limit, might turn out to be contradictory.
A truly fundamental science which knows only too well how illusory the
irrepressible human tendency toward objectivation, substantialisation and ab-
solutisation of the theoretical models and dominant scientific paradigms of a
given epoch (yesterday Lapacian mechanism, today the mechanism of Turing)
can be, is not afraid of appearing to progress backwards by systematically referring
back its “explanatory” and “predictive” concepts to their conventional
and so largely arbitrary principles of construction, the field of its “real” objects
to the geometry it makes use of, its “exact” measurements to the limited power
of resolution of its instruments. Apparently this is still not the case in neuroscience,
where the same dogmatic defenders of the genetic determinism of the
cerebral thinking machine with its cognitive programmes also want to present
themselves as heralds of epigenesis and of the history of the development of
the individual. And the very persons who, in the course of 20 years, have revolutionised
cerebral cartography, demonstrated the inanity of its traditional
concepts “map”, “somatotopy”, “representation”, “coding”, etc., and so laid the
basis for the next functional neurodynamics, habitually employ a language
that preserves and perpetuates the prejudice of a (or even many) homunculi in
the brain.
The format of scientific journals which print in small letters the technical
account of the cell recordings, the image analysis or the method by which the
published “maps” are constructed, leads one to separate these products from
their mode of production, thereby incurring the risk of their being envisaged
as maps in the brain. But that nothing like such maps is found in the brain
is something that can be persuasively upheld. The following items related to
maps are evidently not found in the brain: readings obtained from the grids of
penetration sites of electrodes in the cytoarchitectonic cortical areas, outlines
of the cutaneous neuronal receptor fields, histograms of the neuronal peristimulus
action potentials, mosaics of the categories of movement evoked by IMS,
electroencephalograms, scintigrams of the rate of consumption of oxygen or
glucose by the regional blood flow, the distribution across the scalp of loci of
stimulation evocative of motor potentials, dipoles of the sources of the induced
electric or magnetic fields, etc. But when one imagines that it might be possible
to “go further” (by extrapolating from the available methods of obtaining
images or representations) there arises a danger of fixing, objectifying or sub-
stantialising the transitory configurations of the functional dynamism of living
organisms. That includes that one misses the essential and persistent feature of
the potential for reconfiguration and functional reallocation which is not limited
to an early age or to the axonal regeneration and functional recuperation
of a lesion.
The challenge is to understand neuro-plasticity without trying to situate
our conceptual instruments in the brain, by talking of “neuronal coding” or of
the “genetically programmed”, and without entering into any collusion with a
neuronal determinism which conceives of the functioning of the brain as the
calculations of a machine that follows a programme that completely specifies
in advance all its transitions from state to state.
Even if linguistic habits have not changed greatly, we cannot but concede
that this challenge has been met from the time of the first work on cerebral
plasticity. In an effort to grasp conceptually the data of Merzernich and his
team, Edelman has advanced the idea of a functional and interactional morphogenesis
by selective stabilisation of the synaptic connection patterns in
conjunction with the activity of the organism (Edelman et al. 1987; Kaas et
al. 1983).While avoiding any reductionist explanation, a computer simulation
of a simplified model of the neuronal network has made it possible to elucidate
analogically and holistically the principles of a dynamicmorphogenesis of
functional topologic maps, by bringing to light certain of the properties established
by deafferentation or amputation of the fingers in the monkey.Without
entering into details, we would like to applaud the spirit in which this model
has been developed, to the extent that its dynamic approach seems to us to contradict
the fixist prejudices conveyed by the language of coding inherited from
a mechanistic conception of cerebral functioning.10