A functional neurodynamics for the own body - III

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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

 

Publié dans philosophie

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