A functional neurodynamics for the own body - I

Publié le par Jean-Luc PETIT

 

                                A functional neurodynamics

                         for the constitution of the own body

                                         Jean-Luc Petit

                                       Body Image and Body Schema

                             Interdisciplinary perspectives on the body

                           Helena De Preester & Veroniek Knockaert eds

                                       John Benjamins Publishing Co

                                       Amsterdam/Philadelphia, 2005

 

1. Introduction

However little philosopher may as yet be aware of this recent development, the

burgeoning field of brain cartography has transformed the traditional dispute

between phenomenology and positive science about the adequate treatment

of the body into an obsolete quarrelling. Up to now, phenomenology used to

dedicate itself to calling attention to the difference (not to say stirring up the

conflict) between the fixity of anatomic Körper structure as an object of science,

and the free fluidity of the meaning patterns of Leib subjective experience.

From now on, one’s inquiry should be whether or not such a contrast is on the

verge of vanishing. In fact, neuroscience has resolutely shaken off its former

belief in a rigidly somatotopic representation of the peripheral organs of the

body within the frontiers of definite somatosensory mapping territories of the

centro-parietal cortex and thalamus. Accordingly, a new methodological approach

is forcing its way through brain science labs, putting on their common

agenda the setting up of a global online recording of constantly moving functional

activation patterns (a “mental cinema”). These patterns transitorily distribute

themselves over varying regions of cerebral tissue at a rate determined

by the demands made upon them by the performance of behavioural tasks.

Such representational plasticity, far from being genetically predetermined in

all its localisational specifics, proves itself to be induced, shaped and modulated

to a considerable extent by the unique experience of the organism in its

environment. Laying our bet on the chances of a new relationship between

phenomenology and objective science, we will take advantage of the opportunities

created by these developments. And we will (allowing ourselves some

speculation) bring together the flow of functional activity of the brain and the

flow of lived experience of the body in an attempt to bridge (or at least narrow

down) the gap between activation patterns and meaning patterns, considering

that they are mutually indispensable correlates underlying the auto-affection of

the acting person.

Dominated as it is by the paradigm of a brain-machine designed to process

information, neuroscience tends to reduce “the body” to one of the representations

in the brain alongside representations of other things. And so it becomes

the representation of that object by means of which it receives information

(mainly tactile) and the muscular movements of which it controls. In one particular

branch of the neuroscience, cerebral brain cartography, a branch which

has made remarkable progress in the last thirty years, the talk is of “somatotopic

coding”, regions of “cortical representation”, “cellular receptor field”,

etc. Apparently, this way of talking is inspired by the fairly traditional ideology

of representation as an unequivocal correspondence (isomorphism) between

the peripheral structure of the body and the central homunculus (or homunculi).

However, belief in the rigidity of this projective relation suggested by the

expression “somatotopic coding” is (at least potentially) contradicted by the

discovery of the representational plasticity of the cerebral tissue, a discovery

made by this same cerebral cartography. The current generalisation of this phenomenon

of plasticity from association to primary areas and to all the sensory

modalities, as well as to the motor function, increases the tension between the

new intuitions and conceptions and the modes of expression still employed.

All the same, the power of the metaphor of the brain-machine upholds the use

of the vocabulary of the code and of somatotopy and delays its replacement

by a conceptual framework better adapted to the functioning of the brain and

to its true relation to the body. With regard to this relation one already suspects

(while waiting for the paradigm change which will make it a legitimate

claim) that, rather than the representation of a body preconstituted prior to

this representation, it will have to take the form of a dynamic interaction between

three terms: the body, the brain and also the world (absent from the

traditional, representational ideology), terms which cannot be taken to exist

prior to this same relation since they bring each other into existence through

their mutual interaction.

 

2. Somatotopic cartography and functional plasticity

A few preliminary remarks are useful to fix the limits of our enquiry. First of

all, research into the functional plasticity of the brain does not stop at the re-

presentations of the body in the somatosensory (SI) and motor (M1) cortices.

It applies equally to the retinotopic representation of visual information in the

striate cortex (V1) and to the tonotopic representation of acoustic information

in the temporal area (A1). We will restrict our attention to the evidence

bearing on the cartography of the body, even though the plasticity of corporeal

representations is not isolated from modifications stemming from exteroceptive

sensory influences. Second, one of the factors responsible for much of the

progress in neuroscience consists in experiments performed on animals and

the transfer of hypotheses or concepts developed in connection with mammals

or primates to human beings. In particular, the rat is currently an object of

intense research, due to the ease with which its sensory system can be manipulated

in experiments, a system whose vibrissae are the peripheric organs and

the barrel cortex the organ of internal representation. Since evidence relating to

a systemas specific as this cannot be directly carried over to humans, we won’t

go into this any more. On the other hand, restricting ourselves to the human

system would put us in a position where we could no longer obtain a global

view, not even a view of detail bearing on plasticity and somatotopy, since

progress in non-invasive techniques of cerebral imagery have not yet made it

possible to reduce the gap between knowledge bearing on the human brain

and knowledge already achieved in connection with monkeys (by means of

recording techniques based on chronic – i.e. permanent – cerebral electrode

implantation). Finally, our interest is in plasticity induced or modulated by

experience, understanding by that experience the one that an individual develops

through a normal use of his or her body, a use which is evidently enriched

and diversified in the course of a learning process. The plasticity that is evidenced

by patients that have suffered a stroke or a surgical amputation of a

limb and reacted to it by a functional reorganisation of their brain, cannot be

described as induced by experience except in a highly extended sense of that

word. In particular, we are not going to take into consideration “the illusion of

phantom limb”, with regard to which the literature tends to be as vast as it is

controversial.However, even if we decided not to take themechanisms brought

into play in that case or the other into account, it would be foolish to ignore

the knowledge obtained by the study of such reorganisational phenomena in

the case of lesions both in humans and animals, because if the word “re-

organisation” tends to be employed in this context while the word “remodelling”

is more frequently used in the context of normal usage, this verbal difference does

not seemto be one which testify of the existence of a distinction in re.

 

3. Penfield’s homunculus and its contemporary “Verification”

Penfield himself is remarkably prudent in his statements regarding the value he

accords to the “sensorial and motor homunculus” (Penfield & Boldrey 1937)

or to “the sensorial homunculus and the motor homunculus” (Penfield &

Rasmussen 1950) as regards the light it throws on cortical topography of the

sensory and motor functional representations. Moreover, the expressions of

“mapping” and “coding” have not yet been used. In the first version, “this

grotesque creature” is only called in to faithfully represent two features. The

first feature is the constant order of succession of the different parts of the body

concerned by the movement provoked or the sensation evoked by an electrical

stimulus applied bit by bit to the cortex, following the edges of the central

sulcus in the medio-lateral direction. These parts are, specifically, the body, decapitated

and inverted, then the head from the front, juxtaposed to the thumb,

then the tongue out of the mouth, etc. The second feature is the relative vertical

extension of that portion of the rolandic cortex devoted to the representation of

each part of the body, which is carried over to the homunculus as the disproportionate

length of the tongue, the face and hands in comparison to the rest

of the body. With the result that, with the exception of these two topographical

constants, all that the outline could save as representative of a man (“as

though representing a man”, says Penfield, cf. Penfield & Boldrey 1937: 431)

with its specific surface, its size, its precise contours (not to mention hair and

skin wrinkles in certain popular illustrations!) had to be treated as arbitrary

and misleading. For in fact Penfield does not try to hide the considerable dispersion

of the points of stimulation evoking motor or sensory responses in

different individuals, and in the same individual from one to another surgical

intervention. Even though he distinguishes a postcentral sensory cortex and a

precentral motor cortex, he admits that he also obtained motor reactions (even

though less frequently) by stimulating the postcentral cortex, and sensory reactions

(more frequently) by stimulating the precentral cortex. If he proposes

a delimitation of the areas responsible for different parts of the body, it is not

for making of them “the borders of the territory of representation” relative to

these parts, but to underline theirmutual interpenetration (Penfield & Boldrey

1937: 430, fig. 25). In the end, he holds back from developing any hypothesis

about the correspondence or lack of correspondence between representations,

whether this be with the cytoarchitectonic regions of the cerebral tissue or with

the distributive density of the sensory captors on the skin of different parts

of the body. Hence the notice to the reader in the work of 1950: “It is a car-

toon of representations in which scientific accuracy is impossible” (Penfield &

Rasmussen 1950: 56).

The development of a technique of non-invasive cerebral imagery at the

end of the 70s and the beginning of the 80s has made possible a certain “con-

firmation” of this classical description of the somatologic organisation of the

functional representation of parts of the body in humans. Measuring the regional

blood flow in the cerebral areas through tomographic recording by

the emission of positrons (PET), visualisation of the structures of the brain

through magnetic nuclear resonance (fMRI), exploration of regions of interest

by subtraction of images,1 the addition of images maximising activations corresponding

to each condition in one subject and to one and the same condition

in all subjects, without taking into account numerous operations of normalisation,

correction, standardisation, redistribution, averaging and calibration, all

of the above adds up to a mass of manipulations each of which rests upon a

questionable presupposition of neutrality and non-interference with the facts

under examination. Since the complexity of the technical apparatus brought

into play and the tacit claim of transparency seem to grow at the same pace,

the apparatus employed tends to disappear behind the publicly communicated

“views of the brain” and their reproduction in works of synthesis.Without going

too far into the much needed criticism of such methodology, let us at least

ask what in fact the procedure adopted has helped to confirm. Essentially two

things: (1) by means of a manual cutaneous vibrator applied successively to the

lips, the fingers and the feet, the latter are stimulated in such a way as to evoke

responses focused in different regions of the postrolandic cortex (SI) strung

out along the central sulcus in a latero-medial order from the parietal opercula

to the interhemispheric wall (Fox et al. 1987); (2) chasing a target moving

randomly about a video screen, using respectively the big toe, an outstretched

arm, the index finger or the tongue, activates precentral zones of the cortex

(MI) which follow upon one another from the dorso-lateral edge of the interhemispheric

fissure to the neighbourhood of the lateral sulcus, passing across

a region of activation where the index finger is superimposed upon that of the

arm (Grafton et al. 1991). One notes that only that aspect which Penfield himself

considered true remains in accord with Penfield’s homunculus, namely, the

sequential order of the functional representations on the medio-lateral axis of

the pre- and postrolandic cortices. However, the limits of this agreement tend

to be concealed by the expressions employed. The talk is of “millimetric localisation”,

even though the average difference between the localisations taken two

by two in the same subject is of the order of 3 mm (Fox et al. 1987: 39). Or

one talks of “detailed examination of the somatotopic distribution” and of

“localisation to predictable sites” even though the variability of the gyri and sulci

from one individual to another involve displacements in the activations, both

in breadth and in depth, which only allow for a range of estimation regarding

their probable occurrence (Grafton et al. 1991: 737; 739, Fig. 3).

 

Publié dans philosophie

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