A functional neurodynamics for the own body - IV

Publié le par Jean-Luc Petit

8. Autonomy and experience in the constitution of the own body

By virtue of its quasi-spontaneous or autonomous character, the correlative

emergence of neuronal groups in the context of the network of neurons and of

neuronal receptor fields in the matrix of the skin captors of the same network

is the best analogy that one could find in contemporary naturalistic science

for the transcendental constitution of the own body in genetic phenomenology.

The use of my hands gives me (in a certain sense) my own body. But

in what sense exactly? According to Husserl’s later manuscripts, the regulated

effectuation of tactile kinesthesia (objectifying) and of motor kinesthesia (deobjectifying)

is the constitutive operation bymeans of which alone I acquire the

sense of being (and from there consciousness) of my own body, both as a body

object, an object among other objects of sensory perception, and as the unique

organ of my voluntary movements. In the very course of its functioning, the

first group of kinesthesia constitutes a continuous and closed surface which

adopts forme the meaning of being “my skin”; the second fills this surface with

a subjectively animated matter which adopts for me the meaning of being “my

flesh”. But neither my skin nor my flesh have anything a priori to do with this

 “mass of flesh and bone that I call my body” (Descartes). They are in essence

the products of a constitution, more specifically, of an active auto-constitution

on the part of the living organism, a self-organising agent, constantly adapting

to its context, moulded by its own history. The organism (as certain eminent

physiologists have said in astonishingly phenomenological terms) makes “an

effort aftermeaning” (Bartlett, quoted in Barlow 1985: 121) and “chooses from

one moment to the next the being it will become” (Merzenich & deCharmes

1995: 76). Ironically, by adopting the hands, which are both sensitive surfaces

and motor organs, as the privileged models for the morphogenesis of the somatotopic

maps (for simple reasons of practical convenience I assume), the

neuroscientists thereby resuscitated the analyses (developed by Husserl and

Merleau-Ponty) of the celebrated example of “my right hand touching my

left hand, the latter, in turn, passing from being passively touched to actively

touching.”

This improbable encounter between a neurodynamic (still in preparation

despite the promising perspectives opened up by the “mental cinema”) and a

genetic phenomenology (unhappily relegated to the field of historical studies)

attests to the possibility of at least breaking the magic circle of representation,

which still holds neuroscience imprisoned in the paradigm of the mechanical

brain and the body representationally intellectualised. What does this opportunity

depend on? On the fact that the emergence of the body schema, on the

basis of the functioning of a dynamic systemin the brain, and the constitution

of our sense of the own body, on the basis of kinesthetic activities of the organism,

are (for the one who places himself or herself in the context of the flux of

experience and not in the position of an external observer) genuine beginnings,

effects without causes, absolute origins. For in fact, for the living organism

caught up in the immanence of its own experience, there is no such thing as a

physical or anatomical body to be represented, a body which would precede in

the order of being its representation, the latter reproducing somewhere in the

mind-brain a cartographic image of this same preconstituted body. The signifying

form, the sense of being a body, arises fromits own operation as self-given

sense. The own body is no more the representation of the physical body than

the functional body is the representation of the anatomical body. The true relation

runs in the reverse direction; first comes the own body, the subjective form

of lived experience or the functional configuration of a living organism. As for

the anatomical or physical body, it is a later product constituted by a procedure

of scientific objectification, and, what is more, a constituted product in

the paradigmatic context of yesterday determinist science, a science of perma-

nent objects, the fixed substrates of properties such as physical, functional or

mental properties, which can always be precisely located.

 

Acknowledgments

I would like to express my appreciation to Dr. Christopher Macann for the

English translation.

Notes

1. The subtraction: image of a brain state of sensory stimulation (ormotor activity) – image

of a state of rest.

2. For any individual neuron in a brain cortical tissue area that functions as territory of somatotopic representation of hand, the surface of hand skin which tactile stimulation induces

the firing of this neuron is its receptive field.

3. Relation between the extension of representative cortical area and the extension of represented

cutaneous area.

4. Cf. Buchner et al. (1995) for modification in the excitation-inhibition equilibrium of the

sensorial input in the case of local anesthetic of fingers 2, 3 and 4. It has been established that

this reorganisation is subject to the modifying influence of attention (Buchner et al. 1999).

For an expansion of the representation of the movement of the finger in neuropathy, both for

active and passive movement cf. Reddy et al. 2001. For the post-operative reorganisation of

the map of the hand in patients suffering from syndactyly, which reflects the new functional

status of the hand cf. Mogilner et al. 1993. For the dynamical re-organisation of muscle representations in the case of amputations, cf.

Cohen et al. (1991).

5. For a progress in dexterity in monkeys, Xerri et al. 1999.

6. Selective loss of finger sensation and difficulty of control of finger coordinatedmovement.

7. For the convergence of results confirming an expansion of the functional representation

of the reading finger with blind readers in Braille, cf. Pascual-Leone et Torres (1993). This

expansion can no doubt be imputed to the intensity and to the selective character of the

sensorial stimulation imposed upon this finger by the rapid and repetitive movements of

the tactile detection of letters in Braille. This expansion is accompanied by a disorganisation

of the somatotopological topography of the representation of the fingers, which no longer

follow one another in the normal latero-medial order, a disorganisation which could be

related to a difficulty in identifying the finger subject to a minimal tactile stimulation arising

from the determination of the threshold of the sensorial sensitivity of these fingers. This

observation poses the question of the adaptive value of the functional plasticity induced by

use (Sterr et al. 1998).

8. For the plasticity of the functional topography of the motor cortex linked to a motor

learning process in monkeys, cf. Nudo et al. (1996).

9. Without calling in question the role of functional plasticity in the learning process, an inverse

result suggests that it would be wrong to dogmatise on the basis of hypotheses drawn

from empirical research. A TMS of the motor output of M1 towards the muscles of the fingers

was practised between blocks of tests. The tests concerned the reaction time for the

appearance on the screen of a computer of the number of the finger which had to be used

to press the reply button. This experiment shows successively (1) a coupling of the progressive

diminution of the reaction time with an amplification of the motor output and an

expansion of the map of the excitable positions on the scalp, (2) an abrupt uncoupling of

this same reaction time, which continues to diminish, and the amplitude and extension of

the representations of motor outputs, which shrink and return to their previous level and

topography. This change (on a smaller time scale than the previous experienced) reflects

the transition from a practical and implicit mode of knowledge to one which is declarative

and explicit as well as the taking over from M1 by other structures (Pascual-Leone et al.

1994, 1999).

10. If the anatomical architecture of Edelman’s “neuronal network” is initially fixed, the

functional properties attributed to the “synapses” are not, but change as a function of the

“cutaneous” stimulation, on the one hand, and of the equilibriumestablished by the exciting

and inhibiting influences that “the cells” exert on each other, on the other hand. The operative

concept is that of the “neuronal group”. Neuronal groups are not anatomical entities

but rather purely functional entities, stabilised patterns of cellular activations distributed

throughout the network. Their process of formation depends uniquely upon the flux of

stimulation of the captors and on the local equilibrium between excitation and inhibition.

By hypothesis, the network is deprived of initial organisation, the connections between cells

being left to chance. In accordance with the theory of “selection of neuronal groups”, three

principles of synaptic functioning make possible the “spontaneous” emergence of these neuronal

groups. (1) The mutual overlapping of the divergent “thalamo-cortical connections”.

(2) “Selection”: the neuronal groups whose activation is more powerful stabilise their internal

connections and refine their receptor fields while the weaker ones tend to dissolve.

(3) On the borders between groups in the course of differentiation, intervening cells form

groups which compete with each other, as a result of which amore precise determination of

their mutual frontiers becomes possible. The organisation of a network of neurons on the

unique and exclusive basis of these three principles results in neuronal groups whose behaviour

simulates some observed functional plasticity phenomena of the somatotopicmaps

of the hand: their expansion under the impact of an abnormal stimulation of a finger, retraction

and substitution of the RFs of the palm by RFs on the back of the hand in response

to a deafferentation of the median nerve.

 

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