A functional neurodynamics for the own body - I
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).