Neurobiological data & Husserlian constitution - III

Publié le

The neurophysiological basis of the constitution of meaning

We should be careful not to confuse incarnation and naturalisation. Incarnation

is a response to the demand that the flux of experience be relived concretely,

that is, freed from the accretions of meaning that come from verbal

expression or logical shaping. Naturalisation is a response to a completely

different demand; that of procuring, for an eidetic description, the meaning

structures belonging to the subjective experience correlated with objects located

within the field of investigation of the cognitive sciences. Recognition

of the role of kinesthesia in constitution amounts to a move in the direction of

the incarnation of meaning or of the possibility of a world figuring within the

corporeal experience of an agent. But it is still well worth asking whether

anything has been accomplished thereby in the direction of naturalisation. To

be sure, the experiences connected with the movement of the organs of perception,

of one’s own body or the body of the other, remain at a distance from

the physical aspects of the corresponding movements. However, this distance

is no longer a metaphysical abyss separating two substances, but a demarcation

line which can be crossed by appealing to some postulate of normality.

Normally, when we raise our arm, when we feel it moving, or rather, when

we feel the I move of our arm, it rises. That this normality is a purely contingent

consequence of the way our body is constructed and that things could

have been otherwise (so that my left arm rose every time I tried to raise my

right arm) does not endanger its constitutive function. Disconnected from the

realm of Platonic Ideas, the apriori of meaning is rooted in a body of flesh

and blood. The visual field belongs to lived experience not to the retina. For

all that, the structure of the retina imposes its own constraints upon the visual

field. It is this predetermination of the phenomenal field by an underlying

organisation which sets the stage for the implementation of a neurobiological

programme.

The theory of constitution makes perception responsible not just for the

reception of the perceived thing, or for its representation, but also for its

givenness. The absence of anything like an external instructor capable of let

ting the brain know whether or not its cellular activity corresponds to a real

external object only makes the problem of cognition for the brain that much

more akin in its radicality to the one of constitution. Everything about the

object, not just its figurative and qualitative properties but also its identity and

individuation, depends upon cerebral activity. On the one hand, constitution

is not to be construed as the solipsistic unfolding of internal representations

but as a matter of a mutual and reciprocal formative interaction between an

active agent and its environment. On the other hand, abandoning the myth of

the grandmother neuron, cognitive neuroscience is moving in the direction

of a more abstract and dynamic conception of the neural pathways responsible

for the treatment of sensorial information by, for instance, identifying the

schemas of activation making possible the identification of a face, schemas

progressively modified with a retroactive feed-back of information upon the

functioning of the primary sensory regions (Rolls and Deco 2002). The same

primacy of central activity is to be found in the interaction between the cerebral

circuits and the effectors and sensors at periphery on one side, and in

the interaction between the intending act of the subject and the body as organ

of its intentions on the other.

 

A neural correlate for the noema of a face

The regions of the cerebral cortex are, from the point of view of their respective

contribution to the perceptual treatment of visual information, divided up

into primary occipital regions and parietal and frontal regions of temporal

association. The primary regions are organised retinotopically and in such a

way that a stimulus appearing in the contiguous regions of the visual field will

fall into the receptor field of contiguous neurones. The regions of association

contain neurones possessing more extended receptor fields, reacting to more

complex stimuli and contributing to the highest levels of the processing of

visual information. In particular, in the case of the monkey, we have been able

to register in the lateral fusiform gyrus, the superior temporal sulcus and the

intraparietal sulcus, cells which are selectively activated by faces (Perrett,

Rolls, and Caan 1982). These cells are not of the type grandmother neuron,

that is, exclusively concerned with the face of one and the same individual, in

abstraction from variations in its mode of presentation. But, unlike the cells

of the primary regions, they are also not restricted to the encoding of an elementary

feature of the visual image in a receptor field limited to a narrow

sector of the retina (e.g., a small luminous band in movement). Their conditions

of activation are associated to varying degrees with (1) a relative invariance

with regard to isomorphic transformations (rotation, colour, size, contrast,

plane or volume, orientation with regard to the ego) and (2) a relative

sensitivity to changing points of view or aspects (face or profile, eyes, mouth,

hair, orientation of the gaze) (Baylis, Rolls, and Leonard 1985; Desimone et

al. 1984; Hasselmo et al. 1989; Rolls and Baylis 1986).

This type of behaviour suggests the existence of a system of analysis and

recognition of faces which proceeds by associative synthesis of any relevant

information and a filtering out of irrelevant information, and not by means of

any simple comparison between a stimulus and a preestablished concept stored

in the memory (Booth and Rolls 1998; Tanaka 1996). In the (non-anatomical)

functional configuration resulting from this process, a configuration which

is sometimes described as the distributed representation of an identity equivalent

to a list of distinctive characteristics, at other times, as a buffer process

capable of absorbing changes in the retinal image of one and the same object,

I would be prepared to see a correlate of the noema of a face. With regard to

this noema, this functional configuration possesses in effect the dialectical

polarity of both of two moments (1) the pure something = X which refers to

the individuality of the transcendent thing in itself in relation to the experience,

and (2) the perceived as such, the object in the how of its determinations

and in-determinations, situated at the very heart of the experiences of

perception and action directed towards this thing.

 

A neural correlate for the noema of one’s own body

The constitution of one’s own body rests upon the distinction and the articulation

of two types of kinesthesia: objectifying tactile kinesthesia which present

this body as a spatial thing, and subjectifying practical kinesthesia thanks to

which these organs are experienced in their very movement as organs of the

ego. As a possible correlate I will this time make use of the phenomenon of

the plasticity of those body maps which are localised in the primary somatosensorial

and somato-motor regions of the cerebral cortex throughout the

central sulcus as well as in the relay regions of the cortical and sub-cortical

sensori-motor pathways activated by motor behaviour (Merzenich and

deCharms 1995).4 In support of this theory of constitution I can think of no

better empirical argument than the cumulative evidence relating to the modifying

effects (already noticeable in the learning process but also more directly)

upon this plasticity of voluntary motor activity and of intentional behaviour.

Corresponding to Husserl’s intuition that the intentionality of action contributes

to the meaning of one’s own body we find the hypothesis that this func-

tional somatotopy is moulded by the intra-cortical signals whether efferent,

reentrant or reafferent, thanks to which the somato-motor and somato-sensorial

maps influence each other in the course of the entire experience of the

subject. That the habitual localisation of subjective properties (qualia) in the

body requires the integration of tactile fields and of practical kinesthesia could

be due to the fact that the somatotopy of the cortical representation of the

sensitive regions of the body is mediated by the use the subject makes of his

body and from there by the motor somatotopy of his voluntary action and the

imprinting of the latter into his memory as a motor schema. At the level of the

cerebral metabolism the complete circuit of these mediations represents the

basis for the spatialisation of the acting subject in its own body.

Whatever might be held to be true of the anatomical form of the body, the

truly real body is both an acting body and a sensibly experienced body, the

acting body constantly modifying the experienced body which, in its turn, anticipates

its own modification. As a result, what we call “the body” emerges

from its permanently internal reconfiguration and this at a variety of levels,

the level of visual, tactile and proprioceptive perception, that of muscular

control and, even earlier, of motor schemas, while further back still we find

the intention. This reconfiguration accompanies an entire series of steps encompassing

the mastery of tasks, the choice of strategies, the level of attention,

the formation, maintenance and reactivation of the intention, the fixation

in memory of the repertory of motor schemas – in a word all the micro-aspects

of action.

 

A neural correlate for the instrumental noema

We employ a tool as a prolongation of our hand, a physical extension complemented

by a perceptual assimilation of this tool to the corporeal schema

of the hand. In a study bearing on the neural correlates of this phenomenology,

monkeys were trained to make use of a rake to drag back towards them

with one hand pellets of food to be grabbed with the other (Iriki, Tanaka, and

Iwamura 1996). The identification of a group of visuo-tactile neurones in the

parietal cortex has made it possible to locate at this point a map of the visual

space centred on the body of the animal. Measurements of the visual receptor

field (RF) of these neurones whose cutaneous RF is localised on the fingers and

the palm of the hand being used has revealed its plasticity and the dependence

of the latter upon usage. This visual RF which is at first superimposed

upon the cutaneous RF gets extended along the axis of the rake at the end of

five minutes of use as if the image of this tool had been incorporated into that

of the hand, then gets retracted just as soon as this usage ceases, thereby recovering

its initial localisation, even if the monkey continues to hold onto the

rake. Proof that this expansion is linked to its intention to make use of this

tool, it legitimises our attempt to trace the origin of this modification (linked

to usage) back to the animating intention.

This hypothesis concerning the neural substrate of the use of a tool has been

confirmed at the clinical level with humans in the case of a patient whose right

hemisphere had been lesioned and who showed a hemineglect restricted to

the immediate vicinity (Berti and Frassinetti 2000). In a line-bisection test on

a piece of paper (the left half of which she could not see) this patient deflected

the centre of these lines towards the right. This deviation was more pronounced

in the foreground where she could touch the lines with her finger than in the

distant background where she used a luminous pointer. It has been noted that

the perceptual distortion characteristic of the spatial foreground reappeared

when she was given a stick rather than a pointer to carry out this test in the

more remote reaches of the spatial field. If we assume that the utilisation of a

stick induces an extension of the cerebral map of the body into a region of

space inaccessible to direct handling and that this map subtends the close-far

difference within our conscious experience of space, this phenomenon of functional

plasticity can be seen as the correlate of the instrumental noema for

whoever makes use of it as a non-kinesthetic extension of one’s own body.

 

Publié dans philosophie

Commenter cet article