L'identification d'une architecture
neuronale spécifique dédiée
à la représentation de
l'état fonctionnel interne de l'organisme
et à son homéostasie permet de
comprendre comment les sensations internes
influencent le comportement et
inter-réagissent avec les
émotions. Cet article propose une vision
possible de la perception du bien-être
ressenti par le bâillement et de la
neurophysiologie de sa contagiosité.
[···] Conclusions
and future directions :
The identification of an entire neural system
hat can be cogently conceptualized as a
representation of the physiological condition of
the material body has several fundamental
implications. It provides a rational explanation
for the long-recognized association of pain,
temperature, itch and other feelings from the
body, separate from the lemniscal system that
represents exteroceptive touch and
proprioception.
It incorporates specific labelled lines for
several physical conditions that generate
distinct feelings, and it substantiates their
common integration in the hierarchical
homeostatic network. It provides the
long-missing peripheral and central afferent
complement to the efferent autonomic nervous
system. These findings reveal a direct
cortical image of the state of the body that
differentiates primates from sub-primates
neuroanatomically. The size and multiple
representations of this interoceptive image seem
to differentiate humans from subhuman primates.
Finally, these findings signify the cortical
representation of feelings from the body as the
likely basis for human awareness of the physical
self as a feeling entity. This association
provides a fundamental framework for the
involvement of these feelings with emotion,
mood, motivation and consciousness. These
concepts emerge directly from the functional
anatomy of the lamina I spinothalamocortical
system, rather than from preconceived ideas.
It is important to recognize that this neural
sensory system is part of an entire network
involved in homeostasis; that is, in the
autonomic, hormonal and behavioural neural
mechanisms that maintain optimal physiological
conditions in the body and that respond in an
integrated and on going fashion to all interior
and exterior environmental challenges, ranging
from exercise, dehydration or altitude to
injury, sepsis or social interactions.
The organization of this network is focused
at the spinal level on cardiovascular and direct
endo organ control at the brainstem level on
integrated control of fluid, electrolyte,
energy, immune and cardiorespiratory balances
the forebrain level in sub-primates on neuro
endocrine and behavioural control, and in
primates in a high-resolution encephalized
representation of all aspects of the condition
of the tissues.
Nevertheless, it is avertically integrated
system, and it is important to recognize that
the basis for feelings from the body in humans
is this hierarchical association with
homeostatic mechanisms.This hypothesis is
supported by the close correlation of brainstem
activity with these subjective feelings in human
imaging studies. This recognition recommends
analysis of the interactions of feelings and
emotion with many aspects of subconscious
homeostasis; for example, in stress or pain or
cognitive behavioural research.
In contrast to the many discriminable
sensations from the body, the subjective
appreciation of visceral sensation is more
diffuse, less well localized, and usually below
perceptive thresholds. This was one of the main
reasons for the long-standing mis-categorization
of pain and temperature as exteroceptive rather
than interoceptive. Although it would be highly
inefficient for gastro intestinal processing to
require constant behavioural supervision, this
perceptual difference remains to be explained
adequately. Notably, many observations indicate
that there is opponent processing between
parasympathetic and sympathetic afferents that
parallels their efferent opponency. For example,
there are obligatory mutual inhibitory
interactions between spinal and vagal
small-diameter afferent activities in the
medulla that are essential for cardiorespiratory
control. Similarly, vagal afferent activation
inhibits both pain sensation and spinal
visceroceptive processing.Vagal stimulation can
reportedly reduce stress and depression
clinically. Similarly, opposing effects on
autonomic function have been elicited by
stimulation of human insular cortex on both
sides, and corresponding cortical lateralization
has been observed with micturition and
gustation.
Such a basic organization would be
parsimonious with many considerations and could
explain the perceptual differences, but this
certainly needs further study, particularly
because of the potential clinical
significance.The association of the
re-representation of the interoceptive pathway
with self-awarenes simplies the existence of
neuroanatomically verifiable correlates of
conscious behaviour. To this end, we are now
comparing the size and cytoarchitectonic
differentiation of the thalamic relay VMpo in
different primate species. Preliminary
observations are supportive; VMpo in the pygmy
chimpanzee, which can recognize itself in a
mirror, is clearly similar to that in the human,
albeit considerably smaller. By contrast, VMpo
in the gibbon,which cannot recognize itself, is
barely recognizable, like that in the
macaque.
Similarly, a unique cytological feature has
been described in the ACC and the anteriori
nsula of human and higher sub-human primates
that is not present in lower animals.Further
anatomical analyses of the interoceptive cortex
in sub-human primates and of the anterior insula
in humans are certainly needed, and correlative
imaging and clinical approaches would be most
useful.
Finally, this conceptual framework has
strong implications for medicine. The integrated
neural representation of all aspects of the
condition of the body in a system responsible
for homeostasis and associated with stress,
including a direct cortical image of physical
well-being, provides a sound epistemological
foundation for integrated approaches to the
treatment of pain, metabolic, eating and
psychosomatic disorders.
For example, this provides an easy
formulation for somatization under emotional
stress. Similarly, these considerations imply
that mysterious pain syndromes, such as
fibromyalgia (deepaches and pains), could be
related to homeostatic dysfunction (for example,
salt or waterbalance or cardiovascular
function), rather than to tissue damage, and
this possibility deserves vigorous study.
Consideration of these findings led directly
to the recent proposal that the central pain
syndrome is a thermoregulatory disorder. The
recognition that sensual touch is incorporated
into the interoceptive system has strong
implications for the neurobiological and health
effects of conspecific contact, visitors to zoos
will remember that monkeys, chimpanzees and
bonobos normally spend an enormous amount of
time grooming and cuddling each other, and
readers will remember the classic studies by
H.Harlow showing the importance of conspecific
contact for emotional development. Last, the
observation that the neuroanatomical substrate
for subjective emotion in humans is based on an
abstracted meta-representationof the
physiological state of the body, consistent with
the conjectures of James andDamasio, provides a
basis for the volitional modulation of feelings,
emotion and efferent activity affecting the
state of the body that is unique to humans, and
clearly emphasizes the role of the body's health
in human consciousness and interaction.
The 'somatic marker' hypothesis of
consciousness
On the basis of neurological analyses of
patients with forebrain lesions, Antonio Damasio
has advanced the 'somatic marker' hypothesis of
consciousness.He proposes that the subjective
process of feeling emotions requires the
participation of brain regions that are involved
in the mapping and/or regulation of our
continuously changing internal states, that is,
in homeostasis. These feelings help to guide
behavioural decisions that affect survival and
quality of life by producing a 'perceptual
landscape' that represents the emotional
significance of a particular stimulus that is
being experienced, or of a projected future
action by means of a further 'as-if-body
loop'mechanism. The feelings are grounded in the
body itself, based on multi-tiered and
evolutionarily developed neural mechanisms that
control the body's state. These feelings
distinguish between inner-world representations
and outer-world representations, and allow the
brain to build a meta-representational model of
the relationship between outer and inner
entities. So, the representational image of the
body's state provides a neural basis for
distinguishing self from non-self, and
re-representations of this image enable the
behavioural neural agent to project the effects
of possible actions onto the state of the body,
as well as the resultant changes in such feeling
states due to interactions with other (external)
agents. This hypothesis posits that degrees of
conscious awareness are related to successive
upgrades in the self-representational maps. The
anatomical features of Damasio's hypothesis
include a central role for the anterior insular
cortex in the representation of such feeling
states.
