indusium griseum

NMF neil.fournier at sympatico.ca
Fri Jan 23 19:39:11 EST 2004


The concept of the hippocampus acting as a integrator of bodily states is
not a recent idea. This discussion has prompted me to think of some earlier
limbic system and circuitry research. (Thus, this post is presented solely
to ignite discussion among the participants within this news group.  It is a
rather long post and I apologize for the digression.  Much of these ideas
was formulated through the discussion with a colleague of mine.  Hopefully,
those of you that find aspects of this post interesting will respond).

It has been suggested, at least over fifty years ago with some of earlier
work of Paul MacLean and Crosby and Humphrey's seminal neuroanatomical work,
that the hippocampus proper and hippocampal formation may be involved in
mediating complex physiological processes, such as breathing, cardiovascular
activity, neuroimmunological regulation, etc.  One aspect that might be of
particular interest (especially pertaining to a limbic system role,
especially hippocampal role, in bodily states representation) is that of the
indusium griseum or supracallosal gyrus (a dorsal extension of the
hippocampus).

You can see this structure quite easily in coronal and sagittal sections.
It is revealed as a symmetric pair of narrow strips of gray matter that runs
along the dorsal extent of the corpus callosum. The indusium griseum
essentially lines the inferior wall of the callosal sulcus.

Typically, the indusium griseum is presumed to be vestigial and
nonfunctional structure of the brain.  That is this structure is not
considered to functional in the adult or shortly after birth.  However,
there is evidence suggesting that functionality (as observed through
intracellular and extracellular recordings) is present during early
embryonic development.

Although the functional role of this structure is currently not known, in
light of the suggestion of a complex role of the hippocampus as a structure
involved in bodily physiological processes, it could be suggested that the
indusium griseum may serve as a homuncular representation of the developing
embryo.  It is also well-established that the fetus has the cerebral
capacity to detect and consolidate experience (Chamberlain, 1987).  In light
of these findings, the indusium griseum may serve two dual functions: first,
as a embryonic equivalent of the adult hippocampus and second, as a
embryonic equivalent of the adult somatotopic representation (i.e.
homuncular somatotopic representation of the fetus, i.e. the embryonic image
would be a small humanoid with a large head, disproportionate sized body,
and large eyes).

Like the postcentral gyrus (primary somatosensory cortex), which maintains a
somatic representation of the adult body, the indusium griseum may in fact
represent the neonatal equivalent of the somatotopic representation of the
adult.  Indirect evidence of this comes from studies with temporal lobe
epileptics, that show a seizure foci within the hippocampus and commonly
report "visitations" by small humanoids (Crosby,  Humphrey, and  Lauer,
1962).  As epileptiform propagate or spread along efferent trajectories
(i.e. hippocampal-amygdaloid pathways), output fibers from the entorhinal
cortex, dentate gyrus, medial septum, and even piriform cortex that
innervate portions of the indusium griseum will activate this structure.
Because the phenomenological experiences associated with the epileptic
discharges (ictal and post-ictal manifestations) should reflect those
structures of the temporal lobe that are most highly metabolically and
electrically labile at the time of the seizure discharges, the most striking
correlative experience in adult would be the report of a presence of a small
humanoid with a large head. (Especially if the seizure foci is confined to
right hemispheric mesial temporal lobe with limited bihemispheric spread.
The experience of the small humanoid would typically be on the left side of
the body and often associated with terror and vigilance).  There is also
evidence that these experiences may also be adult modifications of perinatal
memories (Lawson, 1984).

In the normal person when would activation of the indusium griseum occur? Is
it possible for this structure to be activated?  Yes and one possibility is
during dreaming.   During dream states, which are characterized by massive
activation of hippocampal-amygdaloid pathways, limbic pathways and their
correlative experiences might be accessed but then modified during the
process of translation to adult neocortical activity.  One extremely likely
activation point, based upon the neuroanatomical organization of the limbic
system, would the be occasional incorporation of indusium griseum activity
by limbic afferents (and thalamic, i.e. reuniens, afferents).  Considering
that the initial function of the indusium griseum may serve for the
consolidation of perinatal memories, the activation of this structure in the
adult during dream states or during transient fluctuations in normal
electrochemical activity following periods of stress, then incorporation and
reactivation of infantile memory sequences into dream processes or during
waking periods associated with extremely arousal would be likely.

Persistent accounts of  phenomenological experiences of visitations by small
humanoids have been commonly reported throughout the history of "man".
These experiences have long been a source of extraordinary creativity and
intuitive insight. Often these experiences occur during nocturnal periods
(or periods of intense psychological grief and stress).    The common
nocturnal reports of cherubs, Harpies, succubi, incubi, and Muses may in
fact have neuroanatomical and neuropsychological correlates.    The modern
equivalent of these traditional nocturnal visitations would have the label
of "extraterrestrial" and "alien abduction".  The themes of these
phenomenological and paranormal experiences would reflect the construction
and organization of the limbic system.






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