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Individual variability of higher brain functions

William Calvin wcalvin at stein1.u.washington.edu
Tue Nov 2 01:22:52 EST 1993

"Richard E. Cytowic MD" <p00907 at psilink.com> writes:
>	Whoever told you that movement and somesthesis are restricted 
>to the pre-and post-central gyrii is misinformed. In 
>marsupials (eg wallably and oppossum) there is CONSIDERABLE overlap of 
>motor and sensory areas; overlap is also seen in cats and other 
>mammals; multimodal neuron pools are found in humans too.

Apropos movement from cortical stimuation, here are a few paragraphs
from W. H. Calvin & G. A. Ojemann, CONVERSATIONS WITH
NEIL'S BRAIN:  Seeking the Narrator of Consciousness (out April
1994 from Addison-Wesley):

The texts show typical maps of the sensory strip and the motor strip,
but patients exhibit a lot of variability.  That's the reason it has to be
mapped carefully in each patient undergoing epilepsy surgery.  Brain
maps are just as variable as faces.  No one knows whether such details
of cortical organization are important -- but they might reflect the
differences between the clumsy and the well coordinated, the articulate
and the tongue-tied.
      There are several more motor maps; they are hard to detect in
the operating room with the stimulating technique, but are known from
laboratory studies of monkey brains.  The motor strip, for example, is
not the exclusive commander of the muscles.  It certainly isn't the
exclusive commander of the neurons in the spinal cord that actually run
the muscles; the premotor regions just in front of the motor strip have
just as many connections down to the spinal cord as does the motor
strip itself.  But the loss of motor strip tends to produce muscle
weakness and, if the damage is extensive enough, paralysis.

Although most cortical movements and sensation relate to the opposite
side of the body, there are connections between primary motor and
sensory cortices and ipsilateral body, especially for face and to a lesser
extent leg.  Only fine finger movement seems to be totally dependent
on contralateral motor cortex.  These ipsilateral connections are
probably important in recovery from damage to primary motor and
sensory areas.  Only very rarely will electrical stimulation of primary
motor or sensory cortex evoke ipsilateral responses.  However,
stimulation of some of the secondary motor or sensory maps will more
often yield ipsilateral effects.

      The comparative neuroanatomist Irving Diamond argues that
the "motor cortex" isn't restricted to the motor strip but is the fifth layer
of the entire cerebral cortex.  This is true because the fifth layer, no
matter where it is, contains neurons that send their mass mailings down
to the spinal cord, with copies to the brain stem, basal ganglia, and
hypothalamus.  Diamond likewise argues that the fourth layer
everywhere is the "sensory cortex" and that second and third layers
everywhere are the "association cortex."  See "The subdivisions of
neocortex: A proposal to revise the traditional view of sensory, motor,
and association areas," in J. M. Sprague, A. N. Epstein (eds.),
Progress in Psychobiology and Physiological Psychology, 8:1-43
(Academic Press 1979).

    William H. Calvin   WCalvin at U.Washington.edu
    University of Washington  NJ-15
    Psychiatry & Behavioral Sciences
    Seattle, Washington 98195 FAX:1-206-720-1989

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