Q; WHY left hemisphere language?

Dag Stenberg stenberg at cc.Helsinki.FI
Thu Feb 16 13:42:03 EST 1995


CYNTHIA J. MONHEIM (cjm6689 at silver.sdsmt.edu) wrote:
} Does anyone have an answer as to WHY language ability initially develops in 
} the left hemispere, without exception?    

I wish I was able to check the literature right now... but:
- first of all, there are hemispheric specializations in other species.
In some (like rats?), laterality preference within the species seems 
to be 50/50. In other, like singing birds, a consistent preference in
the species for one side is found - even one (left) side, and only
males.
  The points here seem to be:
1) there must be some advantage in hemispheric specialization, which is
not restricted to that of language functions in humans. Maybe the
lateralization of some other (spatial? - make a good map on one side, do
not divide it up into two "pages") function is really the important
one even in humans.
2) in some species one of the two possibilities has gradually been
eliminated from the population. Why is right called "right", and left in
many languages is synonyme to "clumsy"? Maybe that terminology is very
ancient, and those people were less likely to survive due to defective
division of tasks between brain parts. 
  Why are l-forms of molecules generally more active biologically? Why
does all DNA turn one way? Is all of life really one-sided?
But why then are both turning preferences equally common in rats, and 
why do monkeys not have a species preference for handedness? So maybe
handedness has nothing to do with levo-dextroforms.

- second, as far as I have understood, several samples indicate that
about 5-10% of humans are left-handers, and of those, about half have a
language specialization in the right hemisphere, without previous
trauma. I wish I knew how large and representative these population samples
are, and how valid the assessment of laterality. Cave paintings from
early Cro-Magnon times indicate 80% right-handers at that time.

- I think already Broca showed that if the language center is lesioned
before about 5 years of age in humans, the specialization will develop
in the opposite hemisphere, but if the lesion is encountered much later,
aphasia will follow. So for a while only during the development of the 
individual, both hemispheres retain the capability to develop language 
function.

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Dag Stenberg     MD PhD                    stenberg at cc.helsinki.fi
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