Aphasia, specifically the type that affects
Broca’s or
Wernicke’s area, is one of the
prime examples used by
linguists as evidence that linguistic ability is lateralized (located almost entirely in) in the
left hemisphere of the
brain. Damage to either region will result in
speech disorder, losing
syntax with
lesions or
injuries to the Broca region (frontal region) and
lexicon when in the Wernicke area (
posterior temporal and
lower parietal regions). Damage to equivalent areas in the
right hemisphere has no affect on lingual ability. (It must be mentioned, though, that the ability to
comprehend semantics may be impaired with damage to the right hemisphere.) Studies of children with
unilateral damage to the left hemisphere exhibited
deficiency in acquiring
language, while those with right hemisphere damage acquired language like
normal children.
Perhaps the greatest piece of evidence, more final than studying brain damage, is the lack of advanced linguistic ability in left hemidecorticates. Hemidecorticates have only one hemisphere of the brain, either by surgical removal or lack since birth. Right hemidecorticates may still acquire language normally, those without a left hemisphere are severely impaired. Furthermore, children born as left hemidecorticates do not develop language normally, proving that language is not only left hemisphere lateralized but innate in human beings.