(BPS Research Digest, 21, November 2013) - Simple facts
about the brain are rare, but one of them is that for most people language
function is located mainly in their left brain hemisphere. The stats vary
according to the measures used, but this is the situation for around 95 per
cent of right-handers and approximately 75 per cent of left-handers. When it
comes to the brain though, few things are straight-forward.
Journal article: Bilateral representation of language: A critical review and analysis of some unusual cases.
If we dig deeper, as Byron Bernal and Alfredo Ardila have
done for a new review paper, we find a more complex, two-sided story. The extent
to which language is dominated by the left hemisphere is not fixed. It
increases through childhood and adolescence, and then this trend reverses in
old age, with signs of greater sharing of language function across the brain
hemispheres in later life.
Moreover, by characterising people in binary fashion
as having their language abilities housed either in their left or right
hemisphere, we ignore those people for whom language is a genuinely
"bilateral function," meaning that both brain hemispheres are substantially
involved.
As Bernal and Ardila point out, a dramatic demonstration of
this comes from the Wada test, named after
Japanese neurologist Juhn Atsushi Wada. With the patient awake,
anaesthetic is injected into the neck or head on one side to effectively shut
down function in that side of the brain. Speech and language comprehension
tests are conducted first with one hemisphere silenced, then the other.
Looking
at the results from 1,799 Wada tests, most of which were conducted with
epilepsy patients prior to surgery, Bernal and Ardila found that 10 per cent of
right-handers and 27 per cent of left-handers (and the ambidextrous) showed
evidence that their language function was supported by both brain hemispheres.
The way that bilateral language function manifests in the
Wada test varies from patient to patient. In some, shutting down one hemisphere
has no effect on their language abilities, while shutting down the other only
partially interferes with language. In other patients, shutting down one hemisphere
completely impairs language, while shutting down the other also has a partial
adverse effect. And in a final group, shutting down either hemisphere results
in only a partial impairment to language.
The reason for these different patterns, Bernal and Ardila
explain, is that there are various ways that language function can be shared
between the hemispheres. Using brain scans from real life case studies, they
show how in some people all functions of language are shared between the left
and right brain, whereas for other people some sub-functions of language are
bilateral, but not others.
For instance, the faculties involved in language
comprehension might be bilateral, but the faculties of language production are
not, or vice versa (similar dissociations can be found for processing sound and
meaning). Related to this, some people show evidence that the different steps
of language function are distributed sequentially between the hemispheres (e.g.
one stage processed on one side, the next stage on the other), so there is no
redundancy, whereas other people show a kind of parallel arrangement where both
hemispheres are able to perform the same steps of language processing.
We need to be cautious when extrapolating from patient
studies to healthy people because it's possible that the brain has altered its
function to adapt to disease. This caveat aside, Bernal and Ardila's
fascinating review is a reminder of the brain's complexity. The factoid that in
most people language is left-lateralized conceals a messy reality. "It is
a frequent understanding that language lateralization is a matter of all or
nothing," write Bernal and Ardila. "However, language dominance is
mostly a matter of hemispheric advantage for a specific multi-modular cognitive
function: language. As such, language in a strict sense is up to a certain
point a bilateral brain function."
No comments:
Post a Comment