Tuesday, 19 November 2013

Where is language located in the brain? There are two sides to this story

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.

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."

_________________________________ ResearchBlogging.org

Byron Bernal and Alfredo Ardila (2013). Bilateral representation of language: A critical review and analysis of some unusual cases. Journal of Neurolinguistics DOI: 10.1016/j.jneuroling.2013.10.002

--Further reading--
The boy who learned to speak again after losing his left brain hemisphere.
Glimpsed at last - the life of neuropsychology's most important patient.
500 Francs Says Language Is Housed in the Frontal Lobes!

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.


Mark said...

Really really interesting read. I used to box and had poor defense to the left and absorbed more blows to this side. I wonder if this is why I am struggling with Spanish night classes so much :)
Thanks for the blog!

Anonymous said...

Would it not be relevant to understand what wad being said? surely different people associate same words differently so would anchor words differently. This is a deeply personal thing and why some comedians are better at puns and wordplay. Since brains set up links dynamically over time relative to exposure very unlikely 2 brains would have same rural pathways. We all know how to get to work but each worker.ay use different route for different reasons. Even metaphors & analogy possibly use right hemisphere more I presume so brains that Cando that may become educators. What were those peoples professions? Its all relative and that's why mapping brains is moot to me.

Abam Panda said...

your blog has inspired mine since months ago. Thank you for that. (not gonna paste the link because it's in a foreign language lol)

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