Young children use both brain hemispheres to understand language, neuroscientists say, which may explain why they appear to recover from neural injury more easily than adults.
Previous brain scanning research and the clinical findings of language loss in patients who suffered a left hemisphere stroke have shown that, in almost all adults, sentence processing is possible only in the left hemisphere.
However, this pattern is not established in young children, according to a team led by Elissa L Newport, from Georgetown University, US. Brain networks that localise specific tasks to one hemisphere start during childhood but are not complete until a child is about 10 or 11.
Scientists have suspected as much, Newport says, but traditional scanning has not revealed the details. It was thus unclear whether strong left dominance for language is present at birth or appears gradually during development.
She and colleagues enrolled 39 children and 14 adults, gave them a sentence comprehension task, then used functional magnetic resonance imaging (fMRI) to examine language activation patterns in each hemisphere of each individual’s brain.
They compared the activation maps for four age groups – 4-6, 7-9, 10-13 and 18-29 – to reveal the percentage of subjects in each age group with significant language activation in each voxel (a tiny point in the brain image, like a pixel on a TV monitor) of each hemisphere
They also performed a whole-brain analysis across all participants to identify brain areas in which language activation was correlated with age.
The findings show that at the group level even young children showed left-lateralised language activation, they say, but a large proportion of the youngest children also showed significant activation in the corresponding right-hemisphere areas.
In adults, the corresponding area in the right hemisphere is activated in different tasks, such as, processing emotions expressed with the voice. In young children, both hemispheres are engaged in comprehending the meaning of sentences and recognising the emotional affect.
Newport is confident the findings, published in the journal Proceedings of the National Academy of Sciences, reflect changes in the neural distribution of language functions and “not merely developmental changes in sentence comprehension strategies”.
“Our findings suggest that the normal involvement of the right hemisphere in language processing during very early childhood may permit the maintenance and enhancement of right hemisphere development if the left hemisphere is injured,” she says.
The researchers are now examining language activation in teenagers and young adults who have had a major left hemisphere stroke at birth.