“Do left-handed people speak with their right hemisphere?” A seemingly absurd question. And yet, the vast majority of the population (90%) prefers to use their right hand. In doing so, these people activate their left cerebral hemisphere, since motor control behavior is “crossed”. On the other hand, many studies have shown that the network of brain areas controlling speech is also lateralized, so that it is situated on the left in 90% of subjects. Does the location of language areas thus correlate with being right or left handed?

To find out, researchers from the Neurofunctional Imaging Group (CNRS/CEA-I2BM/Université de Bordeaux) recruited nearly three hundred participants, half of them left-handed. All were subjected to functional MRI while they performed tests of language. Three major modes of lateralization of language areas thus emerged. The vast majority of subjects, whether left- or right-handed, show a “typical” lateralization with a largely dominant left hemisphere. In the “ambilateral” mode, which only concerns a minority of left- and right-handed individuals, neither hemisphere clearly dominates. Finally, the right hemisphere dominates in 7% of left-handed people. Statistical analysis shows that the correlation between the dominant hemisphere for manual activities and that for language is the result of chance, except for this small fraction of the population (less than 1% of the total) in which the right hemisphere is dominant for both language and for the hand.

What is left now is to understand the origin of this “very atypical” lateralization of language. More generally, this result shows the importance of recruiting cohorts “enriched” with left-handed people for revealing sources of variability in the structural and functional bases of the human brain.