fMRI and Database Show a Dominant Side of the Brain for Handedness and Language

Utilizing a large psychometric and brain imaging database, researchers have demonstrated that the location of language areas in the brain is independent of left- or right-handedness, except for a very small percentage of left-handed individuals whose right hemisphere is dominant for both manual work and language.

Investigators from the Groupe d'Imagerie Neurofonctionnelle, CNRS (Centre National de la Recherche Scientifique)/CEA, Université de Bordeaux (France) published their finidngs June 30, 2014, in the journal PLOS One.

Image: Probability map of the brain regions activated in functional magnetic resonance imaging (fMRI) by a silent sentence production task in a group of 144 right-handed individuals. The color scale indicates the percentage of subjects with significant activation in this area during the task (green: 50%, blue: 65%, red: 80% or more). Note the high asymmetry of the map in favor of the left hemisphere) [Gauche = Left, Droite = Right] (Photo courtresy of Groupe d\'Imagerie Neurofonctionnelle, CNRS/CEA/Université de Bordeaux, France).

Human beings are the only species in which asymmetric motor behavior is preponderant: 90% of individuals prefer to use their right hand and 10% their left hand. This motor behavior is “cross-lateralized”: when using the right hand, the dominant left hemisphere is stimulated. In addition to the motor behavior, language is one of the most lateralized functions of the human body: the networks of brain areas controlling language are located asymmetrically in the brain’s left or right hemisphere. Many studies have revealed that the left hemisphere is dominant for language in 90% of instances, as it is for motor behavior.

The investigators were trying to determine if the 10% of people who are left-handed and of those whose language is situated in the brain’s right hemisphere are the same, and if the location of language areas in the brain was associated with handedness. To solve these questions, researchers from the Groupe d'Imagerie Neurofonctionnelle recruited a large sample of participants (297) including numerous left-handed participants (153). Whereas most other studies only relate to right-handers (which includes most of the population), these researchers examined language lateralization for the first time in a large number of right- and left-handed individuals.

The study participants in this sample were evaluated using functional magnetic resonance imaging (fMRI) while they performed language testing. Three types of language lateralization were then revealed from the images obtained: “typical” with a dominant left hemisphere (present in 88% of right-handers and 78% of left-handers), “ambilateral” without a visibly dominant hemisphere (present in 12% of right-handers and 15% of left-handers), and “strongly atypical” with a dominant right hemisphere (present only in 7% of left-handers). Statistical analysis of this distribution demonstrates that concordance between the dominant hemisphere for handedness and that for language is random, except for a small amount of the population (less than 1%) for whom the right hemisphere is dominant for both language and handedness.

These findings show that knowing an individual’s preferred handedness it is not sufficient to determine their dominant hemisphere for language, according to the investigators. Researchers will now attempt to elucidate why only a small group of left-handers have a dominant right hemisphere for language, especially by determining whether there are genetic variants that might clarify this phenomenon. The study also demonstrated that by contrast with a sample including most of the right-handed people, a sample supplemented with left-handers composed from a large database showed variability factors in the brain structural and functional correlates: determining the sources of this variability in language lateralization may help to clarify language pathologies.

Related Links:
Groupe d'Imagerie Neurofonctionnelle, CNRS/CEA, Université de Bordeaux