A Dominant Hemisphere: For Language and Handedness

Lefty or right handed? The location of language areas. And the way human brain works. The speculated correlations have always raised plenty of interest in the field of Neuroscience.

Based on this work published in the journal Plos One through an innovative approach using a large psychometric and brain imaging database, researchers in the Groupe d'Imagerie Neurofonctionnelle (CNRS/CEA/Université de Bordeaux) have demonstrated that the location of language areas in the brain is independent of left- or right-handedness, except for a very small proportion of left-handed individuals whose right hemisphere is dominant for both manual work and language.

Studies confirmed that humans are the only species in which asymmetric motor behavior is preponderant: 90 per cent of people prefer to use their right hand and 10 per cent their left hand. This motor behavior is "cross-lateralized," while using right hand, the dominant left hemisphere is activated. Along with motor behavior, language is one of the most lateralised functions of the human body, the networks of brain areas controlling language are located asymmetrically in the brain's left or right hemisphere.

It was further established that the left hemisphere is dominant for language in 90 per cent of cases, as it is for motor behaviour.

Now this 10 per cent of people who are left-handed and of those whose language is located in the brain's right hemisphere, are they same? Is the location of language areas in the brain correlated to handedness?

In order to address these questions, in this study, a large sample of participants (297) was recruited, including numerous left-handed subjects (153). "The subjects in this sample were assessed using functional MRI while they were performing different language tests. Three types of language lateralization were extablished from the images obtained; 'typical' with a dominant left hemisphere (present in 88% of right-handers and 78% of left-handers), 'ambilateral' without a clearly 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 shows that concordance between the dominant hemisphere for handedness and that for language is random, except for a small fraction of the population (less than 1 percent) for whom the right hemisphere is dominant for both language and handedness.”

These results showed that knowing an individual's preferred handedness it is not sufficient to determine their dominant hemisphere for language.