No relationship between left-handedness and cognitive efficiency
Because there is a long-standing debate about supposedly less good performance in left-handers, we have studied the association between cognitive abilities and manual preference. We found that there was no difference between left-handers and right-handers, although individuals with low manual lateralization and at least one left-hander in their families had slightly lower performance in both verbal and spatial domains (Mellet 2014).
80% of left-handers have the language controlled by the left hemisphere, like right-handers
In a second study, we addressed a debated issue of the existence of an association between manual preference and lateralization of language. Indeed, since in more than 90% of cases, language is driven by the left hemisphere and more than 90% of human beings are right-handed, the most widespread idea is that there is an association between the two. Thanks to the large sample of left-handed people we were able to show that the association between the preferred side of the hand and the strength of hemispherical lateralization for language is not significant (Mazoyer 2015). Importantly, we also observed that left-handed people typically lateralized on the left at the hemispheric level are identical to right-handed people also at the level of asymmetries in the language regions (Tzourio-Mazoyer 2016).
But we also discovered particularities of the organization of the language of very strongly left-handed people for the use of their dominant hand. This population hosts a small group of individuals lateralized on the right for language (0.7% of the general population) as well as individuals without cerebral lateralisation (or ambilateral).
Individual activation maps obtained in fMRI while participants generate sentences (compared to word list generation). At the top is the image of a typical left-hander with stronger activations in the left hemisphere, almost identical to the activation maps seen in a right-hander. Second raw shows a participant with an inversion of the lateralization of the sentence generation with stronger right hemisphere activations, in symmetrical regions.
This description of the broad spectrum of variability in language lateralization variability in healthy subjects allowed us to look for potential differences in intrinsic connectivity depending on the type of lateralization of individuals. We were thus able to show that there is a trace of the lateralization of language in rest networks (Joliot 2016).
There is an association between lateralization of language and attention only in strong left-handers
Another example of the information provided by BIL&GIN concerns the relationship between cerebral lateralization of complementary cognitive functions. Specifically, we address the question about the relationship between the cerebral lateralization of language and attention within individuals. Some authors propose that this complementary hemispheric specialization of these two functions is not associated and are independent the one from to the other, others believe, on the contrary, that the two cerebral lateralization are dependent and causally related. Notably, the different studies leading to these two different hypotheses included different proportions of left-handed and right-handed people. We therefore tested the hypothesis that the complementary HS could be different for right-handed and left-handed people. We observed that the increase in functional asymmetry in the left hemisphere during language production is associated with an increase in right asymmetry during a visual-spatial attention task, only in left-handed subjects having a strong leftward manual preference. Thus, left-handed people who exclusively use their left hand to perform the unimanual gestures of daily life have a different brain organization from other subjects (right-handed or left-handed people less lateralized for their hand). This result could reflect a different set-up of HS during development related to genetic differences (Zago 2016).