I am both left handed and dyslexic and so a recent paper on the connection in hemispheric dominance for hand and for language was a paper I had to read. The Mazoyer study seems to be the first to use a reasonable number of left- and as well as right-handed people to look at language lateralization. (citation below).
Whether someone was left-handed or right-handed was determined by the self-reported category (the LH and RH identifier in the paper). However, the subjects were also given the Edinburgh questions which give an index between -100 (most left-handed), +100 (most right-handed) with 0 as perfectly ambidextrous. This was used as a measure of the extent and direction of lateralization of the hand’s motor control. This index need not tally with self-reporting, but actually does quantify the lateralization. They used fMRI measurements for the lateralization of language. Reciting a very over-learned list (like the months of the year) is almost symmetrical (not lateralized) and so it was used as a base compared to forming a sentence which varies in lateralization. Language is usually biased to the left hemisphere as is hand control in right-handed people.
Their conclusion was: “This study demonstrates that, except in a small sample of strong LH with rightward asymmetry, concordance of hemispheric dominance for hand and for language production occurs by chance. The present result thus questions the existence of a link between control of the hand and of language by the same hemisphere, while indicating that a rightward representation of language, although rare, is a normal variant of language lateralization.”
At first glance this is not what the graph appears to show. But if you ignore the white data points at the bottom then it appears that the amount of language lateralization (y axis) is heavily biased to the left hemisphere but the amount of bias is evenly spread across the hand lateralization (x axis). The white data points on the other hand show that extreme right hemisphere lateralization of language only seems to occur in a small group of extremely left-handed people. These people would be approximately 1% of the population. This group was also identified by Gaussian analysis which found 4 peaks, the 4th being this group of atypical left-handed people. Without this group the peaks for left and right-handed people were not statistically different.
Lateralization of language plotted against lateralization of hand control: “Figure 5. Plot of hemispheric functional lateralization for language as a function of manual preference strength. Manual preference strength was assessed using the Edinburgh inventory, ranging from 100 (exclusive use of the right hand) to -100 (exclusive use of the left hand). Subjects also self-reported whether they consider themselves as right- handed (RH, squares) or left-handed (LH, circles). HFLI, an index of hemispheric functional lateralization for language measured with fMRI during covert generation of sentences compared to covert generation of list of words, was used for classifying subjects as « Typical » (HFLI>50, bright color symbols), « Ambilateral» (-20<HFLI<50, pale color symbols), or « Strongly-atypical » (HFLI<-20, open symbols).”
Personally I find this very interesting. I have to assume I am in this small strongly atypical group. I score -100 on the Edinburgh test and have fought with dyslexia all my life. But from a more general perspective it is interesting that the lateralization of language has a natural spread without regard to another lateralization that gives handedness. Another interesting piece of data is that left-handed people appear (on the surface) to not be as left-handed as right-handed people are right-handed. The crossover seems to be at Edinburgh 50 (not 0 or -50). This may be an artifact. Left-handed people may learn to do a number of tasks in a right-handed manner because of the general handedness of the environment. A right-handed person has no incentive to do any particular task with the left-hand. We may be looking at motivation rather than anatomy. Finally, although this is a good start to looking at the lateralization of language, language is a complex function and there may be a lot of detail hidden in a single fMRI procedure. This authors mention this. “Because typical subjects represent 90% of the population, it is important to assess whether or not they constitute a homogeneous group with respect to hemispheric dominance. Gaussian mixture model suggests the existence two distinct subgroups of typical individuals, having strong and moderate left language lateralization, respectively, this holding both for RH and for LH. ”
Here is the abstract:
“Hemispheric lateralization for language production and its relationships with manual preference and manual preference strength were studied in a sample of 297 subjects, including 153 left-handers (LH). A hemispheric functional lateralization index (HFLI) for language was derived from fMRI acquired during a covert sentence generation task as compared with a covert word list recitation. The multimodal HFLI distribution was optimally modeled using a mixture of 3 and 4 Gaussian functions in right-handers (RH) and LH, respectively. Gaussian function parameters helped to define 3 types of language hemispheric lateralization, namely ‘‘Typical’’ (left hemisphere dominance with clear positive HFLI values, 88% of RH, 78% of LH), ‘‘Ambilateral’’ (no dominant hemisphere with HFLI values close to 0, 12% of RH, 15% of LH) and ‘‘Strongly-atypical’’ (right-hemisphere dominance with clear negative HFLI values, 7% of LH). Concordance between dominant hemispheres for hand and for language did not exceed chance level, and most of the association between handedness and language lateralization was explained by the fact that all Strongly-atypical individuals were left-handed. Similarly, most of the relationship between language lateralization and manual preference strength was explained by the fact that Strongly-atypical individuals exhibited a strong preference for their left hand. These results indicate that concordance of hemispheric dominance for hand and for language occurs barely above the chance level, except in a group of rare individuals (less than 1% in the general population) who exhibit strong right hemisphere dominance for both language and their preferred hand. They call for a revisit of models hypothesizing common determinants for handedness and for language dominance.”
Mazoyer, B., Zago, L., Jobard, G., Crivello, F., Joliot, M., Perchey, G., Mellet, E., Petit, L., & Tzourio-Mazoyer, N. (2014). Gaussian Mixture Modeling of Hemispheric Lateralization for Language in a Large Sample of Healthy Individuals Balanced for Handedness PLoS ONE, 9 (6) DOI: 10.1371/journal.pone.0101165