Language function distribution in left-handers: A navigated transcranial magnetic stimulation study

Recent studies suggest that in left-handers, the right hemisphere (RH) is more involved in language function when compared to right-handed subjects. Since data on lesion-based approaches is lacking, we aimed to investigate language distribution of left-handers by repetitive navigated transcranial magnetic stimulation (rTMS). Thus, rTMS was applied to the left hemisphere (LH) and RH in 15 healthy left-handers during an object-naming task, and resulting naming errors were categorized. Then, we calculated error rates (ERs=number of errors per number of stimulations) for both hemispheres separately and defined a laterality score as the quotient of the LH ER RH ER through the LH ER + RH ER (abbreviated as (L-R)/(L+R)). In this context, (L-R)/(L+R)> 0 indicates that the LH is dominant, whereas (L-R)/(L+R) <0 shows that the RH is dominant. No significant difference in ERs was found between hemispheres (all errors: mean LH 18.0 +/- 11.7%, mean RH 18.1 +/- 12.2%, p=0.94; all errors without hesitation: mean LH 12.4 +/- 9.8%, mean RH 12.9 +/- 10.0%, p=0.65; no responses: mean LH 9.3 +/- 9.2%, mean RH 11.5 +/- 10.3%, p=0.84). However, a significant difference between the results of (L-R)/(L+R) of left-handers and right-handers (source data of another study) for all errors (mean 0.01 +/- 0.14 vs. 0.19 +/- 0.20, p=0.0019) and all errors without hesitation (mean -0.02 +/- 0.20 vs. 0.19 +/- 0.28, p=0.0051) was revealed, whereas the comparison for no responses did not show a significant difference (mean: -0.004 +/- 0.27 vs. 0.09 +/- 0.44, p=0.64). Accordingly, left-handers present a comparatively equal language distribution across both hemispheres with language dominance being nearly equally distributed between hemispheres in contrast to right-handers. (C) 2016 Elsevier Ltd. All rights reserved.