Hemispheric consistency in language production, comprehension, and reading in typically and atypically lateralized left-handers: Implications for reading performance.

Language is a complex cerebral function comprising several processing components, such as speech production, auditory comprehension, and reading. While speech production is functionally specialized in the left hemisphere for most of the population, recent findings suggest that receptive components may follow an independent pattern of hemispheric lateralization. In this study, we used fMRI to analyze a sample of 69 left-handed participants, preselected to include a higher-than-usual proportion of people with atypical hemispheric lateralization. We assessed language functional lateralization across three components: speech production (Broca's area), auditory comprehension (Wernicke's area), and overt reading (visual word form area). Using the Laterality Index-a measure of functional hemispheric lateralization-we found a remarkably strong concordance between speech production and auditory comprehension (ρ = .653), with reading emerging as the least concordant of the three language components (ρ = .382 and ρ = .307, with production and comprehension, respectively). Based on these results, we categorized participants as either consistently lateralized (all components lateralized to the same hemisphere; n = 44) or inconsistently lateralized (one component diverging from the others; n = 25). Using this classification, we found that hemispheric inconsistency was predominantly characterized by an overall rightward dominance of language (two components lateralized to the right and one to the left), with reading being the most common diverging component. As a result, a left-lateralized function diverging from an overall rightward dominance was more common than the reverse pattern. Crucially, we also observed that hemispheric inconsistency-both as a categorical and continuous measure-was associated with slightly slower reading speed, particularly when the inconsistency occurred between speech production and reading components. In conclusion, our findings challenge previous accounts by showing that speech production and auditory comprehension lateralize quite uniformly, and that exceptions to this-inconsistent patterns mostly involving a divergent lateralization of reading-are associated with a slight reduction in reading speed.