Gregory Kroliczak, Agnieszka Nowik, and Lukasz Przybylski
Faculty of Psychology and Cognitive Science, Adam Mickiewicz University, Poznan, Poland
Little is known about the cerebral polymorphisms for disparate praxis and language skills because there are no studies testing simultaneously the multitude of variants in their functional lateralities. Hence, the prevalence of their most common atypical forms is yet to be established. Here, using functional magnetic resonance imaging (fMRI) in 62 individuals with different handedness status (assessed by multiple behavioural tests), we studied the phenotypes of functional organisation of two different skilled action tasks (praxis) and two disparate language tasks, as well as the laterality/organization of simple visual processing of tool images. In two study sessions, using either the right or left hand (counterbalanced) in response to visually presented stimuli, participants 1) planned functional grasps of tools vs. simple grasps of non-tool objects, and 2) executed tool use pantomimes vs. manually counted parts of non-tools. Moreover, participants 3) performed a subvocal word generation (verbal fluency) test, and 4) completed a receptive language (auditory semantic decision vs. tone decision) task. In our sample of 34 non-righthanders (adextrals), as many as 41% of participants had atypically organised neural activity underlying functional grasp planning, vs. 35% with atypically organised tool use pantomimes, regardless of the used hand. Interestingly, only 24% of adextrals had both of these praxis skills atypically organised. Critically, out of merely 18% of adextrals who had all of the studied functions (i.e., including the two language tasks) atypically organised, only 12% had them exclusively right lateralised. Additional 6% of adextrals with typical, left-lateralised functional grasp planning, had the remaining 4 studied functions in an atypical form. In sharp contrast, only 7% of righthanders had both praxis skills atypically organised, and just 4% showed right lateralisation of all the studied functions. Thus, our research shows that reversed-typical lateralisation of all these functions – expected to be left lateralized in the majority of the population – is not that rare, and can be even found in righthanders, too. The prevalence of these polymorphic forms will be discussed in the context of genetic models (e.g., dextral-chance modelling) based on the studies of handedness.
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