Praxis and language are linked: evidence from co-lateralization in individuals with atypical language dominance

We determined the neural correlates of word generation and tool use pantomiming in healthy subjects with typical (n=10) or atypical (n=10) language dominance to investigate similarities in response pattern and hemispheric specialization between language and praxis. All typical language dominant volunteers also revealed left hemisphere changes during tool use pantomiming in prefrontal, premotor, and posterior parietal regions. All atypical language dominant participants displayed right hemisphere engagement for tool use. Co-lateralization of the language and praxis networks was observed on group and individual level, regardless of the participant's handedness. Activation maps of the word generation and tool use pantomiming contrasts displayed overlap in five cortical regions: supplementary motor area, dorsal and ventral premotor cortex, dorsolateral prefrontal cortex, and posterior parietal cortex. Individual lateralization indices were calculated for each region and revealed significant positive group correlations between .51 and .95 with every other region within the paradigms. Positive cross-task correlations ranged between .72 (supplementary motor complex) and .97 (dorsal premotor cortex) and illustrate that the strength of hemispheric specialization of one task significantly predicts the side and degree of lateralization of the other task, suggesting a functional and topographic link between language and praxis. These findings support models that link gestures and speech to explain the evolution of human language. We argue that the existence of a common and co-lateralized network underlying the production of complex learned movement, whether it be speech or tool use, may represent the evolutionary remnant of a neural system out of which proto-sign and proto-speech co-evolved.