PRAXIS AND LANGUAGE ORGANIZATION IN LEFT-HANDERS

Abstract

The performance of learned manual gestures (praxis) and the production of speech are thought to depend on related neural processes. If this relationship is not invoked by an unknown, third variable then shifts in their laterality, including dissociations of these two functions, would be unlikely unless the sharing of some neural resources with other functions is advantageous. This could be the case in lefthanders, in whom actions requiring manual precision are controlled by their right hemispheres, and whose representations could attract the control of skilled gesture. Functional neuroimaging (fMRI) was used to study praxis and language functions. Their lateralization indices were measured in 56 consecutively tested lefthanders (28 females), with the mean age of 23.3±4.9 years (range 18.4 – 47 years), and an Edinburgh Handedness Inventory quotient between –100 and –55.6 (with the mean of –83.8±14.2). We show that atypical, bilateral organization or right-lateralization of praxis is more common than atypical organization/lateralization of language, observed, respectively, in 23 (41%) vs. 15 (26.8%) of cases. Specifically, we found: (a) seven cases (12.5%) of clear, and an additional three cases (5.4%) of less pronounced dissociations of atypically represented praxis from typically represented language; (b) 13 cases (23.2%) with atypically organized praxis also associated with atypically organized language, and (c) only two cases (3.6%) of rather strongly atypical lateralization of language, yet with quite typical lateralization of praxis. These outcomes are consistent with an idea that, in some lefthanders, the guidance of skilled manual actions can profit from tighter links with the right hemisphere, whose motor specialization is linked in this particular population to manual precision, but in general to attentional resources, visuo-spatial processing and even bimanual coordination. Because of the presumed links of praxis with productive language, such transfers are often, and unsurprisingly accompanied by the reorganization of the latter. Yet, the very rare cases of reversed language functions, without any pronounced shifts in representations of praxis, indicate that such a pattern of segregation – or inverse dissociation – of these two functions could be maladaptive.