Brain Asymmetries: the lateralization of three different cognitive processes

The hemispheric lateralization has received attention across years because its implication in determine behavioural and individual differences. Specifically, the left hemisphere’s expansion and specialization in the course of the evolution received a lot of attention for its involvement in language functioning. Nevertheless, also the right hemisphere expanded and, in particular, the right parietal lobe increased its dimension and its connections, becoming a specialized area for the multisensory integration mechanisms. Even if the parietal lobe has been extensively studied, there are still unanswered questions about the lateralization of different cognitive processes related to this area. Given the broad implication of the parietal areas in several cognitive processes, in this thesis three stand-alone studies on healthy population are presented. The studies are independent even if all of them aim at exploring the lateralization of cognitive processes typically related to the parietal areas. A hierarchical structure was followed from the most basic to the most complex cognitive process: (i) visuo-spatial attention, (ii) motor behaviour and (iii) body representation. In the first study, thirty-six right- and left-handers participants were tested during perceptual (Line Bisection) and representational (Mental Number Line) tasks. Taking into consideration the attentional bias (pseudoneglect), results showed a greater difference between the perceptual and representational domains in the left-handers group. This difference is led by a bias farther to the left in the representational domain, but not in the perceptual one, compared to right-handers. These results suggest that right- and left-handers are differently affected by attentional asymmetries depending on the domain being representational or perceptual. The second study aimed at exploring the lateralization of the bimanual temporal coupling effect. Thirty-two participants were tested during a bimanual task while the transcranial Direct Current Stimulation was applied over right and left parietal area. Results highlight different effects of the stimulation depending on the level of complexity of the movement. In the third study, eighteen participants were tested during implicit (not requiring awareness) and explicit (requiring awareness) body-schema related tasks while a cathodic or sham transcranial Direct Current Stimulation was delivered on the right parietal area. Results highlight an effect of stimulation specific for the implicit task, and selective for identification processes rather than widespread to motor related aspects. These results challenge the general idea of a right hemispheric dominance, and rather suggest that body schema processes rely on a bilateral network dependent on awareness encompassing both the left and the right hemisphere. The three studies together highlight that a black or white hemispheric lateralization of cognitive processes related to parietal areas is not feasible at this point. Indeed, specific features (domain, complexity and awareness) play an important role in concluding for a right or left dominance. For this reason, future studies are needed in order to clarify their weight using different techniques together.