Hemispheric asymmetries in the control of upper limb movements.

This chapter deals with the unique human abilities of using tools, imitating others' gestures, drawing, and building complex items. Herein, after a brief overview of clinical manifestations and assessment of disorders of tool use and imitation (upper limb apraxia) and of the impairments in drawing and assembling multipart objects (constructional apraxia), brain asymmetries are discussed mainly starting from the neuropsychologic studies on patients with focal brain lesions, although both upper limb apraxia and constructional apraxia are often observed during the course of neurodegenerative diseases. Although no room is allowed here for a full discussion of brain-behavior relationships, relevant functional neuroimaging findings in healthy individuals are considered. The data presented in this chapter clearly demonstrate that tool use, gesture imitation, and "formative" activities (i.e., drawing and assembling) require the interplay of several brain areas and neural networks distributed over both hemispheres. Nonetheless, gesture processing exhibits a quite strong lateralization to the left hemisphere in typical right-handers. As regards "formative" activities, the neural networks in both hemispheres seem to provide complementary contributions, although the left inferior parietal lobule might play a specific role. The convergence of gesture processing and "formative activities" in the parietal lobes might be related to their considerable expansion in our species and could suggest that some shared basic, still unknown, computational processes allowed the development of these skills in modern humans.