Manual lateralization in infancy

INTRODUCTION Most people are right-handed and leftcerebrally dominant for language and this brain dissymmetry has been based on “which” type of information each hemisphere controls (Corballis, 2012): In general terms, the left hemisphere (LH) is usually specialized in auditory information, specially linguistic stimuli, whereas the right hemisphere (RH) would be specialized in visuo-spatial information. In another approach, brain dissymmetry depends on “how” each hemisphere processes information: While the RH would process the visual spatial data in a more global, simultaneous and holistic way, the LH would process the linguistic data in a more analytic, sequential and serial manner. Here we will focus on the haptic modality because it has a different status from the visual and auditory modalities, which makes of it a special case that can shed light on the origins of manual laterality. In fact, the arm-hand system fulfills both a perceptual function by gathering information on objects, and an instrumental function by transporting objects in space (Hatwell, 1983). Moreover, one of the particularities of the haptic modality during the fine manipulation of objects is that it involves a perceptual-motor coupling difficult to dissociate. Since the arm-hand system is specialized in the treatment of spatial information like the visual modality, the RH should play a dominant role. Because this is a contact modality, in which the hand processes spatial information sequentially like the auditory modality, it would be the LH that should intervene in this function. Notwithstanding the difficulty of disentangling the perceptual and motor aspects from one another, we argue that thanks to the methodological tools used in infancy it is possible to dissociate them. In fact, when holding an object, infants are evaluated in their motor aspects (holding time, strength of the grasp, etc.), while when infants are habituated to an object and then tested on the recognition of its properties, it is the perceptual-cognitive aspects that are revealed with this procedure. Therefore, we assume that the process of lateralization manifests itself in a clear way both from the perception as from the motor function of the hands. We will review and discuss the literature on the motor aspects of manual asymmetries, and will focus on the methodological importance in order to disclose the perceptual aspects of these asymmetries. Finally, we discuss the manual perceptual asymmetries, and the findings and implications in the context of hemispheric specialization.