Since human beings have two hands which do not interact in the same way with the environment, the emergence and evolution of manual skill in children has always been a controversial topic in philosophers, anatomists, biologists and psychologists for centuries (cf. Harris 1983). The question as to why the great majority of humans (between 85% and 95%) show a right-handed preference in numerous activities, such as writing, drawing, teeth brushing, etc. and, conversely, a left-handed preference in perceptive activities, remains a polemical subject today.
Given the interest in manual skill that has been shown over the years, it is surprising to note that the question of its origin in babies was first studied only two decades ago (cf. Young, Segalowitz, Corter, & Trehub 1983). One of the main reasons for this is that the emergence of manual skill was examined as an epiphenomenona linked to language acquisition, and not really studied as a behavior in its own right. Yet the question is important because behavioral asymmetries are regarded as a reflection of the asymmetries in the working of the two hemispheres. The demonstration of these asymmetrical behaviors in babies, and indeed fetuses, would be a sign of an early functional specialization of the brain hemispheres. However, the nature of the relationship between functional asymmetries, brain morphology and behaviors at the beginning of a child's development are still not widely known. But, thanks to recent methods in functional brain imagery (cf. Chapter 3), this relationship has become visible directly and is no longer inferred from clinical or experimental observations. There are two main approaches to brain dissymmetry or hemispherical specialization (cf. Bradshaw & Nettleton 1981). In the first, brain dissymmetry is based on the type of material to be handled. In this case, it seems that the left hemisphere is specialized in the sequential handling of information, and more particularly in linguistic data, whereas the right hemisphere is specialized in the handling of spatial data. This dichotomy has principally been studied via two sensory systems, vision for spatial data, and hearing for linguistic data. In the second approach, brain dissymmetry is based on the way the information is handled, globally or analytically. The left hemisphere works in a more analytical, sequential, serial and focal way and uses this skill to handle verbal data. The right hemisphere works in a more global, simultaneous, parallel and holistic way and is thus better adapted to the handling of spatial data. The study of the visual and hearing modalities has also provided answers in this second approach.
The haptic modality has a different status from the visual and hearing modalities, and certain difficulties arise when faced with these two dichoto-mous approaches (Verjat 1988). On the one hand, the arm-hand system fulfils two functions, the first one is perceptive and the second is instrumental in transporting objects in space. On the other hand, the manual exploration of objects involves a perception/motor coupling difficult to dissociate. Thus, for the majority of people, the motor and sequential skills which emerge during writing activities, and also during exploration tasks, are achieved better by the right hand. These activities, in a similar way to the auditory perception of word sounds, should be taken care of by the left hemisphere. Otherwise, the haptic modality apprehends spatial properties of objects (shape, orientation, localization; cf. Chapter 8) in the same way as vision, and this function should be taken care of by the right hemisphere. The study of the haptic system is also contradictory regarding the global vs. analytical handling of data. The arm-hand system apprehends the subject's near external space (localization and object taking) and the space of the object (shape and orientation). The right hemisphere should play a dominant role in these activities. But as a contact modality, the hand apprehends spatial data sequentially, and hardly in a holistic way. The left hemisphere should therefore also intervene in this function. How can the moment when the hand is treating information globally be distinguished from the moment when it is detecting more local information? How can spatial data handling be distinguished from sequential handling? Is one hand more skilled in the haptic perception of shape and orientation, whereas the other is more skilled in fine motor activities?
Healey, Liederman and Geschwind (1986) proposed that the adult manual skill be characterized according to the musculature involved in carrying out a task (distal or proximal muscles). With regard to the arm-hand complex, the distal musculature, limited to the hand, is involved in fine manipulation movements and is strongly lateralized. Steenhuis and Bryden (1989) and Bryden,
Singh, Steenhuis and Clarkson (1994) contest this classification, suggesting that it is the complexity of the gesture which should be considered, rather than the muscles involved in its organization. For example, fewer motor skills are necessary to pick up an object than to manipulate or use it. The picking up of an object is also characterized by a less lateralized hand preference than using or manipulating tools. If we are interested in hemispherical specialization in haptic activities, and if we want to know when it emerges, we must direct ourselves towards the study of the fine perceptive/motor performances of the hand, and not towards those mobilizing the global arm-hand motor system.
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