Perceptual skills and handedness

The literature is saturated with inconsistencies and experimental difficulties regarding the perceptive laterality question. When differences are found, the results suggest that men are more strongly lateralized than women, with the general hemispherical dichotomy that verbal functions are taken on by the left hemisphere and non-verbal or spatial functions are more effectively carried out by the right hemisphere (McGlone 1980). The data gathered on subjects having left or right damages or split-brain patients clearly reveal a right hemisphere superiority in the haptic processing of geometrical shapes, whereas healthy subjects (the control group) show no difference in skill between the two hands (Nebes 1971; Ledoux et al. 1977; Franco & Sperry 1977; cf. Chapter 3). More specifically, Nebes's 1971 study suggests that right hemisphere superiority is the result of its capacity to treat information more globally than the left hemisphere.

In healthy subjects, whether the situation be intramodal monohaptic (H-H condition) or intermodal (H-V condition), the results often show a superiority in the performance of the left hand (cf. Lacreuse, Fagot, & Vauclair 1996). Other work has led to contradictory results and no difference is observed in the skills of each hand (Yamamoto & Hatta 1980; Webster & Thurber 1978) or a right hand advantage is observed (Cranney & Ashton 1982). In the dichaptic intermodal situation (H-V), although the classic left hand advantage is observed in research by Dawson (1981) and Fagot, Hopkins and Vauclair (1993), contradictory data have also been observed. In the same way, gender effects on asymmetrical strength and direction in manual perception are not clearly established. A global effect favoring the left hand seems to emerge in the tactile processing of geometrical shapes (cf. Fagot, Lacreuse, & Vauclair 1997; Verjat 1989).

According to Lacreuse et al. (1996), the lack of homogeneity in these results may be explained by the exclusive choice of just one type of measure. The differences in the competence of each hemisphere are inferred from the comparison of the scores of each hand, and not from the analysis of the processes underlying these performances. These authors tested the hypothesis that exploration strategies (the duration and spatial distribution of finger contact), rather than performance, would better display underlying cognitive operations and their lateralization. Their aim consisted of looking for asymmetries both during the initial exploration phase and during the recognition phase of geometrical shapes, and this according to the mode of manual exploration (mono-or dichaptic). The left hand advantage only appears in dichaptic exploration situations. The strategies are especially used during the learning phase and not during the recognition phase. The dichaptic mode favors the appearance of differences in inter-manual performances. Finally, according to the authors, these differences are imputable more to the cognitive constraints of the task (shared attention, memory load) and less to the emergence of an inter-hemispherical competition.

Currently, research is oriented towards the relation between certain cognitive and manual skills. For example, using a monohaptic procedure with stimuli made up of dots which become more and more complex, Ernest (1988) examined manual asymmetry in male and female subjects having weak or strong visuospatial and verbal skills. The results reveal a close relation between cognitive abilities, manual lateralization, and the subjects' gender (cf. Benoit-Dubrocard, Liegois, &Harlay 1997; Halpern, Haviland, & Killian 1998).

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