A lateralized comparison of handedness and object proximity

Abstract This study was motivated by the emerging hypothesis that right-handers are more strongly lateralized and perform better on various aspects of functional asymmetry than do left-handers. Right- and left-handers were observed for hand selection responses to a unimanual task of reaching for a small cube in positions of right- and left hemispace, prompting hemispheric decision-making related to hand dominance and attentional (visuospatial) stimuli. As predicted, left-handers did not respond with their preferred limb as consistently across positions as did right-handers. Additional inspection of the task suggests that being less lateralized may not be a disadvantage in this context, and that environmental influence may play a significant role in hand selection for a particular motor event. The purpose of the present investigation was to examine hypothesized differences between right- and left-handers on a unimanual task involving object proximity. A rather common theme that seems to be emerging is the observation that hemispheric asymmetry for a typical group of left-handers is not as large as for a group of right-handers (Bishop, 1990; Hellige, 1993; Iaccno, 1993). In essence, it appears that left-handers are more variable and "less lateralized" in various aspects of functional asymmetric behaviour compared to right-handers; this conclusion is a result of research involving a wide variety of paradigms and measures. In addition to more recent attention to the comparison of basic perceptual abilities (i.e., specific aspects of dichotic listening and processing of visuospatial stimuli) (Bub & Lewine, 1988; Brysbaert, 1994; Hellige, Bloch, Cowin, Eng, Eviatar, & Sergent, 1994), such observations have also been reported for an array of motor tasks (e.g., Borod, Caron, & Koff, 1984; Curt, Maccario, & Dellatolas, 1992; De Agostini, Pare, Goudot, & Dellatolas, 1992; Gabbard, Hart, & Gentry, 1995). Unfortunately, the literature for motor behaviour is not clearly supportive of a right-hand advantage (e.g. Gabbard et al., 1995; Peters, 1990; Peters & Servos, 1989) due most likely to inconsistencies in method used for classifying hand groups (e.g., right-, mixed-, left-handed) and to the diversity of tasks assessed. However, there have been indications that left-handers are significantly less lateralized and more widely dispersed than right-handers in regard to the magnitude of preference and manual skill (Peters, 1987; Tan, 1983; in addition to the citations already noted). Complementing this observation is the general finding that there are more strong right-handers than strong left-handers (Annett, 1985; Borod et al., 1984; Curt et al., 1992; De Agostini et al., 1992). Theoretical explanations for this phenomenon are usually associated with genetics (e.g., right shift theory, Annett, 1985) and cultural influence (e.g., right-side world hypothesis, Porac & Coren, 1981; Porac, 1993; Provins, 1992). Therefore, the present study was motivated by the hypothesis that right-handers are more strongly lateralized, and as a result, perform better on various aspects of functional asymmetry than left-handers. An experiment was designed to assess limb preference using a unimanual task requiring hand selection in different positions of right- and left hemispace. We selected this task because it provides a reasonable balance between hemispheric decision-making related to hand dominance (motor asymmetry) and attentional (visuospatial) stimuli. With this hypothesis and experimental paradigm, we would predict that left-handers responses would be more dispersed by not being as consistent as right-handers with the preferred limb across positions. This paradigm also allows for further inspection of the contemporary notion that handedness may not be a genetically invariant behaviour; rather, hand selection may be influenced significantly by attentional control related to particular task demands (Bradshaw, Nettleton, Pierson, Wilson, & Nathan, 1987; Hellige & Kee, 1990; Peters, 1995; Verfaellie & Heilman, 1990). …