Measures Of Hand Skill Research Articles

Is laterality adaptive? Pitfalls in disentangling the laterality–performance relationship

Unlike non-human animal studies that have progressively demonstrated the advantages of being asymmetrical at an individual, group and population level, human studies show a quite inconsistent picture. Specifically, it is hardly clear if and how the strength of lateralization that an individual is equipped with relates to their cognitive performance. While some of these inconsistencies can be attributed to procedural and conceptual differences, the issue is aggravated by the fact that the intrinsic mathematical interdependence of the measures of laterality and performance produces spurious correlations that can be mistaken for evidence of an adaptive advantage of asymmetry. Leask and Crow [Leask, S. J., & Crow, T. J. (1997), How far does the brain lateralize?: an unbiased method for determining the optimum degree of hemispheric specialization. Neuropsychologia, 35(10), 1381–1387] devised a method of overcoming this problem that has been subsequently used in several large-sample studies investigating the asymmetry–performance relationship. In our paper we show that the original Leask and Crow method and its later variants fall victim to inherent nonlinear dependencies and produce artifacts. By applying the Leask and Crow method to random data and with mathematical analysis, we demonstrate that what has been believed to describe the true asymmetry–performance relation in fact only reflects the idiosyncrasies of the method itself. We think that the approach taken by Leask in his later paper [Leask, S. (2003), Principal curve analysis avoids assumptions of dependence between measures of hand skill. Laterality, 8(4), 307–316. doi:10.1080/13576500342000004] might be preferable.

Principal curve analysis avoids assumptions of dependence between measures of hand skill

The relationship between laterality and performance is unclear, and using laterality indices to explore this relationship can be misleading. An alternative approach is to consider laterality as a bivariate construct. A scatterplot of measures on each side will allow the distribution of values in a population to be visualised. However, determining where mean values lie in a population is problematic. Even nonparametric regression techniques such as lowess make assumptions about one variable being independent and the other dependent. Such assumptions are arbitrary in the case of measures by side.Assumptions of dependence are avoided using principal curve analysis. The results of two nonparametric fitting techniques (lowess regression and a principal curve analysis) are compared using hand skill data on 12,782 11‐year‐olds from a UK national birth cohort. The lowess regression is misleading. The principal curve analysis shows that absolute right–left performance differences increase with hand skill to a maximum about the point of average hand skill, and the difference is then constant up to extremes of hand skill.

Principal curve analysis avoids assumptions of dependence between measures of hand skill

Principal curve analysis avoids assumptions of dependence between measures of hand skill

Predicting reading ability from handedness measures.

Word reading, hand skill on a peg-moving task and hand preference were measured in 203 children aged from 11-13 years. Differences in skill between the hands were largely attributable to variations in left-hand skill, consistent with the view that manual and cerebral asymmetry are due to the loss (pruning) of cells in the right hemisphere during development-a mechanism that may be dependent on the "right shift' (rs+) gene postulated by Annett (1985). There was only weak evidence for the curvilinear relation between differential hand skill and reading predicted by Annett's (1985) theory of balanced polymorphism, which postulates a heterozygotic advantage. Reading was better predicted by absolute measures of hand skill than by the difference between the hands, especially among boys. For the boys, word reading was predicted significantly from right-hand skill, while for girls left-hand skill predicted slightly more strongly than right-hand skill. This may be related to differential growth gradients in the two hemispheres, and different environmental influences for boys and girls during a period of right hemispheric growth.