"Does degree of asymmetry relate to performance?" A critical review

In a recent paper, Chiarello, Welcome, Halderman, and Leonard (2009) reported positive correlations between word-related visual field asymmetries and reading performance. They argued that strong word processing lateralization represents a more optimal brain organization for reading acquisition. Their empirical results contrasted sharply with those of another such large-scale study, by Boles, Barth, and Merrill (2008). We reported negative correlations between asymmetry and performance when both were measured using the same visual lexical tasks. Most recently, within-task negative correlations were also reported by Hirnstein, Leask, Rose, and Hausmann (2010). Here two major differences between studies are explored. Task purity refers to the influence of the same mental processes on both the asymmetry and performance measures, and is arguably maximal in studies measuring both within the same task. The other difference concerns the measurement of asymmetry. Linear corrections for ceiling and floor effects were used by Chiarello et al. and Hirnstein et al., while we used a more appropriate nonlinear one. Their results are difficult to interpret for those reasons. The operation of a third variable to which both asymmetry and performance are positively correlated could also be a factor in the Chiarello et al. findings. The Boles et al. findings reflect a negative correlation between an asymmetric visual lexical process and performance measured within the same task.