The measurement of left-right asymmetries in the Simon effect: a fine-grained analysis.

The spatial Simon effect is often asymmetric, being greater on one side than on the other. To date, not much attention has been paid to these asymmetries, and explanations of the Simon effect do not take them into account. In the present article, we attempt to clarify the statistical implications of the asymmetries so as to provide a useful tool for future empirical investigation. Starting with examples from our laboratory and previous well-known studies, we point out the consequences of ignoring the asymmetries in the Simon effect. We suggest an alternative data analysis that might render the results clearer. Finally, through a comparison of left- and right-handed subjects, we demonstrate that asymmetries in the Simon effect are linked to the lateralization of processes involved in the Simon task--that is, attention and response selection. This approach provides a strong argument against collapsing data from the two sides to measure the Simon effect.