The measurement of left—right asymmetries in the Simon effect: A fine-grained analysis

Abstract

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.

Exploring handedness asymmetries in the Simon effect

“…They concluded, BThere is evidence … of an attentional bias centered on the dominant hand^(p. 1067), and this bias is due to Ba larger cortical representation of the dominant hand^(p. 1068). Tagliabue et al (2007) reported similar experiments on the Simon effect asymmetry. They also found that for righthanded participants the Simon effect was larger for the right stimulus location than for the left one, whereas for left-handed participants the opposite relation held.…”

“…Although Tagliabue et al (2007) interpreted their results as agreeing with an attention shift account and against a referential coding account, they also noted that the existing theories to explain the Simon effect were lacking an explanation for the asymmetry. In a later study, Spironelli et al (2009) developed a connectionist model for righthanded participants to explain the asymmetry.…”

“…Therefore, in Experiment 2 we tested only right-handed participants to determine whether vision of the hands and response apparatus was necessary to obtain the asymmetry. In both experiments we also explored whether the asymmetry is truly an inseparable part of the Simon effect, as claimed by Rubichi and Nicoletti (2006), Tagliabue et al (2007), and Spironelli et al (2009) in postulating that a dominant-hand centered bias influences the attention shifts between fixation and stimulus. There is room for doubt in this regard as the asymmetry was mainly found for corresponding trials between stimulus locations in Rubichi and Nicoletti's (2006) Experiment 1, though not in their Experiment 2.…”

“…However, studies reported within the past 10 years have questioned this practice, noting that the Simon effect is often asymmetric across the left and right positions (Rubichi & Nicoletti, 2006;Spironelli, Tagliabue, & Umiltà, 2009;Tagliabue et al, 2007). Those authors have also argued that accounts of the Simon effect that ignore the asymmetry are incomplete and have proposed that the asymmetry reflects stronger coding of stimulus location in the visual hemifield corresponding to the dominant hand, as a consequence of an attentional bias.…”

“…We analyze the results with the factors of stimulus location and response location rather than stimulus location and correspondence, as Rubichi and Nicoletti (2006) did. The two analyses yield the same F ratios, just rearranged in terms of their assignment to effects (see Table 1), but Tagliabue et al (2007) noted that the stimulus and response locations version leads to a more straightforward interpretation of the data than does the stimulus location × correspondence version. The reason is that the latter version confounds the hand distinction with the Simon effect: For the stimulus on the side of the dominant hand, corresponding trials with the dominant hand are compared to non-corresponding trials with the nondominant hand, whereas for the stimulus on the opposite side, corresponding trials with the non-dominant hand are compared to non-corresponding trials with the dominant hand.…”