The components of visual attention and the ubiquitous Simon effect

Spatial responding is influenced by the degree of correspondence between the stimulus-response (S-R) code activated by the target's task-irrelevant location and the S-R code activated by the target's non-spatial, task-relevant feature. A generally accepted explanation of this "Simon effect," named after its discoverer, is that there is a natural tendency to respond towards the source of stimulation. First we will review the ubiquity of the Simon effect. Then we will review the literature, including our own studies when appropriate, that has explored the relationship between the Simon effect and the components of attention: alertness, orienting and executive control, with an emphasis on visual orienting. The Simon effect is reduced when participants are not alert and when executive control is effective in filtering out the irrelevant location information. When attention is oriented endogenously, or is captured exogenously by uninformative peripheral stimulation, the Simon effect is additive with attentional facilitation (i.e., the Simon effect is the same magnitude for targets presented at attended and unattended locations). Yet, some forms of orienting, such as orienting directed by gaze and biased by inhibition of return, modulate the Simon effect. We will explore the implications of these patterns of additivity and interaction for our understanding of both the Simon effect and spatial attention.