Multiple spatial codes and temporal overlap in choice-reaction tasks

Experiments by Umiltà and Liotti (1987) and Lamberts, Tavernier, & d'Ydewalle (1992) examined the Simon effect (an influence of irrelevant stimulus location on reaction time) as a function of multiple frames of reference. The Simon effect was absent for all reference frames in the former experiment, leading Stoffer (1991) to propose that a spatial code is formed only if the last step in directing attention to the imperative stimulus is a lateral shift. However, the Simon effect was evident for all frames in the latter experiment. Hommel (1994) proposed that the multiple spatial codes implied by Lamberts et al.'s findings were also activated in Umiltà and Liotti's experiment but had decayed by the time the relevant stimulus information had been identified. Experiments 1, 2, and 3 examined these accounts of attention shifting, multiple codes, and temporal overlap for variations of the Simon task in which the stimulus could occur in one of either eight or four possible stimulus locations. Three stimulus sets that differed in ease of discriminability were used in each experiment. Experiments 1 and 2 were replications and extensions of those of Lamberts et al. and Umiltá and Liotti, respectively. In both experiments, two boxes, with a stimulus inside of one, appeared simultaneously, and the subject was to respond to the identity of the stimulus. Experiment 3 used a procedure in which the four stimulus locations were demarcated by three vertical lines. Two of the three experiments showed Simon effects with respect to multiple frames of reference, and the magnitude of these effects was a decreasing function of the difficulty of stimulus discriminability. Spatial compatibility proper was examined in Experiment 4 using the same layout as Experiment 3. In this case, only the relevant frame of reference was coded. On the whole, the results indicate that multiple codes are formed, but not automatically, and that those codes decay when irrelevant.