The Simon effect occurs relative to the direction of an attention shift

UniversitA di Padova We investigated whether the Simon effect depends on the orienting of attention. In Experiment 1, participants were required to execute left-right discriminative responses to 2 patterns that were presented to the left or right of fixation. The 2 patterns were similar, and the discrimination was difficult. A letter at fixation signaled whether the current trial was a catch trial. The results showed a reversal of the Simon effect. That is, spatially noncorresponding responses were faster than spatially corresponding responses. In Experiment 2, the discrimina- tion of the relevant stimulus attribute was easy. In Experiment 3, the discrimination of the relevant stimulus attribute was difficult, but the stimulus exposure time was long. In either experiment, the regular Simon effect was reinstated. In Experiment 4, the letter that signaled a catch trial appeared to the left or right of the imperative stimulus. The Simon effect occurred relative to the position of the letter. Stimulus location is coded and cannot be ignored in many situations for which it is irrelevant to the task to be performed (e.g., Tsal & Lavie, 1993, and the references therein). The effects of irrelevant location codes emerge clearly in the Simon and spatial Stroop tasks (for reviews, see Lu & Proctor, 1995; MacLeod, 1991; Umilt~t & Nieo- letti, 1990). In a Simon task, the stimulus features relevant for selecting the correct response are nonspatial (e.g., two colors or two shapes). These nonspatial features are assigned to spatially defined responses (typically keypresses; e.g., left and right or above and below), and the location (e.g., left and right or above and below) in which the stimulus is shown is task irrelevant. The Simon effect refers to the fact that responses are faster when the stimulus location corresponds to the location of the assigned response than when it does not correspond. For instance, participants are presented with two different patterns and are required to press the right-side key in response to one pattern and the left-side key in response to the other. Responses are faster when the location of the stimulus and the location of the response key are located on Sandro Rubichi and Cristina Iani, Dipartimento di Scienze Biomediche, Universi~ di Modena, Modena, Italy; Roberto Nico- letti, Institnto di Psicologia, Universit,,3 di Urbino, Urbino, Italy; Carlo Umilt~, Dipartimento di Psicologia Generale, Universith di Padova, Padua, Italy. This study was supported by the Consiglio Nazionale delle Rieerche (Grants 93.00752.PF41, 94.00667.PF41, and 95.00923.PF41) and ~ Ministero della Universi~ e della Ricerca Scientifica e Teenologiea. We thank Sandro ~lla for valuable help in planning the experiments. Correspondence concerning this article should be addressed to Carlo UmiltL Dipartimento di Psicologia Genemle, Universit~ di Padova, via Venezia, 8, 35131 Padua, Italy. Electronic mail may be sent via Internet to umilta@psico.unipd.it. the same side (e.g., right-right or left-left; i.e., correspond- ing stimulus--response [S-R] pairings) with respect to the condition in which they are positioned differently (e.g., stimulus on the right side and response left or vice versa; i.e., noncorresponding S-R pairings). Thus, in the Simon task, the irrelevant information concerning stimulus location is processed even though participants are clearly instructed to select the response exclusively on the basis of the relevant, nonspatial stimulus dimension. Most accounts of the Simon effect share two basic assumptions (for reviews and discussions of the various accounts, see Lu & Proctor, 1995; Umilt~ & Nicoletti, 1990; for dissenting views, see Hasbroucq & Guiard, 1991; Simon, 1990; Verfaellie, Bowers, & Heilman, 1988). One assump- tion is that a spatial code is generated for the irrelevant stimulus location attribute. In fact, multiple locational codes, all of which perhaps contribute to the Simon effect, are likely produced for a single stimulus (De Jong, Liang, & Lanber, 1994; Lamberts, Tavernier, & d'Ydewalle, 1992). In most situations, spatial coding is a function of the location of the target stimulus relative to the location of the alternative stimulus (e.g., Umilt/t & Liotti, 1987; Umilt~t & Nicoletti, 1985). Spatial coding is also thought to occur in terms of egocentric hemispace (e.g., Umilt~ & Liotti, 1987), relative to the position of the attentional focus (Nicoletti & Umilt/t, 1989), or as a function of configural cues in the display (Hommel, 1995). The other widely shared assumption is that the Simon effect occurs at the response-selection stage. The idea is that, provided there is enough similarity between the (irrelevant) spatial stimulus dimension and the (relevant) spatial re- sponse dimension (i.e., the two dimensions overlap; see Kornblum, 1994; Kornblum, Hasbroucq, & Osman, 1990; Kornblum & Lee, 1995), a stimulus automatically activates its spatially corresponding response code (also see Eimer, 1995; De Jong et al., 1994). For trials in which the 1353