This is an Accepted Manuscript of an article published by Taylor & Francis in The Quarterly Journal of Experimental Psychology.

This is an Accepted Manuscript of an article published by Taylor & Francis in The Quarterly Journal of Experimental Psychology. Treccani B., Cubelli R., Della Sala S., Umiltà, C. (2009). Flanker and Simon effects interact at the response selection stage. Quarterly Journal Of Experimental Psychology 62:1-21 Published article is available at http://dx.doi.org/10.1080/17470210802557751 Flanker and Simon effects Interact at the Response Selection Stage Barbara Treccani*, Roberto Cubelli, Sergio Della Sala, and Carlo Umiltà* *Dipartimento di Psicologia Generale, Università di Padova, Italy Dipartimento di Scienze della Cognizione e della Formazione, Università di Trento, Italy Human Cognitive Neuroscience - Psychology, University of Edinburgh, UK Keywords: S-S congruency, S-R spatial correspondence, Simon effect, Flanker effect, Response selection Correspondence should be addressed to: Barbara Treccani, Dipartimento di Psicologia Generale, Università di Padova, via Venezia 8, 35131 Padova (Italy) tel: +39-0498276941 fax: +39-0498276600 e-mail: barbara.treccani@unipd.it

Acknowledgements: We wish to thank Alessandra Girardi for her help in data collection. Barbara Treccani and Carlo Umiltà were supported by grants from MIUR and the University of Padova.

Abstract: The present study aimed at investigating the processing stage underlying stimulusstimulus (S-S) congruency effects by examining the relation of a particular type of congruency effect (i.e., the flanker effect) with a stimulus-response (S-R) spatial correspondence effect (i.e., the Simon effect). Experiment 1 used a unilateral flanker task in which the flanker also acted as a Simon-like accessory stimulus. Results showed a significant S-S congruency S-R correspondence interaction: An advantage for flanker-response spatially corresponding trials was observed in targetflanker congruent conditions, whereas, in incongruent conditions, there was a non-corresponding trials advantage. The analysis of the temporal trend of the correspondence effects ruled out a temporal-overlap account for the observed interaction. Moreover, results of Experiment 2, in which the flanker did not belong to the target set, demonstrated that this interaction cannot be attributed to perceptual grouping of the target-flanker pairs and referential-coding of the target with respect to the flanker in the congruent and incongruent conditions, respectively. Taken together, these findings are consistent with a response-selection account of congruency effects: Both the position and the task-related attribute of the flanker would activate the associated responses. In noncorresponding/congruent trials and corresponding/incongruent trials, this would cause a conflict at the response-selection stage.

Introduction Many psychological paradigms are effective in showing that the presence of shared properties among the elements of stimulus and response sets can affect response accuracy and speed. These phenomena are known as compatibility effects. Kornblum and his colleagues (Kornblum, Hasbroucq, and Osman, 1990; Kornblum, Stevens, & Requin, 1999) proposed a taxonomy of the possible stimulus-stimulus (S-S) and stimulus-response (S-R) compatibility effects that may be observed in experimental tasks. The core of this taxonomy is the idea of dimensional overlap (i.e., a perceptual, structural or conceptual similarity) between the task-relevant and irrelevant stimulus and response dimensions. Different mechanisms are proposed to explain the various compatibility phenomena according to the type of dimensional overlap occurring in the ensemble of stimuli and responses used in the tasks. While there is a general consensus about the mechanisms underlying the effects that derive from an S-R dimensional overlap (hereafter, S-R correspondence effects), the functional locus of effects resulting from an S-S dimensional overlap (S-S congruency effects) is still debated. According to some authors, the two types of effects originate from two different processing stages. In contrast, other theorists maintain that S-R correspondence and S-S congruency effects occur at the same processing stage. These hypotheses make different predictions about whether S-R correspondence and S-S congruency interact in tasks that involve both compatibility factors. The aim of this study was to shed light on this controversial question by using an ensemble that presented both S-S and S-R dimensional overlaps and analyzing the interaction between the processes underlying the two effects. In Korblum et al. s (1990, 1999) taxonomy, Type-1 ensembles are characterized by the absence of dimensional overlap in both the relevant and irrelevant dimensions. No compatibility effects are observed with this kind of ensemble. Type-2 and Type-3 ensembles are characterized by a dimensional overlap between a stimulus dimension and the relevant response dimension, the difference being that in the former the dimensional overlap occurs in a relevant stimulus dimension, whereas in the latter the overlap occurs in an irrelevant stimulus dimension. Probably the better

known examples of these two types of ensembles are the spatial compatibility proper task and the Simon task, respectively. In both tasks, the response dimension, along with one stimulus dimension, is spatial, and typically, the task requires pressing a left- or right-sided key to stimuli presented to the left or right of the fixation point. In tasks of spatial compatibility proper, the stimulus position signals the correct response, that is, participants have to press the key on the same side of the stimulus or on the opposite side, according to the mapping, compatible or incompatible, assigned to them. In contrast, in Simon tasks the stimulus position is irrelevant and responses are chosen based on some non-spatial stimulus attribute (e.g., the colour of the stimulus). Regardless of stimulus position being or not relevant, in both tasks responses are faster, and often more accurate, when the position of the stimulus corresponds to the position of the response key (e.g., right stimulus - right response) than when it does not (e.g., right stimulus - left response). In Korblum et al. s (1990, 1999) Dimensional Overlap model, the locus of S-R correspondence effects (in both Type-2 and Type-3 ensembles) is assumed to be the response selection stage. Indeed, most of the studies that investigated the processes underlying these effects have provided evidence in support of this assumption. Regarding spatial correspondence effects, there is converging behavioural and electrophysiological evidence that they result from the automatic coding of the stimulus position, which, in turn, automatically activates the response spatially corresponding to the stimulus, thus causing a competition at the response selection stage between the spatially corresponding response and the response required on the basis of task instructions (see Lu & Proctor, 1995; Stoffer & Umiltà, 1997, for reviews). Type-4 ensembles include those tasks in which there is a dimensional overlap only between the relevant stimulus dimension and an irrelevant stimulus dimension. For example, Stroop-like tasks (Stroop, 1935) in which participants have to respond with keypresses to the ink colour of stimuli consisting of names of colours <FOOTNOTE 1>. In this type of tasks, the response dimension (the key position) does not overlap with either the relevant or irrelevant stimulus dimension, which both refer to colours. In these tasks, responses are typically faster when the two

stimulus attributes are congruent (e.g., the word red printed in red) than when they are not (e.g., the word red printed in green). Consistent with the widespread belief that S-S congruency influences the stimulus processing stage (cf., e.g., Sternberg, 1969; Miller, 1988), Kornblum et al. (1990, 1999) attributed Type-4 compatibility effects to processes occurring at the level of stimulus identification: Congruent and incongruent irrelevant attributes would produce perceptual facilitation and perceptual interference respectively, speeding up or slowing down the identification of the relevant target feature. The dimensional overlaps occurring in the remaining categories of the taxonomy represent combinations of those of types 2-4. In accord with the perceptual interpretation of S-S congruency effects, the compatibility phenomena observed in tasks in which the dimensional overlap occur on both S-S and S-R dimensions (e.g., the colour Stroop task proper) are thought to involve both stimulus-identification and response-selection processes. However, the perceptual account of S-S congruency effects is controversial, and some authors have interpreted also these effects in terms of processes occurring at the response selection stage (e.g., Cohen & Shoup, 1997). Korblum et al. s (1990, 1999) hypothesis is based on the assumption that when the stimuli (e.g. coloured names of colours) are unrelated to the responses (e.g., keypresses), no response-activation and response-competition processes, able to produce compatibility effects, can occur. Nevertheless, it has been demonstrated that, not only long-term S- R associations pre-existing to the task (e.g., left stimulus left keypress), but also short-term associations created on the basis of the task instructions (e.g., red stimulus left keypress), can give rise to such processes (e.g., Tagliabue, Zorzi, Umiltà, & Bassignani, 2000). A well known example is the so-called Eriksen flanker task (Eriksen & Eriksen, 1974). In the standard flanker task, participants have to discriminate a target stimulus (e.g., the letter F or G) presented at the centre of the screen, by pressing one of two keys. The target is flanked by distractor stimuli (usually two or more). The flanker effect refers to the fact that participants are faster and more accurate when the target and flankers are congruent (i.e., the flanker letters are identical to the target; e.g., G G G ) than when the target and flankers are incongruent (i.e., the flanker letters are identical to the

alternative target; e.g., F G F ). Reaction times (RTs) in the neutral condition, in which the flankers are not associated with assigned responses (e.g., N G N ), are typically intermediate, indicating that the flanker effect entails both facilitation and interference (e.g., Cohen & Shoup, 1997). According to Eriksen and coworkers (Eriksen & Eriksen, 1974; Eriksen & Schultz, 1979), both target and flankers activate the responses associated with them on the basis of the task instruction, and the interaction of these response codes underlies the flanker effect. More precisely, the target and flankers would activate a common response code on congruent trials and competing response codes on incongruent trials. The typical facilitation of congruent flankers compared to neutral flankers would then be the result of an increased activation of the response code associated with the target, while the interference caused by incongruent flankers would occur because the appropriate response must be selected among competing codes. This view is supported by a remarkable amount of behavioural and psychophysiological evidence (see Cohen & Shoup, 1997; Sanders & Lamers, 2002, for reviews). For example, Miller (1987) showed that, when two different letters are mapped to each response (e.g., both the letters G and F are assigned to the left keypress, while the letters N and T are assigned to the right keypress), responses are faster when the target letter is flanked by letters assigned to the same response (e.g., F G F ) than when the target is flanked by letters to which a different response is assigned (e.g., N G N ). More recently, De Houwer (2003) obtained similar results in a flanker version of a colour Stroop task (i.e., a task in which participants were presented with three words names of colours - and had to press one of two keys according to the identity of the central word; Experiment 1). Even when the flanker and target words were different, De Houwer observed faster RTs when they were assigned to the same response than to alternative responses. The same findings were obtained in a more conventional Stroop-like task in which participants responded with keypresses to the ink colour of names of colours (Experiment 2). Despite these results that strongly suggest a response-selection origin of S-S congruency effects, contradictory evidence comes from studies involving an orthogonal variation of S-S

congruency and S-R spatial correspondence, that is, studies that combined flanker or Stroop-like tasks with Simon tasks. S-R ensembles in which both S-S congruency and the correspondence between the irrelevant stimulus and response dimensions are varied, in the absence of dimensional overlap between the relevant stimulus and response dimensions, are classified as Type-7 ensembles in Korblum et al. s (1990, 1999) taxonomy. These authors explicitly excluded an interaction between S-S congruency and S-R correspondence in this type of ensemble. Their Dimension Overlap model assumes that stimulus processing, which is thought to be affected by S-S congruency, and response selection, which is thought to be affected by S-R correspondence, are two distinct, sequential and independent stages. Most results from studies employing type-7 ensembles are consistent with the Dimensional Overlap model s independence assumption. Combining a Simon task with a Stroop-like task (i.e., participants had to respond to either the ink colour or the meaning of laterally-presented colour words), Simon, Paullin, Overmyer and Berbaum (1985), Simon and Berbaum (1990), Kornblum (1994), and Hommel (1997a, Experiment 1) all found additive effects of spatial S-R correspondence and S-S congruency. With a different procedure, Stoffels and van der Molen (1988, Experiment 1) also found additive effects. They employed a flanker paradigm in conjunction with an auditive accessory-stimulus version of the Simon task, in which an irrelevant sound was presented to the left or right ear during target and flankers presentation. Unlike the standard Simon task, in which both the relevant cue (e.g., stimulus colour) and the irrelevant one (stimulus location) are borne by the same object (they are two attributes of the target stimulus), the accessory-stimulus Simon task uses accessory lateral signals, which convey the response-irrelevant spatial information, co-occurring with response-relevant stimuli. With this type of tasks, responses are faster when the accessorystimulus location corresponds with the response location (e.g., Acosta & Simon, 1976; Hommel, 1995; Mewaldt, Connely, & Simon, 1980; Notebaert & Soetens, 2003). Stoffels and van der Molen failed to find an interaction between the flanker effect and the accessory Simon effect. According to Sternberg (1969) additive factors logic, this finding suggests that S-S congruency and S-R

correspondence affect two different serial stages: While S-R correspondence would affect response selection, S-S congruency would affect an earlier stage, likely, stimulus identification. Actually, some studies did find interactions between these two factors. Hommel (1997a, Experiment 2) observed an underadditive interaction between target-flanker congruency and spatial S-R correspondence: He found a decrease of the correspondence effect (i.e., the spatial correspondence between the response position and the position of a target letter, which was presented to the left or to the right) from the congruent condition (i.e., the target letter was flanked by letters identical to the target) to the incongruent condition (i.e., the target letter was flanked by letters identical to the alternative target). Kornblum et al. (1999, Experiment 2) presented irrelevant colour words that accompanied task-relevant coloured patches, which were presented to the left or to the right. They found an underadditive interaction between S-S congruency (i.e., congruency between the patch colour and the colour meant by the word) and spatial S-R correspondence (i.e., correspondence between the response and patch positions), but only when a certain delay was added between presentation of the two irrelevant attributes (i.e., word and patch positions) and that of the relevant attribute (patch colour), that is, when the word was initially presented together with a grey patch that was subsequently filled by the colour that participants had to judge. Korblum et al. (1999) claimed that these interactions do not imply that S-S congruency and S-R correspondence affect the same processing stage, nor they undermine the assumption of seriality of stimulus and response stages. Unlike the earlier versions of the Dimensional Overlap model (Kornblum, 1992, 1994; Kornblum et al., 1990), Korblum et al. s (1999) version of the model does not assume independence between these two stages. In their revised version, which includes the principle of temporal overlap proposed by Hommel (1993a), the size of the S-R correspondence effect is proportional to the amount of activation that the unit corresponding to the irrelevant spatial stimulus attribute has when the response selection stage begins. The activation of this irrelevant stimulus unit (i.e., the stimulus spatial code, in Hommel s terminology) would decay over time. Therefore, the longer the relevant stimulus is processed, the later in time the response

selection takes place, the smaller is the impact of the spatial stimulus code and the resulting spatial correspondence effect. The underadditive interaction between congruency and spatial correspondence would then be accounted for by the influence that congruency exerts on the length of the stimulus processing phase, on which the beginning of the response selection phase depends: The lengthening of the stimulus identification stage in incongruent trials would imply a delay of the response selection stage, thus causing a decrease of the spatial correspondence effect (see also Hommel 1997a, 1997b for a similar account). A Congruency Correspondence interaction was also found by De Jong et al. (1994, Experiment 3) who employed a spatial Stroop-like task in which participants had to discriminate (by pressing a left- or right-side key) the vertical position of a word ( high or low ), which appeared either above or below a mark composed of plus signs. The word was also presented to the left or right of fixation but its position on the horizontal plane, as well as its meaning, was not relevant to the task. A standard correspondence effect (i.e., faster RTs when the word horizontal position corresponded to the response position) was found in congruent conditions (e.g., the word high presented above the mark), while a small reverse correspondence effect (faster RTs in noncorresponding trials) was observed in incongruent conditions (e.g., the word high presented below the mark). Similarly to Hommel s (1997a, 1997b) and Kornblum et al. s (1999) findings, the interaction observed by De Jong et al. s (1994) is compatible with a perceptual explanation of the S- S congruency effect. These authors interpreted the reversal of the correspondence effect on the basis of the logical recoding hypothesis proposed by Hedge and Marsh (1975). In Hedge and Marsh s study, participants were required to press two coloured buttons (red and green, one on the left and the other one on the right) in response to the colour (red or green) of a left- or right-side visual stimulus. Participants were to press either the button of the same colour as the stimulus or the button of the alternative colour. A standard Simon effect was observed when the same colour rule was in effect, whereas a reverse Simon effect was found with the alternate colour rule. According to

Hedge and Marsh, stimulus position was subjected to a transformation or recoding of the same logical type (identity of reversal) as that required by task instructions for the relevant stimulus attribute, that is, stimulus colour. The resulting code (i.e., same position or alternate position) would then prime corresponding responses in the case of the same colour rule and non-corresponding responses in the case of the alternate colour rule. Following the same logic, De Jong et al. (1994) argued that a salient but irrelevant stimulus dimension (the word meaning in their Experiment 3) is subjected to an automatic and opposite recoding when it conflicts with the relevant dimension (the word vertical position in their case). For example, the word high presented below the positional mark would be recoded as not (or, it is the opposite of) HIGH (= LOW). The recoding would serve to bring the conflicting dimension into correspondence with the relevant stimulus dimension, allowing an unambiguous classification of the stimulus. Generalization of this recoding to the irrelevant stimulus spatial code would result in a reverse Simon effect for incongruent stimuli. De Jong et al. s (1994) account has not had a large following in the literature. Given the small size of the reverse effect observed (i.e., -4 ms), their pattern of results has been taken as indicative of the elimination (and not the reversal) of the effect (Hommel, 1997a, 1997b). De Jong et al. s findings would then represent another example of underadditive interaction, which, being RTs in the incongruent condition significantly slower than in the congruent one, was referable to different time courses of the response in the two congruency conditions (i.e., the temporal-overlap account, Hommel, 1997a). In sum, there is converging behavioural and psychophysiological evidence that S-S congruency effects arise at the response selection stage and perceptual factors either have a minor influence (De Houwer, 2003) or do not even contribute to them (Cohen & Shoup, 1997; Sanders and Lamers, 2002). However, results from studies that used Type-7 ensembles (i.e., Simon tasks in conjunction with either flanker or Stroop-like tasks) are either consistent or reconcilable with a

perceptual account of S-S congruency effects (e.g., Simon et al., 1985; De Jong et al., 1994; Hommel, 1997a, 1997b; Korblum et al., 1999). The present study intended to re-examine the issue of the processing stage underlying the S- S congruency effects by exploring the possible interaction between S-S congruency and S-R correspondence in a paradigm that presents features of both Simon and flanker tasks. In particular, we employed a visual accessory-stimulus Simon paradigm (e.g., Moore, Lleras, Grosjean, & Marrara, 2004; Proctor, Pick, Vu, & Anderson, 2005; Treccani, Umiltà, & Tagliabue, 2006) in conjunction with a unilateral flanker paradigm (Cohen, Ivry, Rafal, & Kohn, 1995). The flanker task was chosen because, among paradigms based on S-S congruency, flanker paradigms are those for which more evidence has been collected suggesting a response-selection origin of the congruency effect. The accessory-stimulus version of the Simon task was the optimal S-R correspondence task to be incorporated in the flanker paradigm. In visual accessory-stimulus Simon tasks, a peripheral visual irrelevant stimulus co-occurs with a central visual target. As in the auditory version of the Simon accessory task, a correspondence effect towards the accessory stimulus location is usually found (e.g., Proctor et al., 2005). In flanker tasks, the irrelevant stimulus attribute that causes the S-S congruency effect and the relevant stimulus attribute to which participants have to respond are conveyed by different objects (i.e., the flanker and target stimuli, respectively). We reasoned that if only one flanker stimulus was presented on the left or on the right of the target, the flanker can also act as Simon accessory stimulus. In the first experiment of the present study, participants had to judge the colour of a central target accompanied by a coloured peripheral accessory stimulus, which could be in the same colour as the target or in a different colour. Unlike previous studies that have used flanker-simon paradigms (Hommel, 1997a; Stoffels & van der Molen, 1988), in the present experiment the same object conveyed both the irrelevant spatial information (i.e., the accessory-stimulus position, which might or might not correspond to the response position and should produce the Simon effect) and the irrelevant feature overlapping with the target feature (i.e., the accessory-stimulus colour, which

might or might not be congruent with the target colour and should produce the flanker effect). Hence, no additional irrelevant cues (such as left- or right-side accessory sounds, Stoffels & van der Molen, 1988, Experiment 1), which might introduce confounding sources, were presented. In addition, this would foster the overlap in time of the coding processes of the two irrelevant stimulus features: It guaranteed that these processes started at the same time. The simultaneous activation of the two irrelevant codes is the prerequisite for the possible interaction between the two compatibility factors to occur. Indeed, different time courses of the coding processes of the S-S overlapping feature and of the spatial S-R overlapping feature might explain the lack of interaction between S-S congruency and S-R spatial correspondence in previous studies (see Keus & Schwarz, 2005, for an analogous hypothesis about the lack of interaction between two different S-R spatial correspondence factors). In prior studies, other factors might have hampered the overlap in time of the two irrelevant features codes. Previous flanker-simon studies used letter discrimination tasks. The letter-coding process is likely to be more time consuming than the coding of the spatial stimulus attribute, and the irrelevant spatial code might then have been available before the target and flanker letters were fully processed. The accessory-stimulus paradigm used here called for a simpler task (i.e., target colour discrimination), and this should make the time windows of two irrelevant coding processes more likely to overlap. In addition, the easy task used in the present study did not force participants to focus their attention too closely on the target (cf. Mack & Rock, 1998). There is indeed evidence that when attention is highly focused on a certain position because of task demands, the simultaneous onset of a peripheral stimulus may not capture attention (e.g., Yantis & Jonides, 1990), thus resulting in the absence of flanker-response correspondence effect (cf., Moore et al., 2004; Treccani et al., 2006). There is also evidence that the flanker effect may be asymmetric when alphanumerical material is used: With standard oriented letters the flanker effect is usually more pronounced for left- than for right-side flankers (e.g., Hommel, 2003). Given the spatial nature of

one of the effects under investigation, possible spatial biases were undesirable and to-be-avoided (see also Treccani, Cubelli, Della Sala & Umiltà, 2008). According to the perceptual account of S-S congruency effects (e.g., Kornblum et al., 1995), these effects occur at the stimulus processing stage, that is, to an earlier stage compared to the Simon effect, and no interaction of the congruency between target and flanker colours with the correspondence between flanker and response positions should be found. This is indeed the conclusion reached by previous studies that employed flanker or Stroop-like tasks in conjunction with Simon tasks (e.g., Stoffels and van der Molen, 1988, Experiment 1). However, as previously pointed out, the bulk of the studies with other paradigms suggest that the interference causing congruency effects arises during response selection (i.e., the same processing stage as the Simon effect) through activation of conflicting response codes (e.g., Sanders & Lamers, 2002). In accord with this account, independently of differences in response speed between the two congruency conditions, a significant interaction between target-flanker colour congruency and flanker-response spatial correspondence should be observed. Experiment 1 Experiment 1 was a unilateral flanker task in which the flanker also acted as a Simon-like accessory stimulus. Given the evidence suggesting that both the Simon and flanker effects occur at the response selection stage (Lu & Proctor, 1995; Sanders & Lamers, 2002; Cohen & Shoup, 1997), a significant interaction between S-S congruency (the congruency between target and flanker colours) and S-R correspondence (the spatial correspondence between flanker and response positions) was expected. While most studies employing Type-7 ensembles actually found these two factors to be additive, only few studies have investigated this issue employing flanker tasks and in one of them (Hommel, 1997a, Experiment 2) a significant interaction was indeed observed. This interaction was explained on the basis of the temporal-overlap principle (Hommel, 1993a), following which the spatial correspondence effect should decrease over time and therefore be larger

in congruent than in incongruent trials, the latter being slower. To control this possible alternative explanation, an analysis of RT distributions (a bin analysis; e.g., De Jong et al., 1994) was planned in the present experiment. This analysis would allow us to check the temporal trend of the correspondence effect. Method Participants Twenty-six participants (aged 17-36) were recruited from the University of Edinburgh undergraduate and postgraduate population. There were 11 females (all right-handed) and 15 males (12 right-handed and 3 left-handed). Participants had normal or corrected-to-normal visual acuity and were not aware of the purpose of the experiment. Apparatus Participants were seated in front of an RM Innovator 14 colour monitor screen, driven by an IBM compatible Pentium IV computer. Participants viewing distance from the screen was about 70 cm. The experiment took place in a dimly lit room and was run using E-Prime (version 1.1.4.1) software system (Schneider, Eschman, & Zuccolotto, 2002a, 2002b). Responses were executed by pressing one of two keys on the computer keyboard. These keys were on the left and on the right of the body midline (the d and l characters, respectively) and were operated with the corresponding index finger. Stimuli A central 0.8 0.8 white cross served as fixation mark. It was accompanied by an 800 Hz warning tone at 70 db. This tone emanated from two loudspeakers that were contiguous to the two sides of the monitor screen (one on the left and one on the right). Target and flanker stimuli were coloured squares of 1.9 1.9 in size (see Figure 1). The target was shown at the centre of the

screen and could be either green or red. The flanker was presented either on the left or on the right of the target (the centre of the flanker being vertically aligned with fixation and 5.7 to the left or right of fixation) and could be green, red, or blue. When it was red or green, it was either congruent or incongruent with the target colour (i.e., it was either the same colour as the target or the colour of the alternative target). Blue flankers served as neutral filler stimuli. Both the fixation cross and stimuli were presented on a black background. A 400 Hz tone served as error feedback. <Insert Figure 1 about here> Procedure Trials began with presentation of the fixation cross. The fixation remained visible for 1500 ms. The acoustic warning tone was delivered at the onset of fixation and lasted 300 ms. At the offset of fixation, the target stimulus appeared, together with the flanker. They remained on the screen until a response was made, but not longer than 1,500 ms. The offset of the target and the flanker was followed by a blank interval of 300 ms. Responses with the wrong key and with latencies in excess of 1,500 ms were counted as errors. In either case, auditory error feedback was given during the blank. Half of the participants were instructed to press the right key in response to the red target and the left key in response to the green target. The opposite mapping was assigned to the other participants. They were told to ignore the peripheral flanker and to respond to the target as quickly and as accurately as possible. There were 300 randomly mixed trials, equally distributed across the 8 types of experimental trials (2 target colours 2 flanker colours 2 flanker positions) and 4 types of neutral filler trials (2 target colours 2 flanker positions). The experimental session was preceded by a practice session composed of 12 trials.

Results Once RTs of erroneous responses were excluded, a cut-off point equal to 2.0 SD from each participant s RT mean was established and outlying data were replaced with that value (cf., Cubelli, Lotto, Paolieri, Girelli, & Job, 2005). Mean correct RTs and error percentages (see Table 1) of congruent and incongruent trials were submitted to analyses of variance (ANOVAs) with two within-subjects factors: congruency between target and flanker colours (congruent, incongruent) and spatial correspondence between flanker and response positions (corresponding, non-corresponding). The ANOVAs revealed only a significant source of variance, that is, the Congruency Correspondence interaction, both for latency, MSE=418, F(1,25)=5.46, p<.05, and accuracy, MSE=8.47, F(1,25)=8.39, p<.01. In the target-flanker congruent colour condition, trials in which the flanker position corresponded to the response position were faster and more accurate than non-corresponding trials (478 vs. 491 ms and 3.2 % vs. 4.2 %). In contrast, in the target-flanker incongruent condition, non-corresponding trials were faster and more accurate than the corresponding ones (489 vs. 494 ms and 3.3% vs. 5.6%). Post-hoc analyses, performed with Fisher s LSD procedure, showed that the corresponding trial advantage in the congruent condition was significant for RTs, while the advantage for noncorresponding trials in the incongruent condition was significant for accuracy (Fs= 5.39 and 8.17, respectively, both ps<.05). Responses were significantly faster and more accurate in congruent than in incongruent conditions, but only in corresponding trials (Fs=8.44 and 8.72, for RT and error comparisons, respectively, both ps<.01). All the other comparisons were not significant. <Insert Table 1 about here> Distributions of RTs before outliers replacement were computed for each participant and each level of both the target-flanker congruency and spatial correspondence factors. These

distributions were divided into five 20% bins (e.g. De Jong et al., 1994; Rubichi, Iani, Nicoletti, & Umiltà, 1997) and mean RTs were computed for each bin. RT data underwent an ANOVA with bin (1st to 5th), congruency, and spatial correspondence as within-participants factors. This analysis revealed, besides the main effect of bin and the Congruency Correspondence interaction, MSE = 2391, F(1,25) = 6.25, p<.05, two other significant sources of variance: the interaction between correspondence and bin, MSE = 410, F(4,100) = 6.34, p<.0005, and the three-way interaction of these variables with congruency, MSE = 586, F(4,100) = 3.56, p<.01. Post-hoc analyses (Fisher s LSD) showed that there were no significant differences between non-corresponding and corresponding RTs at the first four bins (the differences were 0, -1, 2, and 5 ms for bins 1-4, respectively), while corresponding trials yielded 23-ms faster RTs than non-corresponding ones in the fifth bin (F>30, p<.001). This overall effect was modulated by target-flanker congruency (see Figure 2). In congruent trials, a corresponding trial advantage was observed that increased across bins (4, 3, 9, 19 and 48 ms for bins 1-5, respectively) and reached significance at the forth and fifth bins (Fs= 8.00 and >30, respectively, both ps<.01). In incongruent trials, the differences between non-corresponding and corresponding trials (-4,-4,-5, -10, and -1 ms for bins 1-5, respectively) revealed a non-corresponding trial advantage that was never significant. <Insert Figure 2 about here> Discussion

Experiment 1 revealed a significant interaction between target-flanker colour congruency and flanker-response spatial correspondence, both in terms of RTs and accuracy. In the congruent condition, there was an advantage of spatially corresponding trials that reversed in a noncorresponding trials advantage in the incongruent condition. The corresponding vs. noncorresponding difference for incongruent flankers did not reach significance in the RT post-hoc analysis, but it was highly significant for accuracy. This correspondence-by-congruency modulation is hardly ascribable to a difference in response speed between the two congruency conditions, that is, to a delay in the response selection for incongruent trials compared to congruent ones, which would allow the irrelevant spatial code to either decay or be inhibited (i.e., the temporal-overlap hypothesis, Hommel 1993a, 1997a, 1997b). In fact, RTs in the incongruent condition were not significantly slower than RTs in the congruent condition. In addition, the distributional analyses showed that, overall, the corresponding trial advantage increased rather than decreased over time. Therefore, in contrast with the predictions of the temporal-overlap hypothesis, an overall regular correspondence effect was found that became larger (rather than smaller or reverse) as responses became slower. Indeed, congruency rather than speed of response seems to have been critical in modulating the correspondence effect. That is, the influence of congruency on the correspondence effect does not appear to have been mediated by response speed, as shown by the fact that the increase in the corresponding trial advantage in slowest bins was specific to the congruent condition, whereas no significant correspondence effects were observed in the incongruent condition, either in the fastest or the slowest bins. Although we were successful in ruling out a temporal-overlap account for these results <FOOTNOTE 2>, the observed interaction does not necessarily mean that the two effects are ascribable to the same processing stage. Another interpretation can be put forward, which would be compatible with a perceptual account of the S-S congruency effect. Indeed, our paradigm might have triggered mechanisms of perceptual grouping (e.g., Baylis & Driver, 1992; Harms & Bundesen, 1983) and referential coding (e.g., Hommel & Lippa, 1995) responsible for the effects

we observed. When the target and the flanker were of the same colour (i.e., in the congruent condition), they could be seen as forming a perceptual group (i.e., they could be seen as one object), shifted to one side of the display (i.e., towards the flanker position). For example, a red target accompanied by a red flanker on the right could be seen as one (red) object shifted to the right. If that were the case, a regular Simon effect towards the position of the perceptual group should be found: Responses should be faster/more accurate when their position corresponded to the perceptual group position, which happened to correspond to the flanker position. In contrast, when the target and the flanker were of different colours (i.e., in the incongruent condition), the location of the target might be coded in terms of its relative position with respect to the flanker, i.e., the flanker would serve as a reference object for the spatial coding of the target (cf., the referential coding hypothesis of the Simon effect, Hommel, 1993b). For example, a red target accompanied by a green flanker on the right might be coded as left, given that it is on the left side relative to the flanker. Therefore, a Simon effect away from the flanker position and towards the center (i.e., the target position) should be found: Responses on flanker-response non-corresponding trials should be faster/more accurate than responses on corresponding trials. A similar interpretation was advanced by Diedrichsen, Ivry, Cohen, and Danzinger (2000). In their study, a central target was accompanied by two lateral flankers presented on the left and on the right side. One flanker was always gray and the other one was either different or equal in color to the target. In this latter condition (i.e., the target-flanker congruent condition), a Simon effect towards the congruent flanker was observed. The authors concluded that when the target and a flanker were equal in colour, they were perceptually grouped and seen as the same object. No perceptual grouping could occur in the incongruent condition, given that both flankers were different in colour from the target. A straightforward way to test this hypothesis was to analyze data from Experiment 1 neutral condition. In it, as in the incongruent one, the flanker and the target were different in colour, the former being always blue and the latter either red or green. No perceptual grouping should occur in

such condition; the target should be coded with respect to the flanker position and a reverse correspondence effect should be observed. That is, according to the perceptual-grouping/referentialcoding hypothesis, the incongruent and neutral conditions would be exactly comparable. RT distributions of neutral trials were computed as before and entered into an ANOVA with bin and spatial correspondence as within-participants factors. In contrast with the predictions of the perceptual-grouping/referential coding hypothesis, this analysis did not show a reverse correspondence effect. In fact, a regular correspondence effect was found, even if only in the slowest portion of the RT distributions. The correspondence main effect was not significant, but it significantly interacted with the bin factor, MSE= 879, F(4,100) = 3.34, p<.05. As shown by Fisher s LSD post-hoc tests, at the fifth bin there was a significant advantage of corresponding trials over non-corresponding trials (34 ms; F=17.17, p<.0001), while the differences between the two conditions (-1, -1, 4, 5) were not significant in any of the first four bins (see Figure 2). The effect of spatial correspondence in neutral trials was also tested for accuracy: A comparison (paired samples t-test) between corresponding and non-corresponding error percentages was performed. No significant difference was observed. That the correspondence factor did not reach significance as main effect in the RT analysis actually contrasts with the results of previous studies that employed visual accessory-stimulus Simon tasks (Moore et al., 2004, Proctor et al., 2005, Experiment 3; Treccani et al., 2006, Experiment 1a). All three studies showed a global effect of correspondence between accessorystimulus and response positions, that is, an advantage of corresponding trials. However, in Moore et al. s study a correspondence effect was observed only after participants were told to ignore the central target, to report whether they saw the accessory stimulus and, if so, which side it was on. This might have affected the irrelevant nature of the accessory stimulus and be critical in inducing the correspondence effect observed on the following trials. In Treccani et al. s Experiment 1a the accessory stimulus was fully irrelevant to the task but did not appear in the same position on the left or right side. The accessory stimulus position was varied on the vertical axis in order to render more

likely the capture of attention by the accessory stimulus, that is, to prevent habituation to this stimulus and make more difficult for participants to ignore it. Such manipulations were not introduced in the present experiment which, in this respect, was more similar to Proctor et al. s Experiment 3, wherein participants had to discriminate the colour of a red or green central circle. The relevant coloured stimulus was accompanied by a white lateralized circle, which was taskirrelevant and vertically aligned with the target. Notably, Proctor et al. performed an RT distributional analysis and, as in the present experiment, a corresponding trials advantage was found that increased across bins: Significant accessory Simon effects were observed only in the two slowest bins for one of the two tested age groups (i.e., 18-23 year old adults). The other group (i.e., 55-81 year old adults) did not show a significant effect at any bin. Nevertheless, at variance with the present experiment, an overall 14-ms Simon effect was observed for the group of younger adults which proved to be significant. The main difference between Proctor et al. s visual accessorystimulus task and the neutral trials of our experiment is that, in the former, the irrelevant stimulus colour was neutral in terms of response assignment in all trials, whereas, in the latter, trials with a neutral flanker colour were intermixed with trials in which the flanker was of a task-relevant colour (i.e., congruent and incongruent trials). This might have induced participants to focus attention more closely on the target, in order to avoid flanker interference, which might have also reduced the correspondence effect produced by the flanker. This possibility was investigated in the following experiment, in which the flanker was always of a task-irrelevant colour. Experiment 2 This experiment was planned to further investigate the origin of the Congruency Correspondence interaction observed in Experiment 1. Experiment 2 basically corresponded to the neutral condition of Experiment 1. Here, however, neutral trials were presented in isolation (i.e., without being intermixed with congruent and incongruent trials). According to the results of other studies employing accessory-stimulus Simon tasks (e.g., Proctor et al., 2005; Treccani et al., 2006),

a correspondence effect towards the flanker position was expected. That would allow us to rule out the perceptual-grouping/referential coding account of the interaction found in the previous experiment: According to this account, indeed, a correspondence effect away from the flanker position should be found whenever the target and the flanker colours were different. Method Participants Ten postgraduates (aged 22-31) from the University of Edinburgh took part in Experiment 2. There were 4 females (all right-handed) and 6 males (4 right-handed and 2 left-handed). Participants had normal or corrected-to-normal vision and were naïve as to the purpose of the experiment. They had not taken part in Experiment 1. Apparatus, stimuli and procedure The apparatus, stimuli and procedure were as in Experiment 1. The only difference was that the flanker stimulus was always blue. There were 200 trials. In half of the trials the flanker position corresponded to the response position. In the other half, the flanker and response positions did not correspond. Results Outlying RTs were corrected as in the previous experiment. Two paired samples t-tests were performed on corresponding vs. non-corresponding RTs (451 vs. 467 ms) and percentages of errors (3.13% vs. 3.68%). While no difference emerged in terms of errors, the RT comparison was significant, t(9)= 2.47, p<.05. RTs were also submitted to the same analysis performed for the neutral trials of Experiment 1, that is, an ANOVA with bin and spatial correspondence as withinparticipants factors. Both the correspondence main effect and Correspondence Bin interaction were significant, MSE=1631, F(1,9) = 5.44, p<.05 and MSE=381, F(4,36) = 7.02, p<.0005. The

corresponding vs. non-corresponding RT difference (for bins 1-5, respectively) was significant for the forth and fifth bins (Fs=6.47 and >30, respectively, both ps <.05), whereas it did not reach significance in the first three bins. Discussion Consistent with the results of previous studies (Moore et al., 2004; Proctor et al., 2005; Treccani et al., 2006, Experiment 1a), a regular Simon effect was found in Experiment 2. We observed the same pattern of results as in Experiment 1: As already obtained by Proctor et al., we found that the flanker-response correspondence effect increased as RTs increased. Although numerous studies have obtained increasing Simon effect functions across RT bins with auditory stimuli, vertical arrangement of stimuli and responses, or when the hands are crossed for responding, decreasing functions are usually observed with visual S-R sets arrayed along the horizontal dimension and the hands in normal placement (cf., e.g., Proctor, Vu, & Nicoletti, 2003; Wascher, Schatz, Kuder, & Verleger, 2001). On the basis of this evidence, Wascher et al. (2001) proposed that, consistently with Hommel's (1993a) temporal-overlap hypothesis, decreasing functions reflect automatic response activation through S-R direct routes, which decays over time. These routes are assumed to be specific for the visual modality, with the automatic activation occurring only when responses are coded as anatomical segments (e.g., the left response is coded as the left-hand response). Therefore, lateralized stimuli automatically activate corresponding responses only when the stimuli are visual, and both stimuli and responses are aligned horizontally, with the left and right hands on the left and right sides, respectively, so that the left response is emitted by the left hand and the right response by the right hand. However, although the visual accessory Simon tasks employed in the present study and in that of Proctor et al. (2005) satisfied the requirements for the automatic activation to occur, increasing Simon effect functions were observed in both studies.

These findings allow the same interpretation that has been proposed elsewhere (e.g., Ansorge, 2003; Mapelli, Rusconi, Umiltà, 2003) to account for the functions of Simon-like effects obtained with stimuli the spatial attribute of which is not physical location (e.g., eyes with a leftward or rightward gaze orientation). Simon-like effects may increase with increasing RTs whenever the processing of the cue having spatial meaning requires additional processing stages other than the simple localization of the target in the display, such as focusing of attention on other locations in the display (e.g., the flanker location in the present study). This explanation is consistent with the interpretation of the visual accessory-stimulus Simon effect proposed by Treccani et al. (2006): When a central target is presented along with a peripheral accessory stimulus, attention shifts from the target toward the accessory stimulus (and that involves the stimulus spatial coding; cf., Nicoletti & Umiltà, 1994), but only after enough perceptual information is extracted from the target to select the correct response (see also, Yantis & Jones, 1991). Accordingly, response selection might have taken placed before the accessory-stimulus spatial coding more frequently when responses were fast compared to when responses were slower (see, however, Zhang & Kornblum, 1997, for an alternative explanation of the Simon effect functions, which is not based on the assumption that these functions reflect the time-course of the effect; see also Footnote 2). Independently of the time course of the difference between corresponding and noncorresponding RTs, and at variance with what we observed with neutral trials of Experiment 1, in Experiment 2 a significant overall effect of correspondence also emerged. When trials in which the flanker was neutral in terms of response assignment were presented in isolation, a 16-ms Simon effect was found that was comparable with that obtained in the congruent condition of the previous experiment. These findings ruled out a perceptual-grouping/referential-coding account of the Congruency Correspondence interaction observed in Experiment 1 for congruent and incongruent trials, which yielded a regular and reverse correspondence effect, respectively. A regular