Our environment contains more information than can be assimilated at a single glance. For this reason, a selective mechanism is crucial for an appropriate interaction with our environment. This mechanism would be essential to isolate important information, improve its perceptual and/or motor processing, and sometimes suppress irrelevant or distracting information (Desimone and Duncan, 1995). Researchers have traditionally considered attention as a mechanism for selection, biasing information processing in the brain. Attention leads to a selective perception of a small subset of the vast amount of information that continually inundates our senses (see e.g., Ruz and Lupiáñez, 2002), and/or biases action plans to the attended information (Allport, 1989).

However, despite attentional processes enhancing the representation of features that are task relevant, and/or suppressing task irrelevant features, the “competition” between attended and suppressed items (see e.g., Treue, 2001) is not an all-or-none phenomenon. In fact, the processing of irrelevant features of target stimuli and/or distractors can take place despite their irrelevance for the task at hand, as demonstrated in the stimulus–response (S–R) compatibility effect, better known as the Simon effect (Simon and Rudell, 1967; for a review). It consists of slower reaction times (RTs) to stimuli presented contralaterally to the predefined response location (i.e., incongruent condition) as compared to ipsilateral stimuli (i.e., congruent condition; see e.g., Lu and Proctor, 1995, for a review; see also Simon and Rudell, 1967). For example, in a discrimination task requiring responses to a laterally presented colored target, participants’ responses are faster when the spatial position of the stimulus (i.e., left or right) is ipsilateral to the position of the manual response key (i.e., either left or right) than when the stimulus is contralaterally presented. This effect is observed despite the fact that the spatial location of the target is completely irrelevant for the task at hand. Thus, the Simon effect reflects an interaction between the response-related spatial representation activated by the task-relevant dimension of the target (i.e., color) and the response-related spatial representation activated by the task-irrelevant dimension (e.g., location) of the target. It has been proposed that the effect is caused by an incongruence between two spatial S–R codes: one that is transiently generated from the spatial location of the target (which is task-irrelevant) and another one that, due to task-demands, would be activated by the non-spatial identity of the target (see e.g., Hommel, 1993; de Jong et al., 1994; Kornblum et al., 1999; Ivanoff et al., 2002).

Although some studies have demonstrated attentional influences on the Simon effect, no agreement has been reached about the role of spatial attention for the generation of the spatial code. This disagreement comes from the two major hypotheses explaining the effect: the attentional-shift hypothesis (see e.g., Umiltà and Nicoletti, 1990; Nicoletti and Umiltà, 1994), proposing a central role of spatial attention on the Simon effect, and the theory of event coding (TEC), proposing that spatial attention is not crucial for the generation of the spatial code in the Simon task, although it could modulate the Simon effect (see e.g., Hommel et al., 2001; Hommel, 2011).

Although many studies have so far explored the relationship between the Simon effect and the different attentional subsystems (see e.g., Hommel, 1993; Stoffer and Yakin, 1994; Pratt et al., 1997; Lupiáñez and Milliken, 1999; Ivanoff et al., 2002; Ivanoff and Klein, 2004; Lupiáñez and Funes, 2005; Van der Lubbe and Van der Helden, 2006; Abrahamse and Van der Lubbe, 2008; Luo et al., 2011), no agreement has been reached about the specific role of neither exogenous nor endogenous spatial attention in the context of the Simon effect. Most studies have explored exogenous attention (see e.g., Pratt et al., 1997; Lupiáñez and Solano, 1998; Lupiáñez and Milliken, 1999; Ivanoff et al., 2002; Ivanoff and Klein, 2004; Lupiáñez and Funes, 2005; Verleger et al., 2005; Luo et al., 2011), generally reporting an additive pattern (see e.g., Hommel, 1993; although see Van der Lubbe and Van der Helden, 2006). In the case of endogenous attention, while most studies have reported non-significant interactions between both factors (see e.g., Verfaellie et al., 1988; Proctor et al., 1992; Zimba and Brito, 1995; Wascher and Wolber, 2004), we are aware of a few studies that have observed a reduced Simon effect at endogenously attended locations as compared to unattended locations (see e.g., Abrahamse and Van der Lubbe, 2008; see also Umiltá and Liotti, 1987; Stoffer and Yakin, 1994; Van der Lubbe and Woestenburg, 1999). Thus, while the effect of exogenous attention is usually additive with the Simon effect (although see e.g., Van der Lubbe and Van der Helden, 2006), this result cannot be easily generalized to endogenous attention (see Klein and Ivanoff, 2011, for a review).

Interestingly, in all previous studies we are aware of, the Simon effect has always been investigated in relation to target locations, which although irrelevant for the task at hand, contained the relevant target for response. The novelty of the present study is to investigate whether fully irrelevant non-target stimuli, considered fully irrelevant distractors because they never shared any feature or location with the target, could modulate endogenous responses. Thereby, the current work presents a two-fold aim investigation: (1) to explore the role of fully irrelevant distractors (i.e., task and target-irrelevant) and (2) to investigate the inconsistent role of endogenous spatial attention on the Simon effect. Results of the present research will demonstrate whether irrelevant distractors produce Simon effect interference, and whether this effect is attentional or motor. If the effect were attentional, then it should interact with endogenous attention (and modulate attentional event-related components, as discussed below).

We assessed the influence of attentional capture by irrelevant distractors and endogenous orienting on discrimination responses to a grating line orientation task (see e.g., Chica et al., 2014; for a review of attentional studies). A target appeared either above or below the fixation point, and either alone or accompanied by an irrelevant distractor. A symbolic precue (valid, invalid, or neutral) preceded the appearance of the target display. In Experiment 1, we manipulated distractor location (always presented on the horizontal axis, i.e., left or right) in order to investigate the Simon effect. We explored if the Simon effect was modulated by distractor location repetition by presenting distractors before target presentation and at the moment of target presentation. In Experiment 2, our purpose was to link behavioral indexes of interference (Simon effect) with their electrophysiological correlates, to gain a better understanding of the neural mechanism/s underlying this effect, and its interaction with endogenous attention. In particular, the lateralized readiness potential (LRP) component represents a well-known index of selective motor activation (Coles, 1989). This component has been considered an important marker for the Simon effect because it implies covert response activation, mainly generated within M1 (see e.g., Leuthold and Jentzsch, 2002). LRP indicates if the correct hand is covertly activated in relation to target responses (see also Leuthold, 2011; for a review), reflecting direct visuo-motor activation evoked by the stimulus. Moreover, other components such as the N2pc, are related to a benefit for correctly allocating attentional resources, facilitating further perceptual processing of stimuli (see e.g., Luck et al., 1994; Hopf et al., 2000; Vogel and Luck, 2000; Hickey et al., 2009). The N2pc reflects attentional selection of objects (see e.g., Hilimire et al., 2010, 2011, for different indexes of target and distractor processing), representing in the present study a direct measure of distractor processing. The component is always related to lateralized stimuli presented on the horizontal axis, where targets were never presented (see e.g., Eimer, 1996, for a review). In the present experiment, targets could not elicit the N2pc, because they always appeared in the vertical axis (see e.g., Hilimire et al., 2010, 2011).

These electrophysiological indexes, beside other attentional ones related to perceptual (P100, N100), or decisional processes (P300), could shed light on the mechanisms underlying the interactions between the Simon effect and endogenous spatial attention.

This experiment replicated the design of Experiment 1 but eliminating the invalid attentional condition (which yield no significant modulation of the interference effect) and the distractor absent condition (which yield similar results to ipsilaterally presented distractors). We recorded high-density electroencephalogram to better understand the neural basis of interference generated by distractors, and its modulation by distractor location repetition and endogenous attention.

The present study aimed at investigating whether fully task irrelevant distractors (appearing at fully irrelevant spatial locations) produce interference (measured as the Simon effect), and whether endogenous attention modulates this effect. High-density EEG was used to explore whether these effects could be accounted for by the modulation of attentional (N2pc) or motor processes (LRP).

Previous studies have reported contradictory results for the interaction between the classic Simon effect and endogenous attention. While most of them have reported non-significant interactions between the two factors (see e.g., Verfaellie et al., 1988; Proctor et al., 1992; Zimba and Brito, 1995), a small number of studies have reported a reduction of the Simon effect at endogenously attended locations (see e.g., Abrahamse and Van der Lubbe, 2008; see also Umiltá and Liotti, 1987; Stoffer and Yakin, 1994). For example, Abrahamse and Van der Lubbe (2008) reported a reduction of the Simon effect at endogenously attended as compared to unattended locations. However, as it has been noted elsewhere (see e.g., Klein and Ivanoff, 2011), these experiments conform a set of design properties that could account for such findings: constant cue-target SOA, relatively short target durations, or targets presented at a larger than usual eccentricity (±8.3^∘); wherein an explanation in terms of overt attention could not be completely ruled out. Additionally, the remaining studies that have shown a reduction in the Simon effect at the attended location have always used 100% valid cues and neutral cues (see e.g., Umiltá and Liotti, 1987; Stoffer and Yakin, 1994; Van der Lubbe and Woestenburg, 1999).

Our behavioral data are partially consistent with previous data showing no modulation of validity over the Simon effect, but also with previous data showing its reduction on valid trials: while we did not find a significant interaction between Validity and Laterality in any of the two experiments, thus conforming to most previous studies (see e.g., Verfaellie et al., 1988; Proctor et al., 1992; Zimba and Brito, 1995), we reported a reduction of the Simon effect at endogenously attended locations (see e.g., Abrahamse and Van der Lubbe, 2008; see also Umiltá and Liotti, 1987; Stoffer and Yakin, 1994, for similar results) when distractor location repeated as compared to non-repeated distractor locations. Thus, distractor location repetition seems to be a pivotal factor, at least in the present series of experiments, to disentangle the role of endogenous attention on the Simon effect. Our electrophysiological data are also contradictory with previous studies suggesting no N2pc modulations associated to the Simon effect (see e.g., Wascher and Wolber, 2004; Cespón et al., 2012, 2013), although coherent with some others demonstrating N2pc modulations (see e.g., Praamstra and Oostenveld, 2003; Hilimire et al., 2010, 2011).

Our results showed that N2pc was modulated by both, endogenous spatial attention and the Simon effect. This suggests a common attentional mechanism of spatial selection. However, as reviewed above, not all studies have consistently demonstrated an attentional modulation over the Simon effect, depending on the specific design used to manipulate attention and interference. We also observed no modulations in any of the components traditionally related to perceptual/attentional processing such as the P100 component and the N100 component (see e.g., Luck, 1995; Hopf et al., 2000). Taken all data together we can conclude that the existing evidence supports the theory of event coding (TEC) (see e.g., Hommel et al., 2001; Hommel, 2011), in which spatial attention is believed not to be crucial for the Simon effect.

Beyond procedure particularities and/or differences in results, the most important difference between all previous studies and the present one is that the above-mentioned studies have always investigated the Simon effect in relation to target features (i.e., target location, although task-irrelevant) while the present study was carried out in relation to completely task-irrelevant and target-irrelevant stimuli, i.e., distractors that never shared location or any other feature with the target. Therefore, comparisons between studies would not be entirely proper and should be done with caution. In the present experiments, the effect produced by distractors was exclusively due to sharing position with the target response location but never with the target location itself (which appeared in a different axis). Thus, while the previous studies produced interference by manipulating the irrelevant target location, we produced interference by manipulating the location of an irrelevant distractor that was presented together with the target; a novel approach to investigate both the Simon effect and its modulation by endogenous attention. Despite being fully irrelevant, distractors were spatially coded (as reflected by the N2pc component), and interfered with the processing of the target response (as reflected by the LRP component). Attentional capture by the distractor, as reflected in the N2pc component, and therefore its interference with target responses, was modulated by distractor location repetition as a function of endogenous attention.

These results lead to the conclusion, that, at least when the task requires selection of information, the Simon effect could be modulated by both endogenous attention and distractor location repetition. On the one hand, in relation to the distractor spatial processing itself, we observed that ipsilateral distractors led to higher distractor processing in terms of spatial selection. This finding fits with the contingent exogenous orienting hypothesis proposed by Folk et al. (1991, 1992). Ipsilateral distractors lead to a higher attentional capture because they share an attribute with the target, i.e., the response hand (see Folk et al., 1992, for a review). Interestingly, the amplitude of the N2pc component was enhanced for ipsilateral as compared to contralateral responses when attention was focused at the validly cued conditions, while no differences in laterality were observed in the neutrally cued conditions, when attention was not focused. Thus, when attention was endogenously focused (i.e., at valid trials) the Simon effect was observed and reflected as a larger attentional selection for ipsilateral than for contralateral distractor locations. Contrarily, when attention was not focused, as it is the case of neutral trials, the Simon effect was not significantly reflected in the N2pc, suggesting that the system could be equally ready for automatically responding to any position, no matter whether it was ipsilateral or contralateral to the target response hand.

Endogenous attention was also reflected in modulations of the LRP component. The larger LRP for valid than neutral conditions reveals a better motor selection for endogenously valid conditions. The LRP component also reflected the expected Simon effect, with larger LRP for ipsilateral than contralateral distractor positions, leading to a better motor selection when distractor location and response hand matched as compared to incongruent conditions, i.e., contralateral positions. Moreover, one of the most interesting findings observed in the present research was the interaction between Validity, Distractor location repetition, and Laterality; this interaction mirrored the behavioral effect reported in both experiments and demonstrated that interference produced by irrelevant distractors depended on endogenous attention. When attention was focused (at validly cued trials), the distractor affected motor stages of processing (LRP) but especially at non-repeated locations. This finding indicates that fully irrelevant distractors can produce both attentional and motor effects: distractors capture attention (as reflected in early stages of the N2pc component) and generate motor interference (as reflected in later stages of the LRP component) at non-repeated locations. When attention was not focused (at neutral cued trials), distractors did not produce effects at attentional stages of processing, but they tended to produce motor effects when distractor location repeated (although results did not reach statistical significance).

Although speculative, we propose that these novel results could be understood in terms of attentional resources (Lavie, 1995; see also Ruz and Lupiáñez, 2002, for a review) or priority allocation. As mentioned above, at valid conditions, where the system knows a priori the target position, it would have more free resources for processing new information (i.e., irrelevant distractors) at early stages, as indexed by the N2pc component. At late stages, indexed by the LRP component, the newest stimuli (non-repeated distractor locations) would lead to the largest interference. In neutral conditions, however, because the target position is unknown in advance, the system could be ready for processing any location, specially the two potential target locations. At the moment of target appearance, attentional resources could be dedicated to target processing, being prioritized and winning the competition with the distractor location. Thereby, at earlier stages, at which the N2pc is observed, there might not be free attentional resources for processing the distractor, thus explaining the absence of distractor laterality effects in the N2pc component on neutral trials. At later stages, where the LRP component is observed, repeated location distractors produce a larger interference, perhaps because the first distractor has not been properly filtered-out, and its effects are enhanced when distractor location repeats.

In summary, our results demonstrate that fully irrelevant distractors that do not share any feature with the target and/or task, capture our attention and modulate our responses, as reflected in different components related to distractor and target processing such as the N2pc component and the LRP component, respectively. When we endogenously attend to a spatial target location (preselecting it before the target appears), there is a larger attentional selection of the distractor (reflected in the N2pc); likewise, larger motor interference (reflected by the LRP) is also observed for the newest information (i.e., distractors presented at non-repeated locations). In contrast, when attention is unfocused (as it happens in neutral trials), the attentional resources would be more focalized to the target location at early processing stages, avoiding distractor processing. At later processing stages, motor interference is larger when distractor location repeats.

The present paper is, to the best of our knowledge, the first one showing how distractors that never share any feature with the target modulate the Simon effect at endogenously attended and non-attended locations. We consider this work as a first step to achieve a better understanding about the interactions between the attentional and motor systems in relation to surprising or irrelevant stimuli.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.