Attention and spatial stimulus coding in the Simon task: a rejoinder to van der Lubbe and Abrahamse (2010)

Hommel (2011-this issue) has reviewed the major lines of research and ongoing controversies on and around the Simon effect. Van der Lubbe and Abrahamse (2010) take issue with Hommel's assessment of the role of attention shifting in the Simon effect. This rejoinder argues that van der Lubbe and Abrahamse's criticism is off target because it (a) fails to distinguish between the attention-shifting account of (spatial stimulus coding in) the Simon effect—which Hommel discusses and criticizes—and the premotor theory of attention—which Hommel does not discuss; (b) confuses the relationship between the attention-shifting account and the referential-coding account of spatial stimulus coding in the Simon effect—the actual topic of Hommel's discussion—with the relationship between the premotor theory and the theory of event coding—which the criticism focuses on; and (c) confuses the uncontroversial role of attention in stimulus selection with the controversial role of attention in the generation of location codes.     

Endogenous orienting modulates the Simon effect: critical factors in experimental design

Responses are faster when the side of stimulus and response correspond than when they do not correspond, even if stimulus location is irrelevant to the task at hand: the correspondence, spatial compatibility effect, or Simon effect. Generally, it is assumed that an automatically generated spatial code is responsible for this effect, but the precise mechanism underlying the formation of this code is still under dispute. Two major alternatives have been proposed: the referential-coding account, which can be subdivided into a static version and an attention-centered version, and the attention-shift account. These accounts hold clear-cut predictions for attentional cuing experiments. The former would assume a Simon effect irrespective of attentional cuing in its static version, whereas the attention-centered version of the referential-coding account and the attention-shift account would predict a decreased Simon effect on validly as opposed to invalid cued trials. However, results from previous studies are equivocal to the effects of attentional cuing on the Simon effect. We argue here that attentional cueing reliably modulates the Simon effect if some crucial experimental conditions, mostly relevant for optimizing attentional allocation, are met. Furthermore, we propose that the Simon effect may be better understood within the perspective of supra-modal spatial attention, thereby providing an explanation for observed discrepancies in the literature.     

Mechanisms underlying spatial coding in a multiple-item Simon task

Choice responses are faster when target position and response side correspond than when they do not, even if target position is response-irrelevant. This "Simon effect" has also been observed in case of multi-item arrays. Generally, it is assumed that an automatically generated spatial response code is responsible for the effect. The referential-coding account assumes that this code is directly related to the target, although the moment of production of the code is not fully clear. The attention-shift account assumes that the code is directly related to the direction of the most recent attentional shift. An experiment was performed in which left or right target locations were indicated by arrows occurring before (precue), simultaneous with (simcue), or after (postcue) six-element arrays. Overt responses and EEG potentials were recorded. The Simon effect was present in all conditions, and decreased when responses were slower. No relation was found between amplitude of posterior lateralized components and the magnitude of the Simon effect. A posterior contralateral negativity was also found after presenting the arrays in the precue condition, which might reflect the reorienting of attention toward the target position. The results are more favorable to the referential-coding account although this account becomes very similar to the attention-shift hypothesis as the moment of formation of the spatial response code is related to effective target onset rather than to stimulus onset.     

Attention shifts and the Simon effect: a failure to replicate Stoffer (1991)

The Simon effect refers to the intrusion of an irrelevant spatial dimension in the selection of a response to a stimulus. Recently, Stoffer (1991) proposed a specific role for attention orienting in the Simon effect. According to Stoffer, attentional shifts from an initial fixation point to a stimulus location are associated with the generation of a spatial code which specifies the position of the stimulus in relation to the last attended location. This spatial code forms the basis for the Simon effect because it is used to specify the selection of the right or left response. Three experiments were performed to examine further the attention-shift hypothesis forwarded by Stoffer. Experiment 1 was an attempt to replicate Stoffer (1991), but failed to do so. Experiments 2 and 3 were designed to optimize conditions for replicating crucial aspects of Stoffer's findings, but also failed to yield support for the attention-shift account.     

A shift of attention may be necessary, but it is not sufficient, for the generation of the Simon effect

The Simon effect is the performance advantage for spatially corresponding, compared to non-corresponding, target-response ensembles when the location of the target is task irrelevant. In four experiments, we tested the predictions of the attention-shift account of the Simon effect. In all experiments, subjects made choice responses with respect to the identity of a central target that followed a spatially non-informative peripheral precue. The first experiment showed a Simon effect away from the precue when the precue was a go/no-go signal: responses to spatially non-corresponding precue-response pairs were faster than responses to spatially corresponding precue-response pairs. The results of the second experiment suggested that this “reverse” Simon effect was not due to inhibition. In the third experiment, a secondary working memory task required the encoding, and later recall, of “oddball” precues. Although the Simon effect was absent, larger Simon effects towards the precue (i.e., responses were faster to spatially corresponding, compared to non-corresponding, precue-response ensembles) were correlated with poorer performance on the memory task. In the last experiment, the identity of completely non-informative precues was congruent, incongruent, or unrelated to the identity of the target. With precues that were unrelated to the identity of the target, there was a Simon effect towards the precues. Conversely, the Simon effect occurred away from the precue when the identity of the precue was related to that of the target. The findings suggest that a shift of attention alone is not sufficient to produce the Simon effect. Rather, the shift of attention must originate from an intentionally defined object. The results are discussed within a framework that integrates the attention-shift and referential-coding hypotheses. Received: 2 December 1999 / Accepted: 23 March 2000     

The role of attention for the Simon effect

Summary It has been claimed that spatial attention plays a decisive role in the effect of irrelevant spatial stimulus-response correspondence (i. e., the Simon effect), especially the way the attentional focus is moved onto the stimulus (lateral shifting rather than zooming). This attentional-movement hypothesis is contrasted with a referential-coding hypothesis, according to which spatial stimulus coding depends on the availability of frames or objects of reference rather than on certain attentional movements. In six experiments, reference objects were made available to aid spatial coding, which either appeared simultaneously with the stimulus (Experiments 1–3), or were continuously visible (Experiments 4–6). In contrast to previous experiments and to the attentional predictions, the Simon effect occurred even though the stimuli were precued by large frames surrounding both possible stimulus positions (Experiment 1), even when the reference object's salience was markedly reduced (Experiment 2), or when the precueing frames were made more informative (Experiment 3). Furthermore, it was found that the Simon effect is not reduced by spatial correspondence between an uninformative spatial precue and the stimulus (Experiment 4), and it does not depend on the location of spatial precues appearing to the left or right of both possible stimulus locations (Experiment 5). This was true even when the precue was made task-relevant in order to ensure attentional focusing (Experiment 6). In sum, it is shown that the Simon effect does not depend on the kind of attentional operation presumably performed to focus onto the stimulus. It is argued that the available data are consistent with a coding approach to the Simon effect which, however, needs to be developed to be more precise as to the conditions for spatial stimulus coding.     

Attention precuing and Simon effects: A test of the attention-coding account of the Simon effect

Simon effects refer to the finding that choice-response latencies to a nonspatial aspect of a stimulus vary depending on the spatial correspondence between the stimulus position and the position of the correct response alternative. Recently, researchers have proposed an attention-coding account of Simon effects whereby the (irrelevant) stimulus spatial code involved in the generation of the effect is formed in the process of attentional orienting to the stimulus. This account predicts that if attentional orienting is unnecessary at stimulus onset, as when the stimulus appears at an attended location, Simon effects will not be observed. This prediction was tested by measurement of Simon effects in an attention-precuing task in which the stimulus was presented at attended and unattended locations. Significant Simon effects were observed independently of the focus of attention. This result was obtained over a large range of precue-target SOAs, and did not depend on whether central or peripheral precues were used to direct attention or on whether the relevant target dimension was color or form. Significant Simon effects were not observed when the precue-target SOA was 50 ms, irrespective of the other precue and task conditions. The data do not support the prediction of the attention-coding account and thus question the generality of the account in its current form. It is suggested that spatial and temporal uncertainties are important factors that influence the pattern of results, and that these factors must be incorporated into attention-coding models of the Simon effect.     

Inverting the Simon effect by intention

Summary The Simon effect indicates that choice reactions can be performed more quickly if the response corresponds spatially to the stimulus - even when stimulus location is irrelevant to the task. Two experiments tested an intentional approach to the Simon effect that assigns a critical role to the cognitively represented action goal (i. e., the intended action effect). It was assumed that the direction of the Simon effect depends on stimulus-goal correspondence, that is, that responses are faster with spatial correspondence of stimulus and intended action effect. Experiment 1 confirmed that the direction of the Simon effect was determined by spatial correspondence of stimulus and intended action effect, the latter having been manipulated by different instructions. Experiment 2 indicated that effects of correspondences unrelated to the action goal (i. e., stimulus to hand location or to anatomical mapping of the hand), contributed additively to the resulting Simon effect. It is discussed how current approaches to the Simon effect can be elaborated to account for these results.     

基于物体的注意对Simon效应的调节

本研究采用双长方形框范式的变式将注意分离为基于空间和基于物体的注意后,探讨了注意对Simon效应的调节是否由基于物体的注意引起。结果发现,在长方形框水平排列时在基于物体的注意条件下Simon效应增大,而在长方形框垂直排列时在基于物体的注意条件下Simon效应没有显著变化。这表明,在长方形框水平排列时基于物体的注意可以对Simon 效应进行调节。实验结果部分支持了动态版的参照编码假设和物体文件整合假设。     

The premotor theory of attention as an account for the Simon effect

The Simon effect refers to the phenomenon that responses are faster when the irrelevant location of a stimulus corresponds with the response location than when these locations do not correspond. In the current paper we examined the viability of an updated version of the premotor theory of attention (PMTA) as an account for the Simon effect. Two predictions were evaluated. First, in the case of focused attention at the relevant target position a strong reduction of the Simon effect should be observed as the Simon effect according to PMTA crucially depends on attentional orienting. Secondly, if attention is directed towards a location then this orienting by itself should already be sufficient for producing a Simon effect, as stimulus presence is not required. Our data confirmed these predictions thereby supporting the relevance of the PMTA for the Simon effect.     

Stimulus congruence and the Simon effect

Summary A standard experimental procedure was implemented with novel response requirements to assess the hypothesis that the Simon effect is attributable not to the irrelevant stimulus-response relationship, but to the congruence between stimulus attributes. The stimulus ensemble consisted of the words LEFT and RIGHT, one of which was presented on each trial to the left or right of a central fixation point. The distinctive feature of the task is that subjects were asked to respond, by laterally placed keys, whether or not the stimulus word was in accord (i. e., congruent) with its location on the display. Asking subjects to judge stimulus congruence directly enables the effect of congruence to be assessed, as well as independent estimates of the two irrelevant S-R relationships that apply in the task — that is, between the response location and (1) the stimulus location (the Simon effect) and (2) the stimulus word (the reverse Simon effect). Marked effects were obtained in all three cases. Stimulus congruence remains in contention as a factor in the explanation of the Simon effect, but the strong effects of irrelevant S-R factors suggest that a more comprehensive account of the Simon effect is needed. An explanation in terms of stimulus salience, based on an interactive activational model, is briefly discussed.     

Attentional focussing and spatial stimulus-response compatibility

Summary The relative functional significance of attention shifts and attentional zooming for the coding of stimulus position in spatial compatibility tasks is demonstrated by proposing and testing experimentally a tentative explanation of the absence of a Simon effect in Experiment 3 of Umiltà and Liotti (1987). It is assumed that the neutral point of the spatial frame of reference for coding spatial position is at the position where attention is focussed immediately before exposition of the stimulus pattern. If a stimulus pattern is exposed to the right or the left of this position a spatial compatibility effect can be observed when the stimulus-response pairing is incompatible. Generalizing from this, one can say that a spatial compatibility effect will be observed if the last step in attentional focussing of the stimulus attribute specifying the response is a horizontal or a vertical attention shift. If the last step in focussing is attentional zooming (change in the representational level attended to), the stimulus pattern is localized at the horizontal and the vertical positions where the last attention shift had positioned the focus. In this case the spatial code is neutral on these dimensions and so no spatial compatibility effect should result. To test this model we conducted two experiments. Experiment 1 replicated the finding of Umiltà and Liotti that there is no Simon effect in the condition with no delay between a positional cue (two small boxes on the left or right of a fixation cross) and the imperative stimulus, whereas in the condition with a delay of 500 ms a Simon effect was observed. In a comparison condition with a single, rather large cue instead of two small boxes (forcing attention to zoom in), no Simon effect was observed under either delay condition. Experiment 2 used a spatial compatibility task proper with the same experimental conditions as Experiment 1. But in contrast to those of Experiment 1, the results show strong compatibility effects in all cue and delay conditions. The absence of a Simon effect in some experimental conditions in Experiment 1 and the presence of a spatial compatibility effect proper in all conditions in Experiment 2 are consistently accounted for with the proposed attentional explanation of spatial coding and spatial compatibility effects.     

The Simon effect and visual motion

Summary S-R compatibility and Simon effects were studied for real visual motion. In Experiment 1, two small stimulus lights were constantly visible, 5° to the left and right of fixation; after a random delay, one began to move at 2°/s. In Experiment 2, a single stimulus light moving at 2°/s suddenly appeared 5° to the left or right of fixation, i. e., motion onset and stimulus onset coincided. In both experiments, subjects responded by a key press with their left or right index finger as soon as they detected motion. In Condition A responses were made to the position (left or right) from which the motion started, irrespective of its direction (position compatibility); in Condition B responses were made to the direction of motion (leftward or rightward) irrespective of whether motion started to the left or to the right of fixation (direction compatibility). The results show strong compatibility effects for both position and direction of motion in both experiments. A Simon effect, however, occurred only when position was task irrelevant in Experiment 1; no Simon effect was found in Experiment 2. The data only partly confirm previous results obtained with apparent motion. The selective lack of a Simon effect supports the integrated model of Umiltá and Nicoletti (1992), which requires orienting of attention for the Simon effect to occur. It is specifically assumed that this attention-orienting is triggered only by the saccade program and does not extend to the pursuit program that is initiated by smooth stimulus motion.     

Interaction of fixation point and stimulus orientation the appearance of rectilinearity

Data from two experiments indicate that the findings of Prytulak (1973) are generalizable to stimulus lines at all orientations: (a) when setting a response dot so that it appears to lie on an imaginary extension of a stimulus line, Ss misplace the dot on the side of the extension opposite the fixation point (a contralateral response, or CR) when the fixation point is in the vicinity of the stimulus line and on the same side of the extension as the fixation point (an ipsflateral response, or IR) when the fixation point is in the vicinity of the response dot; (b) CR is larger than IR; (c) fixation of the stimulus line or any point on its extension produces relatively accurate settings. New findings were: (a) the magnitude of CR decreased as the orientation of the stimulus line approached the vertical or horizontal; (b) the magnitude of CR on or near the horizontal was weaker than on or near the vertical; (c) the magnitude of IR was greater - for stimulus lines lying in the upper half of the visual field than for stimulus lines lying in the lowerhalf. The difficulty - - of accounting for fixation point effects by means of physiological optics was discussed.     

S-R compatibility effects due to context-dependent spatial stimulus coding

Responses are faster with spatial S-R correspondence than with noncorrespondence (spatial compatibility effect), even if stimulus location is irrelevant (Simon effect). In two experiments, we sought to determine whether stimuli located above and below a fixation point are coded as left and right (and thus affect the selection of left and right responses) if the visual context suggests such a coding. So, stimuli appeared on the left or right eye of a face’s image that was tilted by 90° to one side or the other (Experiment 1) or varied between upright and 45° or 90° tilting (Experiment 2). Whether stimulus location was relevant (Experiment 1) or not (Experiment 2), responses were faster with correspondence of (face-based) stimulus location and (egocentrically defined) response location, even if stimulus and response locations varied on physically orthogonal dimensions. This suggests that object-based spatial stimulus codes are formed automatically and thus influence the speed of response selection.     

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.     

Intentional pre-cueing does not influence the Simon effect

Choice reactions can be performed more quickly if the response corresponds spatially to the stimulus, even when the stimulus location is irrelevant for the task (Simon effect). It is assumed that the Simon effect is related to interference between spatial stimulus and response codes in a response selection stage. A central finding for such a response selection account is the increase in the effect if the most probable response location is given in advance by an intentional pre-cue. However, Hasbroucq and Possamaï (1994) assumed that the increase in the Simon effect in such a task may be due to an unmeant pre-cueing of the stimulus location, which has been recently supported by an electroencephalography (EEG) study by Wascher and Wolber (2004). In the present study this notion has been tested experimentally. In Experiment 1, a centrally presented symbolic cue served as an intentional cue. As a result, the enhancement of the Simon effect in valid cueing almost disappeared. When tactile cues were used (Experiment 2), the increase in the Simon effect disappeared completely. Thus, the influence of intentional cueing reported in previous studies can be assigned to attentional factors and does not support a response selection account.     

Transfer of response codes from choice-response to go/no-go tasks

The present study investigated the conditions for observing the Simon effect in go/no-go tasks. The Simon effect denotes faster and more accurate responses when irrelevant stimulus location and response location correspond than when they do not correspond. In four experiments, participants performed both in a choice-response task (CRT) and in a go/no-go task, and we varied the order and the similarity of the tasks. In the CRT, participants pressed a left key to one stimulus colour and a right key to another stimulus colour; in the go/no-go task, participants pressed one (e.g., left) key to one stimulus colour and refrained from responding to the other stimulus colour. As expected, Simon effects were consistently observed in the CRT. In contrast, Simon effects in the go/no-go task were only observed when it followed the CRT and when the mapping of stimulus colours to response locations was preserved between tasks (i.e., in Experiment 4). Results suggest that transfer of a particular S–R rule including response location from the CRT to the go/no-go task was responsible for the Simon effect in the latter task. In general, results are consistent with a response-discrimination account of the Simon effect.     

Is direction position? Position- and direction-based correspondence effects in tasks with moving stimuli.

Five experiments were carried out to test whether (task-irrelevant) motion information provided by a stimulus changing its position over time would affect manual left-right responses. So far, some studies reported direction-based Simon effects whereas others did not. In Experiment 1a, a reliable direction-based effect occurred, which was not modulated by the response mode--that is, by whether participants responded by pressing one of two keys or more dynamically by moving a stylus in a certain direction. Experiments 1a, 1b, and 2 lend support to the idea that observers use the starting position of target motion as a reference for spatial coding. That is, observers might process object motion as a shift of position relative to the starting position and not as directional information. The dominance of relative position coding could also be shown in Experiment 3, in which relative position was pitted against motion direction by presenting a static and dynamic stimulus at the same time. Additionally, we explored the role of eye movements in stimulus-response compatibility and showed in Experiments 1b and 3a that the execution or preparation of saccadic eye movements--as proposed by an attention-shifting account--is not necessary for a Simon effect to occur.     

Sequential analysis of a Simon task--evidence for an attention-shift account

 We investigated the attention-shift hypothesis of the Simon effect by analysing the effect of repeating relevant colour or irrelevant location of the stimulus in four serial reaction time tasks. In Experiment 1 with short response-stimulus intervals (RSI), we assume that there is no time to engage attention at the fixation cross before the onset of a new stimulus. In agreement with the hypothesis, Experiment 1 reveals no Simon effect when the stimulus location is repeated. In Experiment 2 with long RSI, we observe a Simon effect for location repetitions and alternations. In Experiment 3 with long RSI, we hinder the disengagement of attention by displaying the stimulus after response execution. As expected, the Simon effect is reduced for location repetitions. In Experiment 4 with stimuli additionally presented at the fixation cross, responses are faster if the attention shift towards the centrally presented stimulus corresponds with the location of the required response. Additionally, we argue that binding of the stimulus features into an object or event file better explains the so-called blocking of the automatic response-priming route after a noncorresponding trial. Received: 2 February 2000 / Accepted: 10 November 2000