sp

r S

shs rerlap with a different high-priority task. In contrast, non-spatial variants of the

spatial variant of the Simon task. In the color version there was no sign of early response priming by irrelevantstimulus features in the LRP. The color compatibility effect was independent of task overlap and reected inthe LRP onset latency. In contrast, in the spatial version, priming by irrelevant stimulus location showed upand was mirrored by early LRP activation. Respon

at varying locations in the visual eld, performance is better well, involving interference between an irrelevant feature of the target,

Acta Psychologica 136 (2011) 4251

Contents lists available at ScienceDirect

Acta Psych

e lswhenever the required response is spatially compatible with thelocation of the stimulus than when it is not. This phenomenon hasbeen called stimulus-response (S-R) compatibility (Fitts & Seeger,1953). In addition to location, several other stimulus and responsedimensions such as modality, color and shape show effects of S-Rcompatibility. For instance, responses are usually faster if the colors ofstimuli and of response keys correspond than when they do not.

Effects of S-R compatibility arise even if the critical stimulusdimension is irrelevant for the task: Consider a situation whereparticipants have to conduct choice responses to the identity of stimuli,

such as color or form, and a corresponding feature of the requiredresponse. Although effects of S-R compatibility have been considered adomain-general phenomenon, most subsequent research focused onspatial interference effects. More recent results, however, suggest thatmechanisms underlying spatial compatibility effects differ from thosefor non-spatial effects.

In a previous study, spatial and non-spatial Simon effects werecontrasted by using both versions of the Simon task as secondary tasksin a dual task paradigm (Magen & Cohen, 2005). In the spatial version,the irrelevant location of the stimulus was varied and could bee.g. pressing a left key to a square and a rigstimuli appear randomly on the left or on the(RT) is typically shorter if the irrelevant stimu

Corresponding author. Humboldt-Universitt zu BUnter den Linden 6, 10099 Berlin, Germany. Tel.: +49 3

E-mail address: carola.lehle@cms.hu-berlin.de (C. Le

0001-6918/$ see front matter 2010 Elsevier B.V. Adoi:10.1016/j.actpsy.2010.09.013and responses play anstance, if stimuli appear

1970; for reviews see, e.g. Lu & Proctor, 1995; Simon, 1990); this is theso-called Simon effect. As Kornblum, Hasbroucq, and Osman (1990)pointed out, the Simon effect generalizes to non-spatial dimensions asShared common properties of stimuliimportant role in human action control. For inpsycINFO classication:2300 Human Experimental Psychology2340 Cognitive Processes2330 Motor Processes2323 Visual Perception2346 Attention

Keywords:Dual task performanceCognitive controlConictElectroencephalographySimon effect

1. Introductionwere present only in case of little temporal overlap with the primary task. The absence of spatial compatibilityeffects at strong temporal overlap suggests that response conicts due to stimulus-related priming depend onthe availability of processing resources.

2010 Elsevier B.V. All rights reserved.

with the response location rather than incompatible (Craft & Simon,ht key to a circle, whereright side. Response timelus location is compatible

compatible or inspatial version, loeither identical oleading to interfdynamics of theperformance depthe two tasks: Thoverlap with the

erlin, Institut fr Psychologie,0 2093 9336.hle).

ll rights reserved.se priming and the corresponding Simon effect, however,Available online 27 October 2010

Simon task appear to be unaffected by task overlap. The present study used the lateralized readiness potential(LRP) within a dual task design to elucidate the dynamics underlying these differential effects for a color and aReceived in revised form 27 September 2010Accepted 29 September 2010

when there is temporal oveDifferential dynamics of spatial and non-effects: A dual task LRP study

Carola Lehle a,, Asher Cohen b, Jrg Sangals c, Wernea Humboldt-Universitt zu Berlin, Germanyb The Hebrew University of Jerusalem, Israelc Humboldt-Universitt zu Berlin, Germany

a b s t r a c ta r t i c l e i n f o

Article history:Received 30 January 2010

Choice reaction times areincompatible. Recent studie

j ourna l homepage: www.atial stimulus-response compatibility

ommer c, Birgit Strmer c

orter when stimulus and response features are compatible rather thanvealed that spatial compatibility effects in Simon tasks are strongly attenuated

ologica

ev ie r.com/ locate /actpsycompatible with the response location. In the non-cationwas constant but stimulus color varied andwasr opposite to the color of the correct response button,erence in the non-corresponding condition. Differentspatial and non-spatial Simon effect showed up inending on the amount of temporal overlap betweene spatial Simon effect strongly decreased as temporalprimary task increased, a result replicating previous

43C. Lehle et al. / Acta Psychologica 136 (2011) 4251reports (Lien & Proctor, 2000; Magen & Cohen, 2005; McCann &Johnston, 1992). In contrast, the size of the non-spatial Simon effectwasindependent of the temporal overlap with the primary task. Althoughspatial information seems to play a special role, themechanisms leadingto different effects of spatial and non-spatial information in dual tasksremain to be elucidated.

The present study aimed at investigating the differential dynamicsof the spatial and the non-spatial compatibility effects by combining adual task or Psychological Refractory Period paradigm (PRP) withrecordings of event-related potentials (ERPs).

In the PRP paradigm, two tasks are performed in close succession(e.g. Pashler & Johnston, 1989; Welford, 1952). The interval betweenthe stimuli for the primary and the secondary task, the Stimulus OnsetAsynchrony (SOA), is varied to examine the dynamics of interferencebetween the two tasks. If the two tasks simultaneously access centralprocessing stages, interference is observed. In most cases, centralprocessing stages of the secondary task are postponed until those ofthe primary task are completed. Response selection is commonlyaccepted to be among the central processes. In contrast, processesprior to and following central stages such as perception and responseexecution may overlap for two tasks without interference.

The PRP paradigm can be also applied to investigate the mentalchronometry of experimental effects based on the locus-of-slack logic(see e.g. Schweikert, 1980). If, at short SOA, the secondary task ispostponed until central processing in the primary tasks is completed,a slack period occurs. Accordingly, any effect related to the durationof pre-central processes in the secondary task may be absorbed intothe slack, that is, it shows up only at long SOAs but not or to a lesserextent at short SOAs. Absorption into slack manifests as anunderadditive interaction with SOA in performance, that is, theexperimental effect is mainly present at long SOAs but small or absentat short SOAs. In contrast to this underadditive interaction with SOA,any experimental effect occurring in or after central stages will beadditive with the SOA effect.

In the PRP study comparing spatial and non-spatial Simon effects(Magen&Cohen, 2005), SOA interactedunderadditivelywith the spatialSimon effect but it was additive with the non-spatial Simon effect. Thelocus of slack logic suggests here that the spatial but not the non-spatialSimon effect is absorbed into the slack. Such an interpretation, however,is implausible given that both spatial and non-spatial compatibilityeffects result from conicts between response categories. A parsimoni-ous alternative explanation would be that specically spatial compat-ibility effects decay over time (De Jong, Liang, & Lauber, 1994; Hommel,1997; McCann & Johnston, 1992): Because response times are usuallylonger at short than at long SOA, a decay of the spatial effect becomesmore likely for shorter SOAs. However, depending on the implemen-tation of the task, the Simon effect does not always decay (Lien &Proctor, 2000; Magen & Cohen, 2005; Strmer, Leuthold, Soetens,Schrter, & Sommer, 2002; Wascher, Schatz, Kuder, & Verleger, 2001).

Therefore, alternative accounts have been proposed to explain thespecic dynamics of the spatial Simon effect in dual tasks. First, theSimon effect is most commonly explained by so called dual-routemodels of response selection (e.g. De Jong et al., 1994; Hommel, 1993;Kornblum et al., 1990). Albeit differing in details, all dual-route modelsconcur in the assumption that stimulus location activates responsetendencies along one of two processing routes, a direct (unconditional)route and an indirect (conditional) route. For the Simon paradigm it isassumed that the location information of a stimulus directly activatesthe motor system by priming the spatially corresponding response. Incontrast, task-relevant information is processed via the indirect route byapplying the stimulus-response mapping rules specied by theinstructions. In compatible conditions both processing routes activatethe same response, whereas in incompatible conditions, opposingresponses are activated, creating a conict during response selection. Toresolve this conict and to select the correct response, additional time is

required as compared with the compatible condition.Two different hypotheses have been suggested to explain the specicdual task results of theSimoneffectwithin thedual-route framework. Theactivation reset hypothesis posits that response priming by spatial locationis reset after response selection for theprimary task (Lien&Proctor, 2000;Schubert, Fischer, & Stelzel, 2008). This suggestion is substantiated asfollows: So-called crosstalk effects have been observed in dual tasksindicating interference between response relevant categories from thetwo tasks (e.g. Hommel, 1998; Hbner & Lehle, 2007; Lehle & Hbner,2009; Miller & Alderton, 2006; Schubert et al., 2008; Watter & Logan,2006). To reconcile these effects with a serial response selection stage,Hommel (1998) argued that a process of stimulus-response translationproceeds automatically and in parallel for the primary and the secondarytask. Subsequently, the process of response selection operates serially ontheoverall pattern of response activations. To explain the dynamics of theSimon effect in dual tasks, it can be assumed more specically that, atshort SOA, the location of the second stimulus activates its associatedresponse before the selection of the rst response starts (Lien & Proctor,2000; Schubert et al., 2008). After the response of the primary task isselected, however, all responses that were triggered before are no longeractive. Therefore, the early priming of the spatially compatible responsehas no direct inuence on response selection of the secondary task. Atlong SOAs, response selection of the primary task is already completed bythe time the second stimulus is presented and the correspondingresponse activation takes its full effect on secondary task performance.Since the activation reset hypothesis makes no assumptions about non-spatial compatibility effects, no explanation is provided for the dynamicsof the non-spatial effect in the PRP paradigm.

A further hypothesiswithin the dual route framework accounts for thedifferent dynamics of spatial and non-spatial Simon effects in the PRPparadigm by considering limited processing resources: It is assumed thatonly action-relevant information such as stimulus location can activate itscorresponding response early after stimulus onset. This is supported byelectrophysiological investigations indicating that stimulus positiondirectly activates corresponding hand areas in primary motor cortex(e.g. De Jong et al., 1994; Valle-Inclan, 1996). Priming by stimulus locationseems to be conned to a limited time window after stimulus onset andcannot be postponed in time. Whether the priming is triggered withstimulus onset or not has been shown to depend on several conditionssuch as indication of S-R rules prior to anupcoming stimulus (Valle-Inclan& Redondo, 1998). As a further precondition the limited resources impedepriming hypothesis introduces the availability of capacity by the time ofstimulus onset, which is not met at short SOA in the PRP paradigm. Incontrast, in the non-spatial Simon task, the response activation is notrelated to stimulus onset and is therefore postponed at short SOA.

Another account, the response discrimination hypothesis (Ansorge &Whr, 2004; Whr & Ansorge, 2007; Whr, Biebl, & Ansorge, 2008),does not assume stimulus-driven priming as a necessary preconditionfor the spatial Simon effect. For the Simon effect to arise, in contrast,the following conditions have to be met: First, a stimulus-locationcode is bottom-up activated and enters working memory. Second,codes representing response location as part of the task-specic S-Rrules are also active in working memory. The Simon effect thenresults from interaction of the codes representing stimulus locationwith those representing response locations. Even though the responsediscrimination hypothesis makes no specic proposals about thedynamics of Simon effects in dual tasks, it predicts that irrelevantdimensions are represented in working memory only if sufcientworking memory capacity is available. This assumption is supportedby recent evidence (Whr & Biebl, in press). At short SOA in the dualtask paradigm, working memory capacity might be scarce, so that noSimon effect arises in contrast to longer SOA. Also the responsediscrimination account does not address the specicity of spatial andnon-spatial dimensions. Within this framework an explanation wouldhave to be found why, in dual tasks, representation in workingmemory and the interaction with SOA plays a role only for codes

representing spatial, but not for those representing color information.

Finally, Magen and Cohen (2002) suggested a mechanism forselecting stimulus features on the basis of their similarity to therequired responses. To explain the divergent dynamics of spatial andnon-spatial Simon effects, Magen and Cohen (2005) emphasized aunique role of location information in response selection. In the spatialSimon task, spatial attention can operate on low-level visual cues to

44 C. Lehle et al. / Acta Psychologica 136 (2011) 4251resolve the (incorrect) response activation inducedbystimulus location.This could be accomplished by focusing on the location of the requiredresponse or by inhibiting the alternative response location activated bythe irrelevant location of the target. Because spatial attention has beenshown to operate concurrently for a secondary and a primary task(Pashler, 1991), the resolution of the stimulus-related responseactivation in the spatial task could be carried out during the slack. Incontrast, in the non-spatial task, attention does not have a direct accessto low-level representations and thus cannot operate within the slack.The stimulus-related activation therefore persists in the non-spatial taskalso at short SOA and leads to a Simon effect that is additive with theSOA. In the following, this notion will be called the resolution by spatialattention hypothesis.

To sum up, four alternative hypotheses were outlined that aim atexplaining the dynamics of the spatial Simon effect in the PRPparadigm. The response discrimination and the activation resethypothesis focus on the spatial conict, whereas the limited resourcesimpede priming and the resolution by spatial attention hypothesisaccount provide explanations for the differences between spatial andnon-spatial conicts. Furthermore, the activation reset and theresolution by spatial attention hypothesis posit that responseactivation by the irrelevant stimulus dimension does occur withequal strength at both short and long SOAs, but is reset or resolvedbefore response selection of the secondary task begins. In contrast,according to the response discrimination and the limited resourcesimpede priming hypothesis, no response is activated by the irrelevantstimulus dimension at short SOAs.

Since the Simon conict at short SOA in dual tasks might bepresent without affecting overt performance, conict indicatorsbesides performance are needed. Therefore, in the present experi-ments, we supplemented the PRP paradigm with recording thelateralized readiness potential (LRP), which is viewed as an index ofeffector-specic motor preparation (e.g. Coles, 1989; De Jong, Wierda,Mulder, & Mulder, 1988; Masaki, Wild-Wall, Sangals, & Sommer,2004; Miller & Hackley, 1992; Osman & Moore, 1993). The LRP iscomputed from the ERP recorded above the hand areas of the motorcortices of both hemispheres. Negative and positive LRP polaritiesindicate activation of the correct and incorrect response hand,respectively. Importantly, the LRP also provides information aboutstimulus-triggered response priming. In the spatial Simon task,incorrect LRP activation is found early after stimulus onset inincompatible conditions taken to reect priming of the associatedresponse (e.g. De Jong et al., 1994; Gratton, Coles, Sirevaag, Eriksen, &Donchin, 1988; Valle-Inclan, 1996). Thus, with the LRP the temporaldynamics of the Simon conict during information processing can beaddressed. In particular, the early LRP deection is useful toinvestigate whether there is similar location-related priming at allSOAs or whether priming itself is modied by the SOA. Furthermore, itcan be assessed whether the early LRP deection mirroring responsepriming is present only in the spatial Simon task or whether it showsup in the non-spatial Simon task as well.

In Experiment 1 of the present study1, the spatial Simon task wasinvestigated in a dual task situation. Here, the Simon task was

1 Prior to the dual task experiments a single task study was performed which aimedto clarify several basic points. We assessed compatibility effects for spatial and colorversions of the Simon task in vertical stimulus-response arrangements, implementedon a touch screen. In the non-spatial and the spatial Simon task, reliable compatibilityeffects of equal size were found. Furthermore, an RT-bin analysis showed that thespatial Simon effect did not dissipate with time in the present implementation of the

task. This single task experiment is described in the Appendix.combined with a structurally dissimilar primary task and the SOAwasvaried. We predicted that the Simon effect in performance shouldinteract underadditively with SOA, replicating previous results.Concerning the LRP, the following results were considered possible:First, a stimulus-related response activation of the spatiallycorresponding response might emerge at all SOAs. If this is correct,a substantial early incorrect activation should be found in the LRP, inits amplitude and latency independent of the SOA. This would bepredicted by both the activation reset and the resolution by spatialattention hypothesis. Alternatively, the strength or timing of thepriming itself might be changed by the SOA variation. In this case onewould expect an SOA-dependent modulation of the early stimulus-related response priming in the LRP. The latter result would supportthe response discrimination and the limited resources impedepriming hypothesis.

The non-spatial Simon task was investigated in a dual tasksituation in Experiment 2. According to previous results, the non-spatial Simon effect was expected to be additive with the SOA. Ofmajor interest was whether an early stimulus-driven LRP activationwould show up in the ERPs similarly as in spatial version. Neither theactivation reset nor the response discrimination hypothesis makesspecic predictions about non-spatial conicts. The limited resourcesimpede priming hypothesis posits that stimulus-driven responsepriming is conned to action-relevant information such as location. Inthe non-spatial task, in contrast, no stimulus-driven priming andtherefore no early deection in the LRP was expected. Finally, theresolution by spatial attention hypothesis postulates that spatialattention can resolve the conict specically in the spatial task, butresponse priming and the respective deection in the LRP shouldoccur with equal strength in both variants of the Simon task.

2. Experiment 1

The present experiment examined the temporal dynamics re-sponsible for the underadditive effects of spatial compatibility andSOA in a dual task paradigm. Here, the Simon task was combined witha tone-foot response task of high priority. In this task the participantshad to vary the force of pedal presses (hard vs. soft) in response to thetone's intensity a task with high stimulus-response compatibility(cf. Romaiguere, Hasbroucq, Possamai, & Seal, 1993). The primary taskwas chosen to minimize structural overlap with the Simon task,precluding crosstalk between the two tasks.

It was predicted that the Simon effect in performance interactsunderadditively with the SOA, replicating previous results. For theLRP, an early deection was expected reecting the preparation of thespatially corresponding response hand during an early time windowafter stimulus onset. If the stimulus location primes a spatiallycorresponding response independent of the temporal overlap withthe primary task, then an early incorrect activation should be found inthe LRP for all SOAs. This would provide strong support for theassumption that the priming of spatially compatible responses bystimulus location itself is not affected by a concurrent task, but thatthere is no (or no longer) conict by the time the second response isselected at short SOA. In contrast, if the stimulus-related responsepriming in the LRP is decreased or absent at short SOA, this wouldindicate that the mechanisms underlying the spatial Simon effect areaffected by temporal overlap with a concurrent task.

2.1. Method

2.1.1. ParticipantsEighteen participants (12 female, 6 male), mean age 25 years, took

part in this study. In this and the following experiment, participantswere recruited from a pool of the Humboldt-Universitt zu Berlin; theywere reimbursed with course credits or 7 per hour. Fourteen

participants were right-handed; the others were left-handed

(questionnaire adapted from Oldeld, 1971). Furthermore, all partici-pants of this and the following experiment had normal or corrected-to-normal vision andwerewithout anyneurological or neuropsychologicaldisorder according to self-report. Prior to the experiments informedconsent was obtained.

2.1.2. Apparatus and StimuliThe visual stimuli and the square elds that served as response

keys were displayed on a touch screen (Keytec Inc.) with the stimuliin the upper half and the response keys in the lower half (see Fig. 1Afor the basic design). The touch screen was positioned at a slantedangle with the stimulus display area at eye level and about 50-60 cmin front of the participants. For the spatial Simon paradigm, the stimuliwere the capital letters S or X, presented in light grey on blackbackground (font size 76 pt). The response keys were verticallyarranged blue or yellow squares (edge length 4.5 cm). The color of theupper/lower key was randomly assigned on a trial-by-trial basis.

The touch screen was covered with a transparent pane of glass onwhich the participants hands rested with an open area at the locationof the response keys, allowing for comfortable operation of theresponse keys with the index ngers. Half of the participants operatedthe upper response key with their right index nger and the lowereld with the left index nger. This assignment was reversed for theother participants. Stimulus and response key presentation and therecording of responses were controlled by Presentation software

45C. Lehle et al. / Acta Psychologica 136 (2011) 4251(Neurobehavioural Systems, Inc.).The spatial Simon task was combined with a primary task in a PRP

paradigm. As stimuli for the primary task, tones with 4 kHz werepresented for 150 ms via two loudspeakers located to the right andthe left of the touch screen. The tones were varied in their intensity(soft=75 dB; loud=85 dB). Foot responses in the primary taskwere recorded with two small pedals embedded into a footrest; thepedals were pressed with the big toes, shoes taken off.

2.1.3. Procedure and DesignThe participants sat in front of the touch screen with hands resting

on the pane of glass. A trial started with the onset of the squareresponse keys on the screen and a small xation mark at the center ofthe stimulus eld (cf. Fig. 1A). After 900 ms, a soft or loud tone waspresented (S1). After an SOA of 100, 400, or 700 ms randomized

Fig. 1. Illustrated is the basic designof the spatial (A) and thenon-spatial Simon task (B) asused in the present study. Response keys appeared as squares on a touch screen, whereasthe color of the upper/lower keywas randomly assigned on a trial-by-trial basis. Both keyshad to bepressed by the indexngers of different hands. The individual hand (left or right)

and key (upper or lower) assignment was balanced across participants.across trials one of the letters X or S (S2) appeared at randomabove or below xation. In the primary task (T1), participants had torespond to soft and loud tones by pressing the foot pedal with weak orstrong force (120-990 vs. N990 cN). Whether participants executedthe pedal presses with the left or right foot was held constant over agiven block of trials, but alternated between blocks.

For the secondary task (T2), manual responses were required tothe visual stimuli. The upper or lower response keys were to betouched to the letters X or S, in counterbalanced assignment. Theresponse keys and the visual stimuli remained on the screen untilboth responses had been executed. If RT1 or RT2 were shorter than150 ms or exceeded 2 s, the response was classied as an error. Thetone-foot and the visual-manual task were rst practiced as singletasks. That is, four to six blocks of practice were conducted in the tone-foot task depending on the participant and two practice blocks inthe visual-manual task. Subsequently, in the main experiment, 14blocks of 48 trials each were conducted with both tasks as dual task.Whether participants started with the left or right foot in the rstblock of T1 was counterbalanced. Between blocks, participants couldmake short self-terminated breaks. Any errors in T1 or T2 weresignalled by a specic message on the screen, e.g. too slow, toofast, or wrong response key. If the manual response was executedbefore the foot response, it was also registered as error.

2.1.4. Electrophysiological RecordingsThe EEG was recorded continuously at a sampling rate of 250 Hz

from 64 Ag/AgCl electrodes mounted in an electrode-cap (ECI Inc.).The vertical and horizontal electrooculogram (EOG) was registeredabove and below the left eye and from the left and right outer canthi.All electrodes were referenced to the right mastoid. Electrodeimpedance was kept below 5 k. Bandpass was set to 0.032-70 Hz,and a 50 Hz notch lter was applied.

2.1.5. Data analysisThe continuous EEG was segmented ofine into epochs of

4400 ms, starting 1400 ms before S1 onset. Eye blink artifacts werecorrected according to the Brain Electric Source Analysis (BESA 2.1,Megis Software GmbH). In addition, global eld power (Lehmann &Skrandies, 1980) of the EEG segments was used to sort out trials withlarge artefacts due to muscular tension (Daubenspeck, Lim, & Akay,2000). All segments for which global eld power, averaged over time,exceeded the individual mean plus two standard deviations wererejected from further analysis. Trials with an error in either T1 or T2were excluded from the analyses.

Stimulus-locked ERPs for the visual Simon task were derived in newepochs of 3122 ms, starting 852 ms before S2 by rst averagingseparately according to the specic response-hand mapping, SOA andcompatibility. All ERP waveforms were aligned to a 100-ms baselineperiod preceding S2 onset. LRPs were averaged separately according tothe specic response-hand mapping and compatibility (compatible,incompatible). The signals from lateral electrode channels wererecalculated by subtracting signals from the recording site ipsilateralto the required response hand from the homologous contralateral site.For example, in trials calling for a right-hand response, the signalrecorded at the position C4 was subtracted from the signal recorded atC3. These difference waveforms were averaged across the responsehands for each participant and experimental condition (cf. Coles, 1989).The latencies of onsets and peaks of the LRPs were computed with thejackknife-based method proposed by Miller, Patterson, and Ulrich(1998). In each LRP waveform, the onset of correct LRP activation wasdetermined as the time point where a relative threshold of 50% of thepeak amplitude was exceeded.

Dependent variables were submitted to ANOVAs with repeatedmeasures on the factors SOA (100 ms vs. 400 ms vs. 700 ms) and

Compatibility (compatible vs. incompatible).

2.2. Results

2.2.1. Performance

2.2.1.1. Task 1. Trials in which an error was made in T1 were notincluded in the analysis of RT. The mean latencies of correct responsesto S1 were 508 ms. The ANOVA did not indicate any signicant effectsof Compatibility (in T2), SOA, or interaction of these factors on RT1.

The mean error rate for T1 was 11.8%. The ANOVA revealed only asignicant main effect of SOA, F(2, 34)=8.4, pb .01, indicating thatthe error rates were higher for the shortest SOA (M=13.5%) than forSOAs 400 and 700 (M=10.9% each).

2.2.1.2. Task 2. Trials in which an error was made in either T1 or T2were eliminated from this analysis. Fig. 2 shows the performance inT2. ANOVA revealed signicant main effects of SOA, F(2, 34)=292.9,pb .001, and of Compatibility F(1, 17)=11.8, pb0.01. RT2 decreasedsignicantly with increasing SOA (M=924, 685, and 563 ms),

Furthermore, the onset of the early incorrect activation wasdetermined: For both SOA 400 and 700, the incorrect activationstarted at around 150 ms after stimulus onset and peaked at 220 ms.An ANOVA with repeated measures for the onsets with SOA as factorrevealed no signicant effect. The scalp distribution of the meanincorrect activation over all lateral electrode positions in the early timeinterval from 190 to 250 ms in SOA 700 is illustrated2 in Fig. 4B.

Fig. 4A shows that the LRP onset related to the correct execution ofthe response in the incompatible conditionwas delayed relative to thecompatible condition, closely resembling the Simon effect in RTs. TheSimon effects in LRP onsets were, on average, 11 ms, 62 ms, and 84 msfor SOA100, 400, and 700, respectively. An ANOVA revealed signicant

2 Since each lateralized ERP (L-ERP) is computed from the difference of a pair ofhomologue electrodes of both hemispheres, it contains no information with respect tothe cerebral hemispheres. Instead, the position of each L-ERP signal is dened withrespect to the anterior-posterior scalp axis (midline) and the perpendicular distancethereof. In order to visualize the L-ERP distribution, they are projected onto the lefthemisphere. Importantly, this arbitrary projection does not imply that the L-ERP

46 C. Lehle et al. / Acta Psychologica 136 (2011) 4251reecting a PRP-effect. Importantly, there was a signicant interactionof Compatibility and SOA, F(2, 34)=9.3, pb .001. The mean Simoneffects for the SOAs 100, 400, and 700 were 4 ms, 7 ms, and 38 ms(M=16.3 ms), respectively. A t-test contrasting the Simon effects atthe SOAs 400 vs. 700 showed a signicant difference, t(17)=5.3,pb .001.

The average error rate for T2 was 3.1%. The ANOVA revealed maineffects of Compatibility F(1, 17)=10.2, pb .01 and of SOA, F(2, 34)=11.6, pb .001. Error rate was higher in the incompatible than in thecompatible condition (M=4.0 vs. 2.3%) and increased with SOA(M=2.4, 2.6, and 4.4%), F(2, 34)=9.7, pb .001. Again, there was asignicant interaction of SOA and Compatibility, F(2, 34)=6.3, pb .01,indicating that also the Simon effect in error rates increased with SOA(see Fig. 2).

An RT distribution analysis was conducted to assess whether thespatial Simon effect dissipates with increasing response time. The RTsfor each SOA and compatibility condition of the spatial Simon taskwere divided into quartiles (see Fig. 3). The interaction of ANOVAfactors RT-Quartile and Compatibility as well as the interaction ofRT-Quartile, Compatibility and SOA missed signicance by widemargins [Fsb1].

2.2.2. Task 2 LRPAs can be seen in Fig. 4A, incorrect LRP activity after incompatible

events was clearly present for SOAs 400 and 700. This indicates that,during an early time window after stimulus onset, the handcorresponding to stimulus location was primed in the longer SOAconditions. However, for SOA 100 there was no discernible incorrectactivation in the incompatible condition.

Fig. 2.Mean response times (RT) and error rates for the Spatial Simon task (secondarytask) of Experiment 1. The results are plotted separately for the compatible and

incompatible condition and the three Stimulus Onset Asynchronies (SOA).In order to statistically test the early LRP activation, the averageamplitudes for the time interval from 190-250 ms after S2 onset weredetermined, that is 30 ms surrounding the mean peak-latency ofthe priming-related activation. An ANOVA of the resulting areas withSOA and Compatibility as factors revealed a signicant main effect ofCompatibility, F(1, 17)=7.6, pb .05, indicating that the average LRPamplitude during this interval was more positive after incompatiblethan compatible events. Moreover, there was a signicant interactionof Compatibility and SOA, F(2, 34)=13.2, pb .001, indicating that theincorrect activation on incompatible trials increased with increasingSOA (M=0.072, 0.314, and 0.635 V).

T-tests comparing the area of early incorrect activation at centralsites against zero indicated no signicant difference for SOA 100(tb1), but signicant differences against zero for SOA 400 and 700,t(17)=2.2 and 2.8, pb .05 and .01, respectively. Finally, two separatet-tests comparing the area at SOA 400 vs. 700 showed no signicantdifference [t(17)=1.4, p=.16]. In addition to the areas, also the peakamplitudes of the incorrect activation were compared in an ANOVAwith repeated measures on the factor SOA (400 vs. 700 ms). As for theareas, no signicant effect was revealed here (Fsb1).

If the incorrect response activation is absent at short in contrast tolong SOA, this should be also the case for the facilitation (priming) ofthe correct response in the early time window. T-tests comparing themean activity (area with negative polarity) during the early timeinterval (i.e. 190-250 ms after stimulus onset) for each SOA againstzero revealed no signicant difference for SOA 100 (tb1), butsignicant effects for SOAs 400, t(17)=1.9, pb .05 (one-tailed), and700, t(17)=4.5, pb .001. Thus, at short SOA, no signs of correctresponse priming were present in the compatible condition.

Fig. 3. Response times (RT) distribution analysis of the secondary (spatial Simon) taskof Experiment 1. The Simon effect is plotted depending on RT-quartiles and separatelyfor the three SOAs and for the compatible, respectively, incompatible condition.sources are left-hemispheric or that the L-ERP is absent on the right hemisphere.

t 1,iffehe S

47C. Lehle et al. / Acta Psychologica 136 (2011) 4251Fig. 4. (A) LRPs (electrode positions C3/C4) of the spatial Simon paradigm in ExperimenStimulus Onset Asynchronies (SOA). (B) Voltage map of lateralized readiness potential dExperiment 1. The scalp topography is averaged here for the incompatible condition at tmain effects of SOA, that is, the PRP-effect, F(2, 34)=31.5, pb .001(M=406, 308, and 275 ms), and of Compatibility, F(1, 17)=12.6,pb .01. Moreover, the interaction of SOA and Compatibility in the LRPonset was signicant, F(2, 34)=5.8, pb .01. Thus, the Simon effectincreased with increasing SOA also when considering the LRP onsets.Separate t-tests comparing the onsets of the compatible versusincompatible condition in the LRP revealed no signicant differencefor SOA 0 (tb1), but signicant Simon effects for SOA 400, t(17)=5.1,pb .001, and SOA 700, t(17)=7.6, pb .001.

2.3. Discussion

The present experiment examined the dynamics of the spatialSimon effect when combined with a primary task at different degreesof temporal overlap. The behavioral data replicated previous resultsshowing that the spatial Simon effect interacts underadditively withthe SOA, i.e. decreases with decreasing SOA. This interaction wassignicant for the RTs as well as for the error rates. At SOA 100, therewas no discernible Simon effect. An RT distribution analysisconducted separately for each SOA showed no dissipation of theSimon effect with increasing RTs (see Fig. 3). The underadditive Simoneffect in the RTs was reected by the onset of the correct LRP activity,which was increasingly delayed in the incompatible compared to thecompatible condition.

Furthermore, at SOAs 400 and 700, the expected priming-relatedearly deection appeared in the LRP, starting about 150 ms afterstimulus onset. Incompatible stimulus and response locations caused amotor activation of the wrong response hand, mirrored by a transientincorrect LRP activity. However, no signs of transient motor activationtriggered by stimulus location were present at SOA 100. Obviously,activation of the spatially corresponding hand by stimulus position

Spherical spline interpolation was used. Positivity is shaded grey.displayed separately for the compatible and the incompatible conditions and the threerence waves in the time window 190-250 ms after S2 onset in the spatial Simon task inOA of 700 ms, the positive polarity indicating activation of the incorrect response hand.depended on the SOA. In case the temporal overlap with the primarytask was high, no response priming by stimulus location was observed.

3. Experiment 2

In this experiment a non-spatial Simon task was investigated in aPRP design, analogous to Experiment 1. In the non-spatial taskimplemented here, which closely matched the task used by MagenandCohen (2005), the irrelevant color of the stimuli could interferewiththe color of the response keys. The non-spatial Simon effect should beadditive to the SOA, thus replicating previous results in behaviouralperformance (Magen & Cohen, 2005).

To our knowledge, there are no previous reports of LRPs in a non-spatial Simon task. Of interest was therefore whether an early LRPdeection could be observed similar to that found in Experiment 1.

3.1. Method

A group of 18 participants (12 female, 6 male), mean age 22 years,participated in the study under the same conditions as for Experiment1. None of the participants had taken part in Experiment 1.

Apparatus, procedure, the primary task (T1), electrophysiologicalrecording and data analysis were identical to Experiment 1, with theexception that now a color version of the Simon paradigmwas used asT2 in the PRP paradigm (cf. Fig. 1B for the basic design). For the non-spatial Simon task, one of the numerals 4 or 8 (font size 76 pt) waspresented at random in yellow or blue at the location of thexation mark on the black background. After conducting T1,participants were requested to press the yellow or blue key if an 8or a 4 appeared, respectively, the mapping being balanced acrossparticipants.

3.2.2. Task 2 LRPAs can be seen in Fig. 6, no LRP deection in an early time window

appeared after incompatible events for the non-spatial Simon task. Tostatistically analyse the phase of priming-related early LRP activation,the areas for the three SOAs were determined according to the criticaltime interval for early priming-related activation as revealed byExperiment 1, i.e. 190 to 250 ms after stimulus onset. As expectedfrom visual inspection of Fig. 6, an ANOVA on the factor SOA (100, 400,or 700 ms) revealed no signicant effect. A t-test comparing the areaat central sites against zero indicated also no signicant effect for anyof the SOAs 100, 400, or 700.

Furthermore, the LRP onset related to the correct execution of T2response was analyzed separately for the different conditions. As isshown in Fig. 6, the onset of the correct activation in the incompatiblecondition was delayed relative to the compatible condition, similar to

48 C. Lehle et al. / Acta Psychologica 136 (2011) 42513.2. Results

3.2.1. Performance

3.2.1.1. Task 1. Overall, mean RT1 was 607 ms. There was a signicantmain effect of SOA, F(2, 34)=15.8, pb .001, indicating that RT1increased slightly with increasing SOA (by 58 ms for the SOA 700relative to SOA 100). There was no signicant effect of T2compatibility on RT1 and also no signicant interaction.

The error rate for T1 was on average 10.7%. A two-factor ANOVAwith SOA and Compatibility in the secondary task only revealed asignicant main effect of SOA, F(2, 34)=18.4, pb .001, indicating thatthe error rates increased with decreasing SOA (M=9.5%, 10.2%, and12.8% for SOAs 700, 400 and 100, respectively). There was no furthersignicant main effect or interaction.

3.2.1.2. Task 2. Fig. 5 shows overall performance in T2. Trials in whichan error had been made in either T1 or T2 were eliminated fromthis analysis. ANOVA of RT2 yielded signicant main effects of SOA,F(2, 34)=519.9, pb .001, indicating the standard PRP-effect, and ofCompatibility, F(1, 17)=20.9, pb .001. The interaction of Compatibil-ity and SOA was not signicant, F(2, 34)=1.8, p=.178. The meanSimon effects for the SOAs 100, 400 and 700 were, in order, 27 ms,21 ms, and 39 ms (M=29 ms).

The average error rate for T2 was 3.1%. The ANOVA revealed only amain effect of Compatibility F(1, 17)=15.2, pb .001. As can be seen inFig. 5, the error rate was higher in the incompatible than in the

Fig. 5. Mean response times (RT) and error rates for the non-spatial Simon task(secondary task) of Experiment 2. The results are plotted separately for the compatibleand incompatible condition and the three Stimulus Onset Asynchronies (SOA).compatible condition (M=4.6 vs. 1.7%). Altogether, performanceresults showed additive effects of SOA and Compatibility in the non-spatial Simon task in response times and error rates.

Fig. 6. LRPs (electrode positions C3/C4) of the non-spatial spatial Simon paradigm in Experimthree Stimulus Onset Asynchronies (SOA).the Simon effect in the RTs. The compatibility effects in the LRP onsetwere, on average, 103 ms, 65 ms and 110 ms for SOAs 100, 400 and 700,respectively. An ANOVA with repeated measures comparing the onsetlatencies on the factors Compatibility and SOA revealed signicantmaineffects of SOA, F(2, 34)=49.1, pb .001 reecting a PRP-effect(M=562, 430, and 342 ms) and of Compatibility, F(1, 17)=52.0,pb .001. Importantly, the two-way interaction between SOA andCompatibility was far from being signicant, Fb1. Similar to theSimon effect in RTs, the difference in LRP onsets between the compatibleand the incompatible condition was rather constant across the threeSOAs.

3.3. Discussion

In Experiment 2 the color compatibility effect was additive with theSOA in RT and error rates. Furthermore, the onset of the correct LRPwassimilarly delayed in the incompatible relative to the compatiblesituation for all three SOAs, mirroring the additive Simon effect in theRTs. The performance results corroborate previous ndings by Magenand Cohen (2005).

The results of the present experiment further underline the differentdynamicsof the color as compared to the spatial Simonconict in thePRPparadigm. In contrast to Experiment 1, no signs of an early priming-related LRP deection were found. Obviously, the conict in the colortask appears to arise independentlyof stimulus-driven responsepriming.

4. General discussion

The goal of the present study was to investigate the mechanismsunderlyingspatial andnon-spatial Simoneffects underdifferentdegreesof temporal overlap with a primary task. Simon effects are based uponconicting response tendencies as activated by different stimulusdimensions. Such central processes related to response selection shouldresult in additive effects in the PRP paradigm. In previous PRP studies,however, the spatial Simon effectswere diminished at short SOA(Lien&

ent 2, displayed separately for the compatible and the incompatible conditions and the

49C. Lehle et al. / Acta Psychologica 136 (2011) 4251Proctor, 2000; Magen & Cohen, 2005; McCann & Johnston, 1992). Incontrast, for a non-spatial Simon task, additive effects with SOA wereobserved (Magen & Cohen, 2005).

Four alternative theoretical positions were considered as explana-tions for the specic dynamics of the Simon effect in dual tasks. Theactivation reset hypothesis assumes that spatial response priming is nolonger active after the rst response selection due to reset of responseactivations (Lien & Proctor, 2000; Schubert et al., 2008). The resolutionby spatial attention hypothesis of Magen and Cohen (2005) posits thatresponse activation triggered by stimulus location is resolved byspatial attention during the slack period. Both the activation reset andthe resolution by spatial attention hypothesis assume that responseactivation does occur both at long and at short SOAs, so that the earlydeection in the LRP, reecting stimulus-related response priming,should be independent of SOA in both amplitude and latency.

According to the response discrimination hypothesis (Ansorge &Whr, 2004), a modulation of response activation with SOA can beexpected. The hypothesis predicts that a compatible response isactivated only when a code representing stimulus location entersworking memory and interacts there with the codes representingresponse location. A high working memory load at short SOA shouldprevent the formation of a stimulus-location code in workingmemoryand thus the Simon effect. Finally, the limited resources impedepriming states in the rst place that only spatial but not non-spatial(color) stimulus features lead to direct priming of motor preparation,conned to a limited time window after stimulus onset. At short SOA,not enough capacity is available for the priming to be triggered. In thenon-spatial task response activation is not related to stimulus onset;therefore, it can be postponed until response selection in the primarytask is completed. The response discrimination as well as the limitedresources impede priming hypothesis assume that there is noresponse activation and thus no corresponding early LRP deectionat short SOA in the spatial task.

In Experiment 1, the different predictions were investigated forthe spatial Simon dual task. The Simon task was combined with astructurally dissimilar primary task and the SOA was varied.Replicating previous results, the Simon effect in performanceinteracted underadditively with SOA. At SOAs 400 and 700 ms,signicant priming effects, closely related to stimulus onset, showedup in the LRP. There was no discernible priming-related deection inthe LRP at the SOA of 100 ms.

Decay of response priming with increasing RT can be excluded asbeing responsible for the underadditive effects with SOA in Experi-ment 1. Analyzing RT-bins separately for the three SOA conditions didnot provide any evidence for a decay of the Simon effect withincreasing RTs. The results of Experiment 1 indicate that theemergence of spatial priming is affected by the temporal overlapwith a primary task, so that the irrelevant location of the Simonstimulus does not trigger the spatially corresponding response atshort SOA. Because no response activation is present in case of hightemporal overlap, early conict resolution by spatial attention or areset of the response activation need not be invoked to account for thepattern of results. Present LRP results therefore contradict theactivation reset and the resolution by spatial attention hypothesis.They are in line, however, with the response discrimination and thelimited resources impede priming hypothesis.

In Experiment 2 where the non-spatial task was used as secondarytask, we observed no priming-related early deection at all in the LRP.Nevertheless, therewas an additive pattern of interference of the non-spatial effect for all SOAs, both in performance and in LRP onsets. Theirrelevant color dimension of the stimulus seemed to impede theselection of the colored response buttons on the touch screenindependent of the temporal overlap with the primary task. Theresults of Experiment 2 in combinationwith those of Experiment 1 aretherefore in accord with the limited resources impede priming

hypothesis which posits that response priming related to stimulusonset should be present for the spatial, but not for the non-spatialtask. In contrast, the response discrimination account does notassume a specic pattern of interference for non-spatial in contrastto spatial conicts.

Although both spatial and non-spatial compatibility manipulationsresulted in substantial overall interference effects, they displayed acompletely different pattern of dual-task interference as apparent inperformance as well as in the LRP. This might be attributed to a specicvisuomotor processing route for action-relevant spatial information inthe brain. Location and other action-related information of an object isknown to activate a dorsal processing stream involving parietal brainareas that establish a visuomotor interface directly preparing visualinformation for action (e.g. Riehle, Kornblum, & Requin, 1997). Astimulus feature not providing action-relevant information, such ascolor, is more likely processed via a different processing routecorresponding to the ventral stream in the brain, which has beenconceptualized as slower and to a larger extent subject to intentionalcontrol (Milner & Goodale, 1995; Mishkin, Ungerleider, & Macko, 1983;for a reviewsee, e.g. Snyder, Batista, &Andersen, 2000). According to thedual-route notion, conicts on the spatial dimension in the Simon taskseem to arise between a direct and an indirect route, the formerpresumably reected by the early LRP deection. In contrast, in the non-spatial task, the conict might arise within the indirect route.

In any case, a theoretical approach accounting for S-R compatibilityeffects in general has to incorporate the differences between compati-bility effects based on action-relevant and other stimulus features. Toexplain the full pattern of Simon effects in dual tasks, the limitedresources impede priming and the response discrimination hypothesismight be integrated: At short SOA, the capacity of the system is taken upbyprocessing the primary task. According to the response discriminationaccount, it is assumed more specically that the formation of anirrelevant stimulus-location code is impaired under conditions of highworking memory load. Whether the critical capacity depletion at shortSOA in dual tasks indeed concerns, in the rst place, working memory(Ansorge & Whr, 2004) or other (central) processes such as attention(Melara, Wang, Vu, & Proctor, 2008) or response selection remains to beclaried in future studies. Nevertheless, the lack of available resourcesimpedes the processing of the irrelevant stimulus dimension in thespatial task; therefore, stimulus location does not prime the motorpreparation of the spatially compatible response. In the non-spatial taskat short SOA, depletion of capacity might also prevent immediateprocessing of the irrelevant dimension. As present results suggest,however, the conict in the non-spatial task does not arise between adirect and an indirect route, the former being activated with stimulusonset. Therefore, processing of the irrelevant, non-spatial dimension andthe corresponding response activation is postponed at short SOA, and sois the Simon conict.

To conclude, the present study replicated previous ndings ofdissociable spatial and non-spatial S-R compatibility effects. Whereasthe former is underadditivewith SOA in a dual task paradigm, the latteris additive. LRP analyses yielded several new results. In the non-spatialtask, there was no sign of early motor preparation related to priming bythe irrelevant stimulus dimension. In contrast, a priming of spatiallycorresponding responses via a direct link to response execution could beobserved shortly after stimulus onset in the spatial task. This priming-related early LRP deection in the spatial taskwas completely abolishedat strong temporal overlap with the primary task. The results of thepresent study emphasize the specic role of space in informationprocessing; spatial and non-spatial compatibility effects differ in theirappearance and temporal dynamics. This aspect is neglected in manystudies and accounts about S-R compatibility effects and should beconsideredmore carefully in future research. The present results furthercontribute to prior observations that the Simon compatibility effect ismodulated by contextual factors, e.g. stimulus-response mapping,readiness to react and conict in the previous trial (e.g. Strmer et al.,

2002; Valle-Inclan & Redondo, 1998). Availability of resources turns out

A.2. Results

in the incompatible than in the compatible condition (5.1 vs. 2.1%; seeFig. 7). The interaction between the two factors failed to reachsignicance [F(1, 15)=3.3, p=.087].

An RT distribution analysis was conducted to assess whether thespatial Simon effect dissipates with increasing response time. The RTsfor each compatibility condition of the spatial Simon task weredivided into quartiles. The interaction of ANOVA factors RT-Quartileand Compatibility failed signicance by a wide margin [Fsb1]. For thepresent implementation of the spatial task, a decay function of theSimon effect with time can therefore be excluded.

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Acknowledgements

This research was supported by a grant from the BerlinerProgramm zur Frderung der Chancengleichheit von Frauen inForschung und Lehre to the rst author.

Appendix A. Single task control experiment

A.1. Method

A.1.1. ParticipantsA group of 16 participants (8 female), mean age 24.8 years, took

part in this study. Participants were recruited from a pool of theHumboldt-Universitt zu Berlin; they were reimbursed with coursecredits or 7 per hour. Fourteen participants were right-handed; theothers were left-handed (questionnaire adapted from Oldeld, 1971).Furthermore, all participants had normal or corrected-to-normalvision and were without any neurological or neuropsychologicaldisorder according to self-report. Prior to the experiments informedconsent was obtained.

A.1.2. Apparatus and StimuliThe visual stimuli and the square elds that served as response

keys were displayed on a touch screen (Keytec Inc.) with the stimuliin the upper half and the response keys in the lower half (see Fig. 1 forthe basic design). The touch screen was positioned at a slanted anglewith the stimulus display area at eye level and about 50-60 cm in frontof the participants. For the spatial Simon paradigm, the stimuli werethe capital letters S or X, presented in light grey on blackbackground (font size 76 pt). For the non-spatial Simon task, thenumerals 4 or 8 served as stimuli, and were presented either inyellow or blue on the black background (font size 76 pt). The responsekeys were vertically arranged blue or yellow squares (edge length4.5 cm). Whether the upper or the lower key was colored yellowchanged randomly from trial to trial.

The touch screen was covered with a transparent pane of glass onwhich the participants hands were rested with an open area at thelocation of the response keys, allowing for comfortable operation ofthe response keys with the index ngers. Half of the participantsoperated the upper response key with their right index nger and thelower eld with the left index nger. This assignment was reversedfor the other participants. Stimulus and response key presentationand the recording of responses were controlled by Presentationsoftware (Neurobehavioural Systems, Inc.).

A.1.3. Procedure and DesignThe participants sat in front of the touch screen with hands resting

on (the glass pane of) the touch screen. A trial started with the onsetof the square response keys on the screen and a small xation mark atthe center of the stimulus eld (cf. Fig. 1). After an interval of 900 ms astimulus was presented: For the spatial Simon task, one of the lettersX or S appeared above or below the xation mark. For the non-spatial Simon task, one of the numerals 4 or 8 was presented atrandom in yellow or blue at the location of the xation mark. Forboth versions of the Simon task, participants had to indicate theidentity of the stimulus with their response, whereas the mappingdiffered depending on the specic version: In the spatial Simon task,the upper and lower response keys were to be pressed if an S or anX was presented, respectively, or vice versa counterbalancedacross participants. In the non-spatial Simon task, the participants

were requested to press the yellow or blue key if an 8 or a 4Overall, mean RTwas 578 ms. In the ANOVA there was a signicantmain effect of Simon Version, F(1, 15)=33.2, pb .001. Mean RT waslonger by 138 ms in the color than in the spatial version of the Simontask (see Fig. 7). Furthermore, there was a signicant main effect ofCompatibility, F(1, 15)=115.5, pb .001. The mean Simon effect in RTswas 48 ms, which did not differ between the spatial and the color task[Fb1].

The error rate was on average 3.6%. A two-factor ANOVA withSimon Version and Compatibility revealed signicant main effects ofeach factor, F(1, 15)=7.2 and 8.9, pb .05 and .01, respectively. Meanerror rates were higher in the color than in the spatial Simon task (4.9vs. 2.3%). Therewas also a Simon effect in error rates, withmore errorsappeared, respectively, the mapping again being balanced acrossparticipants.

The visual stimuli were shown until a response was given or until2 s had passed. The latter case was classied as an error. An error wasalso registered if response time was less than 150 ms. Errors were fedback by a message on the screen. The experiment consisted of 10blocks of 56 trials each. Half of the blocks administered the spatial, andthe other half the non-spatial Simon task. The order of taskadministration was balanced across participants. Additionally, twopractice blocks one for each task were conducted but excludedfrom the analyses. After each block of trials, the participants couldtake a self-determined break.

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Differential dynamics of spatial and non-spatial stimulus-response compatibility effects: A dual task LRP studyIntroductionExperiment 1MethodParticipantsApparatus and StimuliProcedure and DesignElectrophysiological RecordingsData analysis

ResultsPerformanceTask 1Task 2

Task 2 LRP

Discussion

Experiment 2MethodResultsPerformanceTask 1Task 2

Task 2 LRP

Discussion

General discussionAcknowledgementsSingle task control experimentMethodParticipantsApparatus and StimuliProcedure and Design

Results

References