Psychological Research (2010) 74:291–301 DOI 10.1007/s00426-009-0247-x
O R I G I N A L A R T I CL E
Compatibility between stimulated eye, target location and response location Andrea Schankin · Fernando Valle-Inclán · Steven A. Hackley
Received: 28 April 2009 / Accepted: 20 May 2009 / Published online: 12 June 2009 © Springer-Verlag 2009
Abstract Responses to stimuli are faster when the stimulus location spatially corresponds to the required response (standard Simon eVect). Recently, a similar eVect has been observed with monocular stimuli. Responses were faster when the response location and the stimulated eye corresponded (monocular Simon eVect). It has been suggested that distinct mechanisms may underlie these two Simon eVects. Here, we attempted to study these two mechanisms simultaneously. For mean reaction time, a Wnding of perfect additivity was obtained. These behavioral data coupled with surface electrophysiological measures support the view that two diVerent mechanisms contribute independently to the monocular and standard Simon eVect.
Introduction Responses to a stimulus are faster if its location corresponds spatially with the side of the response, even when the stimulus location is irrelevant to the task (Simon &
A. Schankin (&) Institute of Psychology, University of Heidelberg, Hauptstrasse 47-51, 69117 Heidelberg, Germany e-mail:
[email protected] F. Valle-Inclán University of La Coruña, Elviña Campus, La Coruña, Spain S. A. Hackley University of Missouri-Columbia, Columbia, MO, USA
Rudell, 1967). This eVect of spatial stimulus–response compatibility (SRC), which has become known as the Simon eVect, has been widely investigated. So far, there is consensus about that the brain produces a spatial stimulus code that, in turn, transiently activates the spatially corresponding response. Simultaneously, the appropriate response is activated as deWned by the stimulus–response (S–R) mapping via a slower and controlled route. The behavioral eVects are caused by the temporal overlap of theses two processes at the response selection stage (de Jong, Liang, & Lauber, 1994; Kornblum, Hasbroucq, & Osman, 1990). However, how the spatial stimulus code is formed and the degree to which the activation of the compatible response should be considered as an obligatory stimulus-driven process is controversial (Proctor & Lu, 1999; Tagliabue, Zorzi, Umiltà, & Bassignani, 2000). Such a dual-route model predicts that the SRC eVects should decrease over time because the response activation driven by stimulus presentation is transient and time locked to stimulus onset (de Jong et al., 1994; Hommel, 1994). This is indeed the usual Wnding as long as the stimuli are presented left and right and the S–R mapping is held constant throughout the task. The model is supported by electrophysiological measures, especially from a scalp recording that is known as the lateralized readiness potential (LRP). The LRP reXects inter-hemispheric diVerences in activation between left and right motor cortex (Coles, 1989; de Jong, Wierda, Mulder, & Mulder, 1988; Gratton, Coles, Sirevaag, Eriksen, & Donchin, 1988). When measured locked to the stimulus onset (s-LRP), it provides a real-time index of response selection even when no response is executed (Miller & Hackley, 1992). On incompatible trials, i.e., when the stimulus location and response location are contralateral, the LRP shows incorrect response activation starting at around 150 ms after stimulus
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onset, which is then followed by correct response activation in a standard Simon task (de Jong et al., 1994; Stürmer, Leuthold, Soetens, Schröter, & Sommer, 2002). However, when the Simon task is slightly changed in certain ways, such as when participants cross their hands, the characteristics of the Simon eVect diVer. In that condition, the instruction given to the participants is exactly the same as in the standard condition, however, the RT distributions show that the eVect does not decay but increases with longer response times (Wascher, Schatz, Kuder, & Verleger, 2001). The s-LRP still shows incorrect response activations but of much smaller size than in the standard task. Similarly, when stimuli and response keys are arranged vertically, an increasing eVect function is also observed, and the s-LRP does not show any evidence of incorrect response activation (Wiegand & Wascher, 2005a, b). Based on these results, it was recently proposed that, depending on the task, two distinct mechanisms, visuomotor and cognitive, underlie the Simon eVect (Vallesi, Mapelli, SchiV, Amodio, & Umiltà, 2005; Wascher et al., 2001; Wiegand & Wascher, 2005a, b). Similar to the dual-route model by de Jong et al. (1994), it was suggested that an irrelevant spatial stimulus feature automatically activates and thus facilitates the corresponding response and that the two spatial codes interfere at the response selection stage. Under some speciWc circumstances, such as lateralized visual stimuli and a corresponding, natural hand position, an automatic activation of the corresponding response results in a facilitation of this response. This activation is thought to be based on information processing within highly privileged visuomotor pathways (visuomotor Simon eVect). In most other situations, for example, crossed hands or vertical S–R relations, the eVect involves spatial stimulus and response codes at a more abstract level which can inXuence selection (cognitive Simon eVect). These two mechanisms can be distinguished by the shape of the eVect function (but see also Roswarski & Proctor, 2003, for a discussion of using eVect functions; Wiegand & Wascher, 2007). The visuomotor information transmission is characterized by a decreasing eVect size as a function of RT quantile (visuomotor Simon eVect), and the code interference process of more cognitive spatial codes is associated with a stable, or even increasing, eVect function (cognitive Simon eVect). Recently, a further Simon-like eVect was observed when monocular stimuli were presented (monocular Simon eVect). In these experiments, the target stimulus, a black or white circle, was presented either to the left or right eye so that the stimulus was perceived by the subjects as centrally presented. Although participants were not aware of which eye was stimulated, reaction times were shorter for ispsilateral than for contralateral stimulus–response conWgurations (Valle-Inclán, Hackley, & De Labra, 2003; Valle-Inclán, Sohn, & Redondo, 2008).
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Purpose and rationale The aim of the present study is to investigate whether the mechanisms underlying the SRC eVect between stimulated eye and response might be the same or whether they diVer from those involved in the standard Simon eVect. This question was Wrst addressed by Valle-Inclán and colleagues (2008). Their experiments showed that the monocular compatibility eVect was constant across the RT distribution and that the s-LRP did not index an automatic activation of the ipsilateral response on incompatible trials. These results suggest that, in contrast to decaying eVects in a standard Simon task, the eye-of-origin information exerts a relatively long-lasting inXuence on response selection and that the monocular Simon eVect might rely on a diVerent mechanisms than the standard Simon eVect. However, because both Simon eVects were investigated independently of each other, little is known about whether they might share a common mechanism or whether they indeed operate independently of one another. In the current experiment, we therefore did not present a monocular stimulus centrally as in previous experiments but to the left or right of Wxation so that a spatial code of the stimulus position is also obtained. Our discriminative reaction stimulus was the letter “M” or “W” presented to the left or right of a Wxation cross. The opposite side contained a Wller, the “#” sign. This display was presented to one eye via a mirror stereoscope. The background was the same for both eyes, a neutral dark Weld, as in previous monocular Simon studies (Valle-Inclán et al., 2003, 2008). If the two spatial codes operate independently, then compatibility eVects on RT should be additive. A similar logic was already applied by Wiegand and Wascher (2005a) in order to disentangle the contributions of horizontal and vertical Simon eVects. The question is how responses are inXuenced by correspondence when they are compatible to both, the spatial code of the stimulus position and the eye-of-origin code. If the monocular and the standard Simon eVect are mutually exclusive, then responses in trials which both dimensions are corresponding should either be aVected by one mechanism or the other. If, however, both mechanisms are simultaneously active, the monocular and the standard Simon eVect should be additive. The use of dichoptic displays allowed us to assess on an exploratory basis whether a manipulation of an additional perceptual stimulation of one eye might inXuence either the standard or monocular Simon eVect. Instead of presenting the stimuli on a homogeneous background, identical for both eyes (as in previous experiments), in some conditions of the experiment a patterned background was presented only to one eye, which leads to a complete suppression of the eye without the pattern. Target stimuli will be consciously perceived when presented to the sup-
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pressed eye due to their abrupt onset. However, electrophysiological data imply that they will be processed diVerently than on trials in which they are delivered to the dominant eye (Mishra & Hillyard, 2008; Roeber & Schröger, 2004; Valle-Inclán, Hackley, De Labra, & Alvarez, 1999). If the interocular suppression diVerentially aVects the standard and the monocular Simon eVects this would constitute further evidence for independence of the two spatial codes.
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side) and a white Wxation cross (1° in diameter) were presented during the whole trial to promote fusion. As noted above, the capital letters “M” and “W” with a size of approximately 1° of visual angle served as target stimuli. They were presented either left or right of Wxation together with a distractor (“#”) of approximately the same size, on the opposite side. These stimuli were white and were presented only to one eye, superimposed upon either a stripe pattern or a black background. The stripe pattern consisted of horizontally arranged red and black stripes or vertically oriented green and black stripes.
Methods Procedure Participants Eighteen volunteers (9 women, 9 men, aged between 18 and 32 years, mean age 20.1 years) participated in the experiment after giving their written informed consent. All of them were right-handed and reported normal or corrected-to-normal vision. Participants received course credits in an introductory psychology class. The experiment was approved by the ethics committee of the University of Missouri-Columbia. Stimuli and apparatus Subjects were seated in a dimly lit, sound attenuated room with response buttons located under their left and right index Wngers (keys “A” and “L” on a keyboard). Stimuli were presented on a 17⬙ computer monitor with gray background that was positioned 35 cm in front of the participants. Subjects viewed the monitor through a mirror stereoscope (Fig. 1). Two white outlined squares (9.7° per
The optimal horizontal distance for fusion was adjusted individually at the beginning of the experiment. The fusion frames and Wxation cross were displayed continuously during a trial, and subjects were instructed to maintain Wxation at the center of the fused image (Fig. 1). Each trial started with the presentation of the fusion frames. One second later, the frame was Wlled in with the background display, either a black background to both eyes (“black condition”), stripe pattern to one eye and black to the other eye (“dominant” or “suppressed condition”, depending on which eye received the target), or the same stripe pattern to both eyes (“color condition”). The color condition was designed to facilitate comparisons of dichoptic trials with the main experimental trials (i.e., the black condition). Except for the black condition, the phenomenological impression throughout was that of a stripe pattern Wlling the fusion frame. The background was displayed for 1,000 ms. Then the letter and the distractor were presented simultaneously for
Fig. 1 Schematic representation of the experimental setup and examples of stimuli. The VGA monitor is viewed through a mirror stereoscope. Stimuli are presented to the left and right eye separately and perceived as one square positioned in the center. If a stripe pattern is presented only to one eye it is dominant while the black background is suppressed. However, as soon as the target stimulus is shown to the suppressed background it becomes dominant and the stripes are suppressed
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120 ms to either the left or the right eye, referred to as “stimulated eye.” On half of the trials an “M” was presented, on the other half of the trials, a “W”. All possible combinations of background, stimulated eye, location of the target with respect to Wxation, and target identity were mixed and presented equally often across the experiment. Participants had to identify which letter was presented as fast and as accurately as possible (e.g., press the left key for a “W” and the right key for an “M”; letters and keys were assigned to each other in counterbalanced fashion across participants). The inter-trial interval varied randomly between 2,000 and 3,000 ms. The experiment consisted of 768 trials, equally distributed across the four background conditions and both stimulus and eye-of-origin compatibilities. The stimulus presentation and response collection were controlled using the Psychtoolbox library (Brainard, 1997) running on a personal computer under Windows XP. ERP recording Electroencephalographic activity was recorded at Fz, C3, Cz, C4, T5, P3, Pz, P4, T6, O1, and O2 according to the International 10-20 System. During acquisition, all scalp electrodes were referenced to the left earlobe. They were then re-referenced oV-line to averaged earlobes. Forehead was used as ground. Horizontal and vertical electro-oculograms (EOGs) were recorded bipolarly from, respectively, sites lateral to the outer canthi of the eyes and above and below the right orbit. Electrode impedance was kept below 5 k. The EEG was sampled on-line at a rate of 500 Hz following ampliWcation with a band-pass of 0.1–30 Hz. Data were subsequently lowpass Wltered oZine with a cutoV of 8 Hz. Data analysis Response parameters Responses faster than 150 ms and slower than 1,000 ms were coded as errors and excluded from all analyses. The RTs were entered into an overall ANOVA including the factors background condition (black, color, dominant, and suppressed), eye-hand congruence (compatible vs. incompatible) and target-hand congruence (compatible vs. incompatible). Subsequently, separate ANOVAs were calculated for each perceptual background condition in order to test for eVects of each perceptual condition on each of the Simon-like eVects and their interaction. These analyses were performed regardless of whether the three-way interaction was signiWcant or not. Additionally, two eVect functions—one for compatibility with respect to the stimulated eye (monocular Simon
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eVect) and one for compatibility regarding the target location (standard Simon eVect)—were calculated by dividing the RT distribution into 5 bins of equal frequency for each participant, separately for each perceptual background condition. Within each bin, mean RTs were calculated (RatcliV, 1979). EVect functions for each compatibility eVect were obtained by subtracting the mean RT for corresponding from non-corresponding trials for each bin. The data were entered into a 4 (background condition) £ 2 (compatibility) £ 5 (bin) repeated-measures ANOVA and analyzed separately for target and eye compatibility. Greenhouse-Geisser epsilon corrections were performed when necessary. EEG parameters EEG was averaged oV-line for epochs of 1,600 ms, starting 100 ms prior to target onset, and ending 1,500 ms afterward. The baseline was corrected according to the activity in the ¡100 to 0 ms interval preceding the target. Trials showing evidence of artifacts (>70 V) in any of the electrodes were excluded from EEG analysis. Transmission of vEOG and hEOG into the EEG was corrected by removing ocular components identiWed by Independent Component Analysis (ICA, Delorme & Makeig, 2004). Similar to previous Simon eVect experiments, contralateral–ipsilateral diVerence potentials on trials without response errors were calculated time locked to the stimulus onset for the symmetrical electrode pairs over the left and right motor cortex (C3 and C4) by subtracting the EEG activity ipsilateral to the response hand from contralateral activity (Lateralized motor Readiness Potential, s-LRP). By subtracting ispilateral activity from contralateral activity, correct response activation produces a negative deXection and incorrect response activation is indexed by a positive going wave. Mean diVerences were calculated separately for left and right target stimuli (standard Simon eVect) as well as left and right stimulated eye (monocular Simon eVect) and subsequently averaged. s-LRPs were formed separately for corresponding and non-corresponding trials for each condition. Mean amplitudes for the s-LRP were calculated in time windows centered around the maxima (230 § 20 ms) that were determined in grand averages across subjects. The time windows selected was in accordance with previous Wndings that showed maxima around 250 ms (Valle-Inclán, 1996; Wascher, Verleger, & Wauschkuhn, 1996; Wascher & Wauschkuhn, 1996). Amplitudes measured in each participant’s data were entered into ANOVAs with the factors S–R compatibility (compatible vs. incompatible regarding stimulus position or stimulated eye) and background condition (black, color, dominant, suppressed).
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Results Behavioral data The main condition of the experiment was designed to assess whether the standard Simon eVect does or does not interact with the monocular variant of this eVect. The reaction time data for the Black Condition in Table 1 show that the two eVects exhibited perfect additivity: The monocular Simon eVect averaged 6 ms whether it was observed in conjunction with either compatible or incompatible trials of the standard Simon eVect. Conversely, the standard Simon eVect averaged exactly 26 ms during trials that were compatible and during those that were incompatible with respect to stimulated eye. Deviations of accuracy means from perfect additivity were trivial. The incompatible eyehand combinations tended to be more accurate, regardless of parafoveal target location (¡0.5 vs. ¡1%; rows), and accuracy was better on compatible than on incompatible trials for the conventional Simon eVect, regardless of stimulated eye (1.1 vs. 1.6%; columns). Main eVects and interactions for the experiment as a whole will now be summarized. There were no diVerences in accuracy among the four background conditions, F(3,51) < 1, = 0.727, 2 = 0.026. As noted above, accuracy was higher on incompatible than compatible trials as deWned with respect to stimulated eye, F(1,17) = 5.1, p < 0.05, 2 = 0.230, but tended to be higher on compatible than incompatible trials when these levels were deWned according to parafoveal target location, F(1,17) = 3.7, p = 0.071, 2 = 0.179. Planned separate analyses of each background condition revealed a signiWcant standard Simon eVect only in the dominant condition, F(1,17) = 6.0 p < 0.05, 2 = 0.261, and a trend in the black condition, F(1,17) = 2.9, p = 0.105, 2 = 0.147, but not in the other
two, both F(1,17) < 1, 2 < 0.044. A compatibility eVect with respect to the stimulated eye occurred only in the suppressed condition, F(1,17) = 4.1, p = 0.059, 2 = 0.194, but did not reach signiWcance in all other conditions, F(1,17) < 2.2, p > 0.156, 2 < 0.115. In none of the background conditions did the two compatibility eVects interact with one another (e.g., for the dominant condition: F(1,17) = 2.4, p = 0.136, 2 = 0.126, all other F < 1, p > 0.400, 2 < 0.043). Although average response times varied as a function of background condition, F(3,51) = 3.9, p < 0.05, = 0.845, 2 = 0.186, subsequent pair-wise comparisons showed only one reliable diVerence. RTs in the black condition were marginally faster than RTs in the color condition, t(17) = ¡3.0, p = 0.052. Responses to spatially compatible target locations were 22 ms faster (SD = 16) than responses to spatially incompatible target locations (standard Simon eVect), F(1,17) = 33.0, p < 0.001, 2 = 0.660. This eVect was similar across the four background conditions, F(3,51) = 1.3, p = 0.284, = 0.832, 2 = 0.071. In contrast, there was no general compatibility eVect of stimulated eye and response hand (monocular Simon eVect), F(1,17) = 3.0, p = 0.101, 2 = 0.150. In this case, the size of this SRC eVect did depend on background condition, F(3,51) = 6.4, p < 0.01, = 0.845, 2 = 0.273. Planned separate analyses for each background condition revealed a signiWcant standard Simon eVect in all conditions, all F(1,17) > 7.88, p < 0.05, 2 = 0.316, and, as already indicated by the missing two-way interaction of target and background mentioned above, this Simon eVect was not modulated by the background condition. A monocular Simon eVect of 6 ms (SD = 12) occurred only in the black condition, F(1,17) = 5.2, p < 0.05, 2 = 0.234. No eVect was observed in the dominant condition, F(1,17) < 1, p = 0.972, 2 = 0.000, and a reversed eVect was obtained in
Table 1 Mean response time (in ms) and accuracy rate (standard errors in parentheses) as a function of background condition, separately for target compatibility and eye compatibility Condition Dominant eye
Suppressed eye
Black eye
Color eye
Comp
Incomp
Comp
Incomp
Comp
Incomp
Comp
Incomp
Comp
550 (17)
548 (17)
554 (18)
547 (17)
541 (17)
547 (15)
561 (19)
553 (17)
Incomp
571 (16)
572 (16)
573 (16)
560 (17)
567 (15)
573 (16)
586 (15)
572 (16)
Comp
95.0 (1.2)
95.1 (1.1)
93.2 (1.5)
94.2 (1.2)
94.3 (1.2)
94.8 (1.1)
93.8 (1.5)
94.1 (1.2)
Incomp
91.9 (1.4)
94.1 (0.9)
91.8 (1.7)
94.6 (0.9)
92.2 (1.4)
93.2 (1.3)
93.0 (1.2)
93.1 (1.4)
Reaction time Target
Accuracy Target
Comp compatible, incomp incompatible
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both the suppressed [¡10 ms, SD = 15, F(1,17) = 8.8, p < 0.01, 2 = 0.341] and the color conditions [¡11 ms, SD = 17, F(1,17) = 7.3, p < 0.05, 2 = 0.301]. Critically, the two types of SRC did not interact with one another, neither in the overall analysis, F(1,17) < 0.5, 2 = 0.026, nor in any of the separate background analyses, all F(1,17) < 1.03, p > 0.324, 2 < 0.058, and the three-way interaction of background, eye compatibility, and target compatibility did not even approach signiWcance, F(3,51) < 0.5, = 0.732, 2 = 0.022. The eVect functions are presented in Fig. 2. The target location SRC eVect declined with increasing RTs, F(4,68) = 14.3, p < 0.001, = 0.453, 2 = 0.456, a pattern that was similar across the four background conditions, F(12,204) < 0.5, = 0.333, 2 = 0.022. In contrast, the SRC eVect of stimulated eye remained more or less constant, F(4,68) = 1.9, p = 0.165, = 0.492, 2 = 0.101, and eVect functions did not diVer across the background conditions, F(12,204) = 1.9, p = 0.118, = 0.379, 2 = 0.099. The proposed independency of the standard and monocular Simon eVect was further explored by computing eVect functions of one eVect while the compatibility relative to the other one was held constant. For example, the eVect function of the monocular Simon eVect was computed once for responses compatible to the target location and once for responses incompatible to the target location (Fig. 3). If the two Simon eVects operated independently, these two eVect functions should be similar. The original data from which the eVect functions were calculated are presented in Table 2. To evaluate the eVects statistically, data were entered into a 4 (perceptual background) £ 2 (eye compatibility) £ 2 (target compatibility) £ 5 (bin) repeated-measure ANOVA. Additional ANOVAs were run separately for each perceptual background, irrespective of whether one or more of the interactions with this factor was signiWcant. Independency of the eVects will be reXected by missing interactions
Fig. 2 Time course of stimulus–response compatibility (SRC) eVect (Y-axis) as a function of the mean response level (X-axis), separately for compatibility according to parafoveal target location (left panel) and stimulated eye (right panel)
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between the factors eye compatibility and target compatibility. As the main eVects are redundant to the analyses described above, only the interaction of the SRC eVects and the three- and four-way interactions with the factors perceptual background and bin will be reported. The overall ANOVA showed neither an interaction between eye SRC and target SRC, F(1,17) < 1, 2 = 0.026, nor any signiWcant three- or four-way interaction between the SRC eVects and the other factors, all Fs < 1, 2 < 0.046. However, separate analysis for each perceptual condition showed a slight trend toward a three-way interaction between the two SRC eVects and bin, F(4,68) = 3.0, p = 0.065, = 0.443, 2 = 0.153, in the dominant condition. Although the interaction is marginally signiWcant, this eVect is relatively small with an estimated eVect size of = 0.18. All other two- and three-way interactions failed to approach signiWcance, all Fs < 1, 2 < .045. Taken together, the eVect functions of one Simon eVect is statistically the same for compatible and incompatible responses relative to the other Simon eVect. Electrophysiological data As noted above, the Lateralized motor Readiness Potentials were calculated stimulus-locked to assess the presence of incorrect response activation on incompatible trials, a phenomenon that is characteristic of the standard but not the monocular Simon eVect. Figure 3 shows the LRP waveforms as a function of compatibility, separately for the standard and monocular Simon eVects. For the standard Simon eVect, within the time window of 210– 250 ms after stimulus onset, an error dip signifying incorrect response activation occurred on incompatible trials, F(1,17) = 50.3, p < 0.001, 2 = 0.747. This eVect did not diVer among background conditions, F(3,51) = 1.1, p = 0.339, = 0.833, 2 = 0.063, and strengthens the assumption of automatic, ipsilateral response activation via the visuomotor pathway.
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Fig. 3 EVect functions for the standard and the monocular Simon eVect, separately for each perceptual condition. The size of one Simon eVect is computed by subtracting response time (RT) to compatible from RT to incompatible trials, separately for compatible and incompatible responses relative to the other Simon eVect. The legend represents the condition, which was held constant, for example, the “Eye SRC eVect when target compatible” line shows the monocular Simon eVect for target compatible trials. If the two Simon eVects indeed operated independently, the two eVect functions for compatible and incompatible trials relative to the other Simon eVect should be the same
Table 2 Mean response time in ms (§SE) as a function of background condition, of compatibility relative to the stimulated eye and to the target location and of bin Bin
Condition Dominant eye
Suppressed eye
Black eye
Color eye
Comp
Incomp
Comp
Incomp
Comp
Incomp
Comp
Incomp
1
424 (10)
421 (11)
426 (12)
424 (11)
419 (11)
421 (10)
433 (12)
425 (12)
2
485 (14)
481 (14)
488 (15)
481 (13)
480 (15)
482 (12)
493 (16)
486 (15)
3
533 (16)
534 (17)
539 (17)
533 (15)
528 (18)
531 (15)
544 (19)
539 (18)
4
590 (19)
594 (21)
602 (21)
595 (20)
583 (20)
587 (19)
608 (23)
597 (22)
5
714 (27)
710 (23)
714 (25)
700 (26)
693 (25)
713 (23)
726 (29)
716 (24)
1
452 (10)
460 (12)
459 (12)
450 (11)
456 (11)
461 (11)
468 (10)
457 (10)
2
516 (13)
517 (13)
516 (14)
505 (13)
516 (13)
513 (13)
525 (13)
517 (13)
3
557 (15)
552 (14)
555 (16)
542 (15)
555 (15)
555 (16)
569 (15)
561 (15)
4
611 (18)
601 (16)
607 (18)
595 (19)
601 (18)
609 (18)
625 (18)
606 (18)
5
713 (25)
725 (24)
724 (22)
705 (27)
705 (22)
728 (25)
740 (24)
718 (24)
Target Comp
Incomp
Comp compatible, incomp incompatible
In the same time window, there was also a compatibility eVect relative to the stimulated eye (monocular Simon eVect), F(1,17) = 5.9, p < 0.05, 2 = 0.259, which was similar across the four background conditions, F(3,51) < 1, = 0.768,
2 = 0.005. In contrast to the standard Simon eVect, the activity tended to be less negative on compatible than incompatible trials. No evidence for automatic activation of the incorrect response on incompatible trials was obtained Fig. 4.
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Fig. 4 Grand averages of the lateralized readiness potential (LRP) time locked to the onset of the stimulus at electrodes placed over the motor cortices (C3, C4)
Discussion The goal of the present experiment was to assess the independence of two putative mechanisms underlying the standard and the monocular Simon eVect. We attempted to engage the two mechanisms simultaneously via a paradigm that combines the monocular version of the Simon eVect (Valle-Inclán et al., 2003, 2008) with the standard version. The results indicated that our combined paradigm did activate both mechanisms. Given the lateralized target stimuli and natural upper-body posture, we expected a standard Simon eVect. Consistent with previous Wndings (de Jong et al., 1994; Hommel, 1994), the diVerence between compatible and incompatible trials declined as a function of mean RT. Furthermore, surface electrophysiological recordings indicated that target onset triggered a brief, automatic activation of the hand on that side. This general pattern was found in all four background conditions. Similarly, we expected based on previous research that the monocular Simon eVect would exhibit a diVerent pattern (Valle-Inclán et al., 2003, 2008). The monocular eVect was obtained only in the main experimental condition, the one using a neutral dark background as had been employed in previous studies (Valle-Inclán et al., 2003, 2008). Importantly, the eVect did exhibit characteristics that points to a mechanism diVerent from the one underlying the standard Simon eVect. SpeciWcally, the size of the compatible– incompatible diVerence did not decay as a function of response time, and no automatic activation of the wronghand cortical region was observed on incompatible trials. Having successfully obtained both eVects in the same experiment, we were able to assess whether or not they interacted. Mean RT in the main experimental condition exhibited numerically perfect additivity, strong evidence
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that both mechanisms can contribute independently to the Simon eVect. The omnibus ANOVA for reaction time provided additional evidence: Neither the two-way interaction of eye compatibility and target compatibility nor the threeway interaction of these Simon eVects with background generated F values that exceeded unity. An unexpected Wnding was that the monocular eVect was strongly inXuenced by background stimulation. When the target and distractor stimuli were shown on black backgrounds presented to both eyes, as in previous studies, an eye-hand compatibility eVect was observed, reXected by shorter RTs for stimulation of the eye ipsilateral to the responding hand. This eVect reversed either when the background presented to the two eyes were the same stripe pattern (color condition) and when the target was delivered to the suppressed eye. The monocular Simon eVect was completely missing when the target was presented to the dominant eye. One possible explanation can be derived from a functional model of response inhibition, described by Houghton and Tipper (1994). In this model, the sequence of “facilitation followed by inhibition” reXects a basic principle in motor control. Response activation and inhibition act as opponent processes, with inhibition as an automatic consequence of response activation. SpeciWcally, self-inhibition terminates response tendencies unless their activation level is sustained by facilitatory input from other sources. Importantly, if perceptual evidence, as a primary source of response facilitation, indicates that a currently activated response is appropriate, a sustained response facilitation will dominate inhibition. If, however, perceptual evidence is eliminated, automatic self-inhibition is not longer counteracted by facilitatory input, and the initial response activation will be subject to inhibition (Eimer & Schlaghecken, 2002).
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This idea is supported by experiments using ‘subliminal priming’ where a masked prime stimulus is presented prior to a response-relevant target. Prime and target can either be mapped to the same response (compatible trial) or to diVerent responses (incompatible trials). Although primes cannot be identiWed by participants, responses to subsequent, clearly visible targets are aVected by prime-target compatibility. Depending on the temporal distance of prime and target, responses in compatible trials are faster and more accurate than responses in incompatible trials if the SOA is short (‘positive compatibility eVect’) or performance is better on incompatible than on compatible trials if the SOA is long (‘negative compatibility eVect’). In both cases, responses are initially activated by the presented prime. In the case of a short SOA, this facilitatory input has direct consequences on the subsequent response; if, however, the SOA is longer, self-inhibitory processes take place resulting in reversed eVects. This interpretation is further supported by eVects on the LRP as an index of selective response preparation and activation (Eimer & Schlaghecken, 2002), showing an initial partial activation of the response corresponding to the prime (Eimer, 1999; Eimer & Schlaghecken, 2002). If the SOA is long, self-inhibitory mechanisms suppress the initial activation; the early partial activation of the response assigned to the prime is replaced by an LRP of opposite polarity (Eimer, 1999; Eimer & Schlaghecken, 2002). This approach of response inhibition may also account for the present Wndings of the monocular Simon eVect under diVerent perceptual conditions. If the target is presented on a black background and a black background is also presented to the other eye (black condition), the perceptual evidence of the eye-of-origin code is quite strong— responses corresponding to this code are not inhibited, resulting in a monocular Simon eVect. In the dominant condition, the stripe pattern itself serves as a mask, and the target is less salient than when presented on a black background. Because the perceptual evidence is smaller than in the black condition, the monocular Simon eVect is reduced or missing in this condition. In the case of the suppressed and the color condition, a stripe pattern is presented to the eye the target is not presented to. This additional visual input may interfere with the eye-of-origin code produced by the target. Because the ‘perceptual evidence’ is quite low this manipulation results in response inhibition and a reversed monocular Simon eVect. A possible explanation for similar eVects in the suppressed and the color condition is that diVerent processes are operating in the two conditions. In the suppressed condition, both eyes are producing eye-of-origin codes; in the color condition, the target is less salient (contrast is reduced) and the eye-of-origin code is not so prominent. The outcome is the same in the two conditions, but the processes ‘masking’ the eye-of-origin are diVerent.
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This interpretation is partly supported by the current LRP data. In all perceptual conditions, the LRP shows an early activation of the wrong response hand for compatible trials which resembles the reversed eVects. In summary, our results support the idea that there are diVerent mechanisms underlying the standard and the monocular Simon eVect. At least Wve arguments support this interpretation: (1) perfect additivity of the two SRC eVects for RT, (2) nearly perfect additivity for accuracy, (3) opposite eVects on the initial LRP deXection, (4) completely diVerent patterns of background eVects for the monocular and conventional Simon eVects, and (5) dramatically diVering slope eVect function slopes for the monocular and conventional Simon eVects. Furthermore, the slope diVerence was in the predicted direction and statistically not distinguishable when controlled for correspondence relative to the other Simon eVect. The conclusion of independent eVects holds regardless of the unexpected eVects of background condition that we observed. Interestingly, our results nicely resemble the postulated diVerential mechanisms involved in the horizontal (or standard) and the vertical Simon eVect (Wiegand & Wascher, 2005a, b). The authors claim that the horizontal Simon eVect relies on a fast visuomotor mechanism, whereas the vertical Simon eVect bases on a cognitive mechanism. Both mechanisms are characterized by their speciWc eVect functions on the one hand and their LRP course on the other hand. Similar to the vertical Simon eVect, we observed a monocular Simon eVect that did not decay over time and did not show an initial activation of the ipsilateral response hand. However, in spite of the similarities, we can only speculate at this point whether the cognitive mechanisms claimed by Wiegand and Wascher also underlie the monocular Simon eVect observed in the present study or whether even a third mechanism may be responsible for a monocular compatibility eVect. It is further unknown how the eye-of-origin information can inXuence response selection processes because it is thought to be lost outside the visual cortex (Graham, 1992) and it is also outside of awareness. One explanation suggested by Valle-Inclán and colleagues is that the V6 complex, located in the parieto-occipital junction, might be involved. V6 contains a retinotopic map of the contralateral visual Weld (Di Russo, Martinez, Sereno, Pitzalis, & Hillyard, 2001), has reciprocal connections with V1 and the premotor cortex (Galletti, Battaglini, & Fattori, 1995), has been implicated in visuomotor coordination, and most of the recorded neurons showed strong eye preference (Shipp, 2004).
Conclusion Spatial codes of task-relevant stimuli, such as their location in the left or right hemiWeld or their eye-of-origin, can
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inXuence the selection of the required responses in diVerent ways. These two codes are not integrated in a common spatial stimulus code but operate independently of each other, as indicated by their eVect function and the stimulus-locked LRP, probably via diVerent neural pathways. Acknowledgments This study was Wnanced by a grant of the Deutsche Forschungsgemeinschaft (German Research Foundation, DFG) to the Wrst author (SCHA 1483/1-1).
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