Functional magnetic resonance imaging (fMRI) studies have identified brain structures involved in detecting/resolving conflict at the stimulus and response levels [1, 2]. In addition, event-related brain potential (ERP) studies have identified a succession of events in stimulus/response conflict resolution [3–5]. However, so far studies have been limited by the fact that there is no independent measure of stimulus/response conflict. Hence, inferences were based on paradigmatic assumptions about the presence/absence of stimulus/response conflict in certain tasks. We have overcome this limitation  by measuring response conflict directly using electro-myography (EMG). Here we extended our previous methodology by manipulating response conflict at the paradigm level and checking the success of this manipulation by EMG. This method enabled us to determine whether a robust ERP marker of conflict detection/resolution, the N450 ERP wave, was related to stimulus or response conflict. The N450 has previously been linked to conflict effects emanating from the anterior cingulate cortex (ACC). Therefore, clarifying the functional significance of the N450 may enable us to time ACC conflict effects detected by fMRI.
In principle, Stroop conflict can appear at the level of stimulus representations (stimulus conflict) or at the level of motor response organization (response conflict). According to the stimulus conflict view, Stroop conflict appears because the representations of parallel processed stimulus dimensions are incongruent with each other . According to the response conflict view, Stroop conflict appears during the organization of motor responses (e.g. vocalizations in oral or button presses in manual Stroop tasks). This so-called 'horse-race model' assumes that motor responses are primed by both task-relevant and task-irrelevant stimulus dimensions [8, 9]. Conflict appears when parallel activated correct and incorrect responses begin to compete to dominate overt response activity. Discriminating between brain markers of stimulus and response conflict has been the subject of several neuroimaging studies.
fMRI studies have pointed to several brain areas involved in conflict processing. The most prominent of these areas is the ACC which has been shown to be active in nearly all studies examining conflict processing. According to a prominent theory, the conflict monitoring view [10–12], the activity of the ACC increases in response to conflicting information. Initially, the conflict monitoring view mostly emphasized the role of ACC in detecting/resolving response conflict [1, 13, 14]. However, evidence has now accumulated demonstrating that the ACC can be activated not only by response but also by stimulus conflict [2, 15–18]. This suggests that the conflict-related activation of the ACC is wider than originally thought, and the ACC is involved in conflict monitoring across several information domains  Naturally, the ACC has not been the only brain area implicated in conflict processing. It has been shown that both the posterior parietal cortex [1, 19] and the inferior parietal cortex  may be related to stimulus conflict processing. Similarly, response conflict has been shown to modulate premotor cortex  and dorsolateral prefrontal cortex activity .
ERP studies have identified a succession of events in paradigms involving conflict processing [4, 6, 21]. These studies have typically compared ERP amplitude in the incongruent vs. congruent conditions of various Stroop paradigms. The timing of these congruency-related effects ranges from relatively early (till about 220 ms after stimulus presentation) amplitude differences [21–23] to later (300-600 ms) amplitude effects [3–6, 24]. Early effects have usually been interpreted as being related to stimulus conflict and late effects as being related to response conflict. The most prominent ERP effect is called the N450 effect. The N450 has been detected in most ERP studies of the Stroop conflict [3, 5, 6, 24, 25]. The N450 has negative polarity in incongruent minus congruent difference potentials, a latency of about 300-500 ms and centro-parietal topography. Previous ERP source localization attempts have consistently found that most variance in the topography of the N450 can be explained by dipoles in the ACC [3, 6, 26]. Henceforth, both the prominence of the N450 in conflict situations and source localization results linking the N450 to the ACC make it a likely conclusion that the N450 may be related to consistent conflict-related ACC activity detected by fMRI studies. However, a better understanding of the functional role of the N450 is hindered by the fact that it is unclear whether the N450 is related to stimulus or response conflict.
With regard to the above question, in some previous Stroop studies we measured the timing of response preparation by employing the Lateralized Readiness Potential (LRP), an ERP measure of correct/incorrect response activation [4, 21, 25, 27]. In these studies the LRP index of incorrect response activation could potentially serve as a measure of response conflict. However, we were able to show an LRP marker of incorrect response activation, ie. response conflict, in correctly responded trials of normal adults only in one of four studies . This was probably due to limitations of ERPs due to volume conduction (see [28, 29] for details). In a recent study  we aimed to solve the above problem by measuring response conflict directly at the effector level by the synchronous recording of electro-myography (EMG) and electro-encephalography (EEG). Our approach was built on seminal studies using EMG in Flanker and Simon tasks [28, 30–33]. We used a numerical Stroop paradigm where participants detected which of two Arabic digits was physically larger . In the congruent condition the physically larger digit was also numerically larger than the other one (e.g. 2 8). In the incongruent condition the physically larger digit was numerically smaller than the other one (e.g. 2 8). EMG detected robust incorrect response hand activity in correctly responded trials of the incongruent condition but not of the congruent condition. This incorrect response hand activity temporally coincided with activity in the correct response hand (there was an overt correct response). This provides unequivocal evidence for the presence of response conflict independent of paradigmatic assumptions in Stroop tasks.1 We also detected an N450 ERP effect in the incongruent, relative to the congruent, condition right after the offset of incorrect hand activity. This confirmed that the N450 is probably closely related to conflict detection/resolution . However, because the focus of our previous study was the direct demonstration of response conflict in Stroop tasks (by comparing the incongruent and neutral/congruent conditions), we did not manipulate the amount of stimulus/response conflict within the incongruent condition. Therefore, we could not effectively test whether the N450 was related to stimulus or response conflict.
Here we have manipulated the amount of response conflict within the incongruent condition in order to determine whether the N450 ERP effect was related to stimulus or response conflict. In order to further generalize our previous findings to a slightly different task than previously used, we used the numerical decision task of the numerical Stroop paradigm. That is, participants decided which digit was numerically larger than the other one. In this task a behavioral response is given with the hand corresponding to the side (left vs. right) where participants see a larger number (Additional file 1: Figure S1). Response conflict was manipulated by applying a cue on a trial-by-trial basis. In 50% of trials numerical stimuli were preceded by an effective cue (an arrow pointing to the left or right) which pointed into the direction of the expected response by 100% accuracy. In 50% of trials numerical stimuli were preceded by a neutral cue (a horizontal line) which did not convey any information about the side of responding. Five percent of trials with effective and neutral cues were no-go catch trials where subjects were instructed to give no response. No-go trials served to force maintaining stimulus analysis in effectively cued trials. In order to further confirm that stimulus analysis happened in effectively cued trials, we also examined numerical and physical size distance effects (distance effects: accuracy and reaction time depends on numerical/physical size discrimination difficulty).
It was expected that response conflict will decrease in effectively cued trials relative to trials with neutral cues because the response can already be fully prepared in effectively cued trials by the time the stimulus appears. In contrast, we expected that stimulus analysis, hence, stimulus conflict, will be maintained in effectively cued trials. We used EMG to provide paradigm-independent direct evidence for the amount of response conflict in effectively and neutrally cued trials. That is, EMG was used in a novel way, to check the success of our response conflict manipulation. Our question was whether the N450 appears both when response conflict is high and when response conflict is low. If the N450 appears equally in both situations, then the N450 can be related to stimulus conflict detection/resolution. In contrast, if the N450 is more expressed when response conflict is high, relative to when it is low, then it is more likely that the N450 is related to response conflict detection/resolution. We also used the peak latency of the P300 ERP wave because several previous Stroop studies [27, 35, 36] used this as a marker of stimulus conflict. However, it is to note that while this view is suggested by some , it is debated by others [38, 39].