Karin Heidlmayr

Maria Kihlstedt

Frédéric Isel
email: fisel@parisnanterre.fr

A review on the electroencephalography markers of Stroop executive control processes

Keywords: Stroop1, executive control2, conflict monitoring3, interference suppression4, conflict resolution5, neurophysiological components6

The present article on executive control addresses the issue of the locus of the Stroop effect by examining neurophysiological components marking conflict monitoring, interference suppression, and conflict resolution. Our goal was to provide an overview of a series of determining neurophysiological findings including neural source reconstruction data on distinct executive control processes and sub-processes involved in the Stroop task.

Consistently, a fronto-central N2 component is found to reflect conflict monitoring processes, with its main neural generator being the anterior cingulate cortex (ACC). Then, for cognitive control tasks that involve a linguistic component like the Stroop task, the N2 is followed by a centro-posterior N400 and subsequently a late sustained potential (LSP). The N400 is mainly generated by the ACC and the prefrontal cortex (PFC) and is thought to reflect interference suppression, whereas the LSP plausibly reflects conflict resolution processes. The present overview shows that ERP constitute a reliable methodological tool for tracing with precision the time course of different executive processes and sub-processes involved in experimental tasks involving a cognitive conflict. Future research should shed light on the fine-grained mechanisms of control respectively involved in linguistic and non-linguistic tasks.

Introduction

Executive control (EC), also called executive function or cognitive control, is constituted of a set of relatively heterogeneous cognitive processes that are involved in many high level functions of human cognition. EC is defined as the ability to flexibly adjust thoughts, actions and the processing of information to the challenges of a task at hand [START_REF] Braver | The variable nature of cognitive control: a dual mechanisms framework[END_REF][START_REF] Diamond | Executive Functions[END_REF][START_REF] Ridderinkhof | The Role of the Medial Frontal Cortex in Cognitive Control[END_REF]. For example, this function allows human beings to flexibly adapt to task demands during goal-directed behavior. EC is thought to be a complex cognitive function composed of relatively separable processes, such as inhibition, shifting and updating [START_REF] Miyake | The Nature and Organization of Individual Differences in Executive Functions: Four General Conclusions[END_REF] and their respective sub-processes, which are recruited to varying degrees in several tasks in order to provide the control required. EC is a core capacity of human cognition notably due to its domain-general nature, i.e. it is involved in numerous cognitive, affective/emotional and motoric processes. For instance, the relevance of executive control has been demonstrated for language processing [START_REF] Fedorenko | The role of domain-general cognitive control in language comprehension[END_REF][START_REF] Hagoort | MUC (Memory, Unification, Control)[END_REF][START_REF] Hagoort | The core and beyond in the language-ready brain[END_REF][START_REF] Ralph | The neural and computational bases of semantic cognition[END_REF], and in particular, for bilingual language use [START_REF] Green | Language control in bilinguals: The adaptive control hypothesis[END_REF]; see also the notion of bilingualism advantage in tasks involving executive functions), for motor and oculomotor coordination [START_REF] Aron | The Neural Basis of Inhibition in Cognitive Control[END_REF][START_REF] Munoz | Look away: the anti-saccade task and the voluntary control of eye movement[END_REF], and for emotion processing and regulation [START_REF] Ochsner | The cognitive control of emotion[END_REF], among others.

Moreover, the executive control capacity and its neural bases are of considerable plasticity.

On the one hand, over the lifespan, the frontal cortex -among other regions that are part of the neurocognitive executive control network -undergoes considerable developmental change from early childhood until young adulthood as well as with age-related alterations in neural processing. These neural changes are related to important functional variations not only in the capacity of executive control itself, but, due to the close interaction with various cognitive domains, such as language, motricity, or emotion, also in the capacity of domainspecific processing [START_REF] Diamond | Executive Functions[END_REF][START_REF] Liu | N450 as a candidate neural marker for interference control deficits in children with learning disabilities[END_REF][START_REF] Zelazo | Executive function across the life span[END_REF]. On the other hand, it has been shown that not only developmental changes shape EC capacity, but also activities that engage EC can strengthen it in the long run. As an example, a rapidly growing body of research has provided evidence that challenging neurocognitive activities, such as bilingualism, musical practice or sportive activity that requires a high degree of coordination, can in the long run lead to an activity-dependent strengthening of EC [START_REF] Aparicio | Inhibition Efficiency in Highly Proficient Bilinguals and Simultaneous Interpreters: Evidence from Language Switching and Stroop Tasks[END_REF][START_REF] Diamond | Biological and social influences on cognitive control processes dependent on prefrontal cortex[END_REF][START_REF] Dye | The development of attention skills in action video game players[END_REF][START_REF] Green | Language control in bilinguals: The adaptive control hypothesis[END_REF][START_REF] Heidlmayr | Neurodynamics of executive control processes in bilinguals: Evidence from ERP and source reconstruction analyses[END_REF]. In contrast, individual differences in EC capacity cannot be fully explained by activitydependent variation, but they are also largely determined by genetic factors [START_REF] Anokhin | Genetics, prefrontal cortex, and cognitive control: a twin study of event-related brain potentials in a response inhibition task[END_REF][START_REF] Friedman | Individual differences in executive functions are almost entirely genetic in origin[END_REF].

Over the past decades, in cognitive neuroscience a rapidly growing literature has improved our understanding of the neurodynamics of executive control processes. The Stroop task [START_REF] Stroop | Studies of Interference in Serial Verbal Reactions[END_REF] is one of the paradigms that allowed gaining most valuable insight.

However, a clear-cut picture of the neurophysiological signatures associated with the different sub-processes constituting the core of executive functioning has still not emerged in this field, certainly in part due to methodological issues, as it is sometime difficult to compare results of studies using different paradigms, tasks and populations, but also different typological distances between the first and second languages, in case bilingualism was under investigation. Given the importance of executive functions in various cognitive domains (e.g. language, emotion, motricity), the aim of the present review is to provide a state-of-the-art overview of the neurophysiological studies that have examined the electroencephalographical (EEG) signatures associated with the most discussed domain-general control processes -i.e., conflict monitoring, interference suppression, and conflict resolution -and their respective neural generators in the Stroop task. Here, we mainly concentrate on the Stroop effect, i.e., one of the most used tasks in cognitive sciences for approaching the issue of executive functioning in human beings. The question concerning the locus of the Stroop effect will be discussed by taking into consideration the neurodynamics as well as the neural bases of the different control processes thought to be involved for performing this experimental task. Our ultimate goal is to propose a precise description of the time course of these different executive processes. This description will be illustrated by a schematic presentation displaying for each process its electrophysiological signature and their possible neural generators.

In the Color Word Stroop task [START_REF] Stroop | Studies of Interference in Serial Verbal Reactions[END_REF], decision making is based on taskrelevant information in the face of distracting information. Indeed, "two conflicting mental representations are active, each associated with a different response, and attention must be paid to only relevant cues" (Bialystok et al., 2006(Bialystok et al., , p. 1342)). Specifically, an ink color must be identified while ignoring the written word itself. Since word reading is more automatic than color naming, executive control is required to override the tendency to respond on the basis of the word rather than the ink color. The need of such control is reflected in slower responses when the word name is competing with the ink color (i.e. incongruent condition like the word green written in red ink) than when it is not (i.e. congruent condition like the word green written in green ink; see Heidlmayr et al., 2013).

EEG recordings of event-related brain potentials (ERPs) allow us to follow with a high temporal resolution, i.e. millisecond by millisecond, the electrical responses of the brain to time-locked events [START_REF] Coles | Event-related brain potentials: an introduction[END_REF]. ERP data completed by a localization of their neural sources are particularly valuable for learning about the fine-grained spatio-temporal pattern of neural activity in different cortical areas sustaining the executive control network.

Moreover, an improved neuro-functional understanding can be obtained when these findings are related to data on a spatially much more fine-grained scale obtained in the neuroimaging literature. For this reason, we refer to this literature in different parts of the current review. To the best of our knowledge, the present review is the first attempt to present a state-of-the art overview of electroencephalographical studies examining a set of specific executive control processes including but extending beyond inhibition, their ERP markers and neural generators. In the first section, theoretical accounts and neurocognitive models of executive control are presented. Then, some of the most discussed executive control processes that play a role in managing conflict in linguistic (but also, marginally in non-linguistic) tasks in particular in the Stroop task, i.e. conflict monitoring, interference suppression, and conflict resolution will be characterized.

Executive control: Theoretical accounts and neurocognitive models

The emergence and crystallization of research on cognitive control historically coincided with the development of connectionism, with both domains undergoing considerable progress since the 1980s [START_REF] Botvinick | The Computational and Neural Basis of Cognitive Control: Charted Territory and New Frontiers[END_REF]. However, the initial theoretical foundations of the two fields are substantially different, in that initial research on cognitive control was grounded in principles of symbolic representation, sequential hierarchical processing, and modularity, and strongly focused on the 'top-down' processes of control. Subsequently, computational modelling had a strong influence on theories of cognitive control. Modelling in this phase took into consideration the role of learning and environmental constraints, and consequently was interested in 'bottom-up' processes and in the way how adaptation takes place in the cognitive control system. Current research is, among others, interested in the reasons why the structure of cognitive control, involving its architecture, representations and operations, and its underlying neural substrate are shaped the way they are. An important yet unresolved question is how the structure of cognitive control reflects the structure of the task environment, which is of core interest given the role of cognitive control in the interaction with naturalistic environments (for a review, see [START_REF] Botvinick | The Computational and Neural Basis of Cognitive Control: Charted Territory and New Frontiers[END_REF]. Currently, most psychological and neurobiological theories do not conceptualize cognitive control either as an unitary instance or as a system fractioned into different sub-processes, but mostly it is attempted to integrate elements of both approaches, unity as well as diversity of executive functions. In this vein, one of the most influential models has been put forward by [START_REF] Miyake | The Unity and Diversity of Executive Functions and Their Contributions to Complex "Frontal Lobe" Tasks: A Latent Variable Analysis[END_REF]. This model postulates a distinction of three main executive functions, namely inhibition of dominant responses ("inhibition"), shifting of mental sets ("shifting") and monitoring and updating of information in working memory ("updating"; see also the "unity/diversity framework" by [START_REF] Miyake | The Nature and Organization of Individual Differences in Executive Functions: Four General Conclusions[END_REF]. [START_REF] Miyake | The Nature and Organization of Individual Differences in Executive Functions: Four General Conclusions[END_REF] claim that according to the level executive functions are looked at, one may find shared characteristics amongst the three of the main executive functions (i.e. inhibition, shifting and updating) or one may be able to subdivide each of the functions into more specific control processes. The different executive functions may be involved to varying degrees according to the experimental task at hand, in order to enable for optimal coordination of control (see also, [START_REF] Diamond | Executive Functions[END_REF][START_REF] Miyake | The Nature and Organization of Individual Differences in Executive Functions: Four General Conclusions[END_REF]. Experimentally, Stroop task is considered to tap interference suppression (Color words are presented in different ink colors; in case of incongruency between color word and ink color, the automatic reading of the color word produces interference on the controlled task of responding to the ink color; [START_REF] Stroop | Studies of Interference in Serial Verbal Reactions[END_REF]; see also, Figure 1; other tasks like the Simon task, [START_REF] Simon | Auditory S-R compatibility: The effect of an irrelevant cue on information processing[END_REF] or the Eriksen Flanker task [START_REF] Eriksen | Effects of noise letters upon the identification of a target letter in a nonsearch task[END_REF]) also involve interference suppression). Here, we will particularly focus on the Stroop task. Interference suppression and response inhibition are generally considered to be reactive control processes, i.e. control processes that are active in reaction to an exogenous stimulus or signal.

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Executive functions: Subprocesses

In the following, some of the most discussed subprocesses of domain-general executive control that play a role to manage conflict in linguistic or non-linguistic tasks will be discussed: conflict monitoring, interference suppression, and conflict resolution. Following the description of each of these control processes, a review for the ERP markers that are frequently associated with the respective process is given, i.e. the N2 component for conflict monitoring and overcoming of inhibition, the N400 component and the late sustained potential for interference suppression and conflict resolution respectively.

Conflict monitoring and overcoming of inhibition: the N2 component

In this section two control processes, i.e. conflict monitoring and in the N2 section also the switching-related overcoming of inhibition will be discussed. In electroencephalographical studies, the ERP marker that is most robustly associated with these control processes is the N2 component and a review over these studies will be presented subsequently.

Conflict monitoring

Conflict monitoring has been defined as the process of monitoring for the occurrence of conflict in information processing and is on the evaluative side of cognitive control. Conflict monitoring serves to translate the occurrence of conflict into compensatory adjustments in control, i.e. the conflict monitoring system evaluates the levels of conflict and communicates this information to systems responsible for control implementation [START_REF] Botvinick | Conflict Monitoring and Cognitive Control[END_REF].

However, conflict monitoring is in most cases one of several control processes involved for realizing a task. Most theoretical frameworks of cognitive control distinguish conflict monitoring from inhibitory control and their respective underlying sources and electroencephalographical markers. However, a strong relation between the two control processes as well as their neural underpinnings is usually assumed. The conflict monitoring theory of cognitive control proposed by [START_REF] Botvinick | Conflict monitoring and decision making: reconciling two perspectives on anterior cingulate function[END_REF] as well as by [START_REF] Carter | Anterior cingulate cortex and conflict detection: An update of theory and data[END_REF] postulates a primordial role of the ACC in detecting conflicts while the dorsolateral prefrontal cortex is thought to modulate cognitive control over the suppression of taskirrelevant information. In the same vein, [START_REF] Macdonald | Dissociating the role of the dorsolateral prefrontal and anterior cingulate cortex in cognitive control[END_REF]see also, Green and Abutalebi, 2013) suggest that a widely distributed neural network may be activated in cognitive control processes but that specific subprocesses of control are reflected by spatially and temporally distinguishable activations, i.e. the anterior cingulated cortex (ACC) shows activation in conflict monitoring while the dorsolateral prefrontal cortex (DLPFC) is active in control implementation. Several fMRI studies lend support to the hypothesis that ACC plays a crucial role in (1) detecting conflict (i.e. conflict monitoring) and processing cognitive conflict as well as in monitoring action outcomes [START_REF] Botvinick | Conflict monitoring and decision making: reconciling two perspectives on anterior cingulate function[END_REF][START_REF] Botvinick | Conflict Monitoring and Cognitive Control[END_REF][START_REF] Carter | Parsing executive processes: strategic vs. evaluative functions of the anterior cingulate cortex[END_REF][START_REF] Chen | The neural mechanisms of semantic and response conflicts: An fMRI study of practice-related effects in the Stroop task[END_REF][START_REF] Gruber | Stroop Performance in Normal Control Subjects: An fMRI Study[END_REF][START_REF] Kerns | Anterior cingulate conflict monitoring and adjustments in control[END_REF][START_REF] Leung | An Event-related functional MRI Study of the Stroop Color Word Interference Task[END_REF][START_REF] Pardo | The anterior cingulate cortex mediates processing selection in the Stroop attentional conflict paradigm[END_REF][START_REF] Peterson | An event-related functional MRI study comparing interference effects in the Simon and Stroop tasks[END_REF]van Veen andCarter, 2005, 2002a;[START_REF] Yeung | Conflict monitoring and cognitive control[END_REF]; for a review on the controversial findings in neuropsychological studies, see [START_REF] Yeung | Conflict monitoring and cognitive control[END_REF] and (2) attentional control in cognitive and emotional processes [START_REF] Bush | Cognitive and emotional influences in anterior cingulate cortex[END_REF][START_REF] Ochsner | Neural systems supporting the control of affective and cognitive conflicts[END_REF].

N2

The N2 (or N200) component is a negative-going component peaking at around 200 ms after stimulus onset. According to task-specificity and topographical distribution at the surface of the scalp, a distinction of three different subcomponents of the N2 has been suggested [START_REF] Folstein | Influence of cognitive control and mismatch on the N2 component of the ERP: A review[END_REF]. These subcomponents vary in scalp distribution and are thought to reflect different cognitive processes: (1) a fronto-central component reflecting novelty or mismatch (i.e. mismatch negativity (MMN); tasks: e.g. oddball task), ( 2) another fronto-central component reflecting cognitive control (e.g. conflict monitoring, response inhibition, response conflict and error monitoring; tasks: Eriksen Flanker task, Stop-Signal task, Go/Nogo task), and (3) a posterior component reflecting some processes of visual attention (for a review, see [START_REF] Folstein | Influence of cognitive control and mismatch on the N2 component of the ERP: A review[END_REF].

In the present review, we will only focus on the control-related N2 component, and more specifically on the N2 to reflect conflict monitoring. The control-related N2 effect thought to reflect specifically conflict monitoring has previously been found in studies using a Stroop task [START_REF] Boenke | Different time courses of Stroop and Garner effects in perception -An Event-Related Potentials Study[END_REF], Simon task (Chen andMelara, 2009), Eriksen Flanker task (van Veen andCarter, 2002a), and the Go/Nogo task [START_REF] Donkers | The N2 in go/no-go tasks reflects conflict monitoring not response inhibition[END_REF][START_REF] Jonkman | The development of preparation, conflict monitoring and inhibition from early childhood to young adulthood; a Go/Nogo ERP study[END_REF], with a large consensus of the ACC, as well as IFC and PFC, as the main neural generators of the N2; for a review, see Table 1. However, even within the controlrelated N2 effects functional distinctions can be found, e.g. despite a large similarity between the control-related N2 reflecting conflict monitoring and an error-related fronto-central negativity (ERN) -which is elicited by participants' errors and by negative feedback about task performance -the two components show a clearly distinct functional sensitivity and are distinguishable by principal component analyses [START_REF] Nguyen | N2 and P3 modulation during partial inhibition in a modified go/nogo task[END_REF] and can thus be considered as two distinct subcomponents [START_REF] Nguyen | N2 and P3 modulation during partial inhibition in a modified go/nogo task[END_REF]. The main neural generator of the control-related fronto-central N2 is the medial frontal cortex, more specifically the anterior cingulate cortex (ACC; [START_REF] Folstein | Influence of cognitive control and mismatch on the N2 component of the ERP: A review[END_REF]van Veen and Carter, 2002a).

Importantly, the ACC is the main neural generator in both, the N2 reflecting conflict monitoring as well as the ERN, though future research may possibly identify specific subregions of the ACC to be involved in each of these processes [START_REF] Folstein | Influence of cognitive control and mismatch on the N2 component of the ERP: A review[END_REF]. In contrast, the main neural generators of the auditory MMN include fronto-temporal regions, i.e. among others superior temporal regions including the primary auditory cortex; interestingly, in MMN paradigms involving a linguistic component, left hemispheric regions are more strongly involved (for reviews, see [START_REF] Garrido | The mismatch negativity: a review of underlying mechanisms[END_REF][START_REF] Pazo-Alvarez | MMN in the visual modality: a review[END_REF].

To sum up, in electroencephalographical studies, the N2 component is the main ERP marker associated with conflict monitoring and with some complex processes involving inhibition, such as the switching-related overcoming of previously applied inhibition, which is required in tasks such as negative priming or in tasks that involve language switching or non-linguistic switching of tasks or task rules. In Table 1, an overview of studies documenting an N2 effect in cognitive control tasks is proposed.

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Interference suppression: The N400 component and the Late sustained potential (LSP)

In this section, we will focus on the control process called interference suppression. The ERP marker usually associated with this process is the N400 component, and in the Stroop literature sometimes also the late sustained potential (LSP). Now, a review of the literature on the functional role of these components will be presented.

Interference suppression

The resistance to distractor interference is the ability to prevent interferences from information in the external environment that is irrelevant to the task at hand and which could disrupt ongoing processes [START_REF] Friedman | The Relations Among Inhibition and Interference Control Functions: A Latent-Variable Analysis[END_REF]. The capacity to suppress distractor interference is usually assessed using tasks such as the Stroop task [START_REF] Stroop | Studies of Interference in Serial Verbal Reactions[END_REF]; see also, Figure 1), the Simon task [START_REF] Simon | Auditory S-R compatibility: The effect of an irrelevant cue on information processing[END_REF] or the Eriksen Flanker task [START_REF] Eriksen | Effects of noise letters upon the identification of a target letter in a nonsearch task[END_REF]. For example, in a Stroop task, color words printed in an incongruent ink colors are presented to the participant (e.g. GREEN), who usually has to manually or verbally indicate the ink color of the stimulus. In this condition, a conflict arises between the automatic language process of word reading, which disturbs another process, of a more controlled nature, i.e. ink color naming. Hence, the interfering automatic process needs to be inhibited for correct performance in the task. Alternative accounts claim that Stroop is not necessarily to be considered as a task on interference suppression, but that it is rather a task involving prepotent response inhibition, i.e. the capacity to deliberately inhibit dominant, automatic, or prepotent responses when necessary [START_REF] Miyake | The Unity and Diversity of Executive Functions and Their Contributions to Complex "Frontal Lobe" Tasks: A Latent Variable Analysis[END_REF]; for a review, see [START_REF] Friedman | The Relations Among Inhibition and Interference Control Functions: A Latent-Variable Analysis[END_REF]. Concerning neuroanatomical localization of executive processes, some fMRI studies have also demonstrated the involvement of dorsolateral prefrontal cortex (DLPFC) activation in tasks evoking cognitive conflict, such as the Stroop task [START_REF] Chen | The neural mechanisms of semantic and response conflicts: An fMRI study of practice-related effects in the Stroop task[END_REF][START_REF] Macdonald | Dissociating the role of the dorsolateral prefrontal and anterior cingulate cortex in cognitive control[END_REF][START_REF] Milham | Practice-related effects demonstrate complementary roles of anterior cingulate and prefrontal cortices in attentional control[END_REF][START_REF] Peterson | An event-related functional MRI study comparing interference effects in the Simon and Stroop tasks[END_REF][START_REF] Van Veen | Separating semantic conflict and response conflict in the Stroop task: A functional MRI study[END_REF]. Moreover, ERP source localization analyses have identified the ACC [START_REF] Liotti | An ERP study of the temporal course of the Stroop color-word interference effect[END_REF][START_REF] Markela-Lerenc | Prefrontal-cingulate activation during executive control: which comes first?[END_REF] and/or the PFC [START_REF] Bruchmann | Individualized EEG source reconstruction of Stroop interference with masked color words[END_REF][START_REF] Qiu | Brain mechanism of Stroop interference effect in Chinese characters[END_REF] as potential neural generators involved in interference suppression. In ERP studies, interference suppression is frequently associated with an effect on the N400 component (cf. section 3.2.2) and a late sustained potential (cf. section 3.2.3).

N400

The N400 (or N4, N450 or Ninc) component is a negative-going component at posterior sites peaking at around 400 ms after stimulus onset. In psycholinguistics, this ERP component has first been shown by [START_REF] Kutas | Reading senseless sentences: brain potentials reflect semantic incongruity[END_REF] see also [START_REF] Federmeier | Time for meaning: Electrophysiology provides insights into the dynamics of representation and processing in semantic memory[END_REF]Kutas andFedermeier, 2011, 2000) as reflecting difficulties of lexical semantic integration (e.g., He spread the warm bread with *socks.) during the visual integration of words in English sentences. (for reviews, see [START_REF] Kutas | Thirty Years and Counting: Finding Meaning in the N400 Component of the Event-Related Brain Potential (ERP)[END_REF][START_REF] Lau | A cortical network for semantics: (de)constructing the N400[END_REF].

However, beyond the sensitivity of the N400 to semantic anomalies, an N400 effect has also been observed in cognitive control tasks involving a linguistic (semantic) component, and it has been interpreted to reflect of inhibitory processes and interference suppression. For example, in language switching tasks, an N400 effect has been suggested to reflect inhibitory processes. More precisely, in semantic comprehension tasks, e.g. a bilingual semantic categorization task [START_REF] Alvarez | Accessing word meaning in two languages: An event-related brain potential study of beginning bilinguals[END_REF][START_REF] Chauncey | Code-switching effects in bilingual word recognition: A masked priming study with event-related potentials[END_REF] or a task involving the evaluation of the semantic expectedness of sentence-final words [START_REF] Proverbio | Language switching mechanisms in simultaneous interpreters: an ERP study[END_REF], a larger N400 was found for language switching compared to repetition conditions, either in one switching direction (L1 to L2; [START_REF] Alvarez | Accessing word meaning in two languages: An event-related brain potential study of beginning bilinguals[END_REF] or in both directions (L1 to L2 and L2 to L1; [START_REF] Chauncey | Code-switching effects in bilingual word recognition: A masked priming study with event-related potentials[END_REF][START_REF] Proverbio | Language switching mechanisms in simultaneous interpreters: an ERP study[END_REF]. Furthermore, an N400 effect was also observed in several EEG studies examining temporal dynamics underlying the interference arising in the Stroop task [START_REF] Stroop | Studies of Interference in Serial Verbal Reactions[END_REF] and has been argued to reflect inhibitory processes or interference suppression. This effect reflects a larger negativity in the incongruent condition in comparison to the congruent condition or a neutral condition (a noncolor word or a string of characters written in one of the ink colors; [START_REF] Appelbaum | Priming and Backward Influences in the Human Brain: Processing Interactions during the Stroop Interference Effect[END_REF][START_REF] Badzakova-Trajkov | An ERP investigation of the Stroop task: The role of the cingulate in attentional allocation and conflict resolution[END_REF][START_REF] Bruchmann | Individualized EEG source reconstruction of Stroop interference with masked color words[END_REF][START_REF] Coderre | Electrophysiological measures of conflict detection and resolution in the Stroop task[END_REF][START_REF] Hanslmayr | The electrophysiological dynamics of interference during the Stroop task[END_REF][START_REF] Liotti | An ERP study of the temporal course of the Stroop color-word interference effect[END_REF][START_REF] Naylor | Cognitive and Electrophysiological Correlates of the Bilingual Stroop Effect[END_REF][START_REF] Qiu | Brain mechanism of Stroop interference effect in Chinese characters[END_REF][START_REF] West | Neural correlates of cognitive control and conflict detection in the Stroop and digit-location tasks[END_REF]; for a review, see Table 2). However, it remains unclear whether this component does reflect partially shared cognitive processes with the classic N400 first identified by [START_REF] Kutas | Reading senseless sentences: brain potentials reflect semantic incongruity[END_REF][START_REF] Silton | The time course of activity in dorsolateral prefrontal cortex and anterior cingulate cortex during top-down attentional control[END_REF]. The so-called N400 Stroop effect usually mirrors the behavioral Stroop effect, i.e. longer response times in the incongruent compared to the congruent condition, response times to neutral stimuli lying in between. A larger negative deflection in the incongruent compared to the congruent and neutral conditions in the time window 400-500 ms post stimulus onset (N400 effect) is interpreted to sign the higher cognitive cost in responding to stimuli in the incongruent condition -usually causing a conflict between the two sources of information, the color word and the ink color. Some studies investigating the localization of the main neural generators of the N400 Stroop interference effect have shown that the difference of N400 amplitude between the incongruent and congruent conditions mainly originates in the ACC [START_REF] Badzakova-Trajkov | An ERP investigation of the Stroop task: The role of the cingulate in attentional allocation and conflict resolution[END_REF][START_REF] Bruchmann | Individualized EEG source reconstruction of Stroop interference with masked color words[END_REF][START_REF] Hanslmayr | The electrophysiological dynamics of interference during the Stroop task[END_REF][START_REF] Liotti | An ERP study of the temporal course of the Stroop color-word interference effect[END_REF][START_REF] Markela-Lerenc | Prefrontal-cingulate activation during executive control: which comes first?[END_REF] and the prefrontal cortex (PFC; [START_REF] Badzakova-Trajkov | An ERP investigation of the Stroop task: The role of the cingulate in attentional allocation and conflict resolution[END_REF][START_REF] Bruchmann | Individualized EEG source reconstruction of Stroop interference with masked color words[END_REF][START_REF] Hanslmayr | The electrophysiological dynamics of interference during the Stroop task[END_REF][START_REF] Liotti | An ERP study of the temporal course of the Stroop color-word interference effect[END_REF][START_REF] Markela-Lerenc | Prefrontal-cingulate activation during executive control: which comes first?[END_REF][START_REF] Qiu | Brain mechanism of Stroop interference effect in Chinese characters[END_REF]. In contrast, the typical semantic N400 is mainly generated by superior and middle temporal, anterior temporal, medial temporal and dorsolateral frontal regions (for a review, see [START_REF] Kutas | Thirty Years and Counting: Finding Meaning in the N400 Component of the Event-Related Brain Potential (ERP)[END_REF]. Table 2 displays a review of the functional interpretation of the N400 effect in the Stroop task and related tasks requiring cognitive control. The largely differing but partially shared (e.g. frontal) localization of the neural generators of the semantic and executive N400s suggests that the underlying processes of these ERP components may be considerably different but also share some aspects of control.

-Insert Table 2 about here --

Late sustained potential (LSP)

In several electroencephalographical studies using a Stroop task -i.e. a cognitive control task considered as involving a linguistic (semantic) component -, a further ERP component was found in the time window of about 550 -800 ms, that is a sustained fronto-central negativegoing potential, i.e. a late sustained potential (LSP; [START_REF] Hanslmayr | The electrophysiological dynamics of interference during the Stroop task[END_REF][START_REF] Heidlmayr | Neurodynamics of executive control processes in bilinguals: Evidence from ERP and source reconstruction analyses[END_REF][START_REF] Naylor | Cognitive and Electrophysiological Correlates of the Bilingual Stroop Effect[END_REF][START_REF] West | Neural correlates of cognitive control and conflict detection in the Stroop and digit-location tasks[END_REF] note that this component has varying names with the different authors, e.g. late negativity (LN; [START_REF] Hanslmayr | The electrophysiological dynamics of interference during the Stroop task[END_REF], sustained negativity (SN; [START_REF] Naylor | Cognitive and Electrophysiological Correlates of the Bilingual Stroop Effect[END_REF], conflict sustained potential (SP; [START_REF] West | Neural correlates of cognitive control and conflict detection in the Stroop and digit-location tasks[END_REF], or late positive complex (LPC; [START_REF] Donohue | The neural dynamics of stimulus and response conflict processing as a function of response complexity and task demands[END_REF]). It is to be noted that some studies also found an additional centro-parietal positive deflection in the incongruent compared in the congruent condition [START_REF] Appelbaum | Priming and Backward Influences in the Human Brain: Processing Interactions during the Stroop Interference Effect[END_REF][START_REF] Coderre | Electrophysiological measures of conflict detection and resolution in the Stroop task[END_REF][START_REF] Hanslmayr | The electrophysiological dynamics of interference during the Stroop task[END_REF][START_REF] Liotti | An ERP study of the temporal course of the Stroop color-word interference effect[END_REF][START_REF] West | Neural correlates of cognitive control and conflict detection in the Stroop and digit-location tasks[END_REF]. The sustained centro-parietal positivity and/or frontal negativity was discussed to reflect either engagement of executive processes [START_REF] Hanslmayr | The electrophysiological dynamics of interference during the Stroop task[END_REF], conflict resolution processes [START_REF] Coderre | Electrophysiological measures of conflict detection and resolution in the Stroop task[END_REF][START_REF] Heidlmayr | Neurodynamics of executive control processes in bilinguals: Evidence from ERP and source reconstruction analyses[END_REF][START_REF] Naylor | Cognitive and Electrophysiological Correlates of the Bilingual Stroop Effect[END_REF][START_REF] West | The effects of aging on controlled attention and conflict processing in the Stroop task[END_REF], semantic reactivation of the meaning of words following conflict resolution [START_REF] Appelbaum | Priming and Backward Influences in the Human Brain: Processing Interactions during the Stroop Interference Effect[END_REF][START_REF] Liotti | An ERP study of the temporal course of the Stroop color-word interference effect[END_REF] or response selection [START_REF] West | Neural correlates of cognitive control and conflict detection in the Stroop and digit-location tasks[END_REF][START_REF] West | The effects of aging on controlled attention and conflict processing in the Stroop task[END_REF]. Source localization has rarely been done for the late sustained negative-going potential but there is some evidence of its main neural generators in the middle or inferior frontal gyrus and the extrastriate cortex [START_REF] West | Neural correlates of cognitive control and conflict detection in the Stroop and digit-location tasks[END_REF]. [START_REF] West | Neural correlates of cognitive control and conflict detection in the Stroop and digit-location tasks[END_REF] suggests that the left middle frontal gyrus and extrastriate cortices are responsive to the presence of conflict, while the right middle frontal gyrus may be sensitive to conflict arising from the less dominant dimension (i.e. color) and may support some aspect of conflict resolution. In Table 3, a brief overview of studies documenting a late sustained potential in tasks involving cognitive control is given.

-Insert Table 3 about here --

A unified neurocognitive model of the executive control time course

The aim of the present article is to provide an overview of the electroencephalographical studies that targeted the examination of ERP signatures associated with the most discussed domain-general control processes (i.e., conflict monitoring, interference suppression, and the switching-related overcoming of inhibition) and their respective neural generators. Figure 2 displays a description of the time course of these different executive processes and their ERP signatures and neural generators. The selection of these specific control processes is justified by the fact that these processes were considered most relevant in the literature, and therefore the most discussed. We agree that further control processes beyond those reported presumably also play an important role and should obtain more focus in future research.

Moreover, it is important to note that there is no single explanation of the control mechanisms in cognition, in particular in bilingual mind, which is characterized by joint activation of the two languages. In a recent review on bilingual adaptation, Bialystok (2017) claimed that attention system should also be considered to better understand the underlying mechanisms enabling bilingual mind to select the appropriate language and avoid interference from an unwanted language. Interestingly, Sholes et al. ( 2007 reported that pupil diameters increased for color distractors that differed from color responses, while they reduced for color distractors that were identical to color responses. The replication of the Stroop effect with recording of pupillary diameter, i.e. a marker of attention-grabbing stimuli (for a review, see [START_REF] Loewenfeld | The pupil: anatomy, physiology, and clinical applications[END_REF] reinforces the idea that attentional resources (here, visual attention) may play a role in interferences control.

-Insert Figure 2 about here -To sum up, the fronto-central N2 component is robustly found in tasks requiring conflict control, e.g. the Stroop, Simon or the Eriksen Flanker task, and is interpreted to reflect conflict monitoring processes as well as switching-related overcoming of previously applied inhibition, as in negative priming or in tasks that involve language switching. The neural generator of the N2 is thought to be the anterior cingulate cortex (ACC), the inferior frontal cortex (IFC) and the prefrontal cortex (PFC). Next, the posterior N400 has also been found in tasks requiring conflict control and has been suggested to reflect different processes, involving conflict monitoring and control implementation, i.e. interference suppression.

However, intriguingly, the N400 effect has mainly been found in cognitive control tasks involving a linguistic (lexical) component, e. From the present review we can draw inference about the following neurocognitive time courses of cognitive control as reflected in ERP patterns (see Figure 2). Based on the present review, a fronto-central N2 component is consistently found to reflect conflict monitoring processes or overcoming of inhibition, with its main neural generator being the anterior cingulate cortex (ACC). Interestingly, three distinct neurocognitive patterns emerge for the subsequent ERP components. On the one hand, for cognitive control tasks that involve a lexical component, the N2 is followed by a centro-posterior N400 and subsequently a late sustained potential (LSP). The N400 is mainly generated by the ACC and the prefrontal cortex (PFC) and is thought to reflect interference suppression, whereas the LSP is probably generated by the middle or inferior frontal gyrus and the extrastriate cortex and plausibly reflects conflict resolution. Moreover, in some studies and paradigms specific ERP components and their functional attribution cannot be unambiguously disentangled (e.g., [START_REF] Donohue | The neural dynamics of stimulus and response conflict processing as a function of response complexity and task demands[END_REF]. This is partially due to a certain heterogeneity in the literature concerning the identification of ERP components and the role that is attributed to them. With the present review article, we hope to have provided a contribution to a clearer characterization of ERP components and the cognitive functions that they have been suggested to reflect.

To conclude, the present review on electroencephalographical markers of executive control processes is intended to provide a comprehensive overview of our current knowledge on ERP markers of control and their neural generators in Stroop task (i.e. conflict monitoring, interference suppression, conflict resolution). Future research should shed light on the finegrained mechanisms of control respectively involved in linguistic and non-linguistic tasks.

But they should also look in to oscillatory activity and their functional significance with respect to cognitive control processes. Recently there has been much progress in our understanding of the functional significance of specific frequency bands with respect to cognitive and motor control, with theta (4-7 Hz) emerging as a marker of cognitive control (e.g., [START_REF] Cavanagh | Frontal theta as a mechanism for cognitive control[END_REF][START_REF] Hanslmayr | The electrophysiological dynamics of interference during the Stroop task[END_REF][START_REF] Mückschel | Interacting sources of interference during sensorimotor integration processes[END_REF], alpha (8-12 Hz) as a marker of inhibition (e.g., [START_REF] Jensen | Shaping Functional Architecture by Oscillatory Alpha Activity: Gating by Inhibition[END_REF][START_REF] Klimesch | Alpha-band oscillations, attention, and controlled access to stored information[END_REF][START_REF] Waldhauser | Alpha/Beta Oscillations Indicate Inhibition of Interfering Visual Memories[END_REF], or beta (13-30 Hz) as a marker of the maintenance of the current sensorimotor or cognitive state (e.g., [START_REF] Engel | Beta-band oscillations-signalling the status quo?[END_REF][START_REF] Heidlmayr | Multiple Language Use Influences Oculomotor Task Performance: Neurophysiological Evidence of a Shared Substrate between Language and Motor Control[END_REF].

Figure captions

Figure 1. The six most common tasks to study conflict monitoring, interference suppression and response inhibition. For each task, the most commonly used configuration and mechanistic interpretation of the underlying control processes is presented, but different variants of the tasks do exist in the literature. Stroop task [START_REF] Stroop | Studies of Interference in Serial Verbal Reactions[END_REF]. The task is to respond to the ink color of the stimulus. In the critical, i.e. incongruent, condition the automatic process of reading the incongruent color word interferes with the more controlled process of responding to the ink color, which is not the case in the control, i.e. congruent, condition where the color word and ink color refer to the same color. Simon task [START_REF] Simon | Auditory S-R compatibility: The effect of an irrelevant cue on information processing[END_REF]. The task is to respond to the direction (left, right) coded by the color of the stimulus. In the critical, i.e. incongruent, condition the automatic process of responding to the physical position of the stimulus interferes with the more controlled process of responding to the direction as coded by the stimulus color, which is not the case in the control, i.e. congruent, condition where the physical position of the stimulus and the direction coded by the stimulus color are identical. Eriksen Flanker task [START_REF] Eriksen | Effects of noise letters upon the identification of a target letter in a nonsearch task[END_REF]. The task is to respond to the direction (left, right) indicated by the middle arrow that is positioned within a string of arrows. In the critical, i.e. incongruent, condition responding to the direction indicated by the middle arrow suffers from the inference from the incongruent direction indicated by the surrounding (flanking) arrows, which is not the case in the control, i.e. congruent, condition where the direction indicated by the flanking arrows is identical with the target arrow direction. Go/Nogo task. The task is to respond to a specific type of stimulus (e.g. circle) but to withhold the manual response when a different type of stimulus is presented (e.g. cross). In the critical, i.e. nogo, condition withholding the manual response requires the suppression of a prepotent response tendency, which is not the case in the usually much more frequent (e.g. 75% of trials) control, i.e. go, condition where the manual response can be executed. Stop-Signal task [START_REF] Logan | On the ability to inhibit thought and action: A user's guide to the stop signal paradigm[END_REF]. The task is to respond to a specific type of stimulus (e.g. circle) but to stop the manual response when a specific signal is presented (e.g. beep). In the critical, i.e. stop, condition stopping the manual response requires the suppression of an already initiated response, which is not the case in the usually much more frequent (e.g. 75% of trials) control, i.e. go, condition where the manual response can be executed. Antisaccade task [START_REF] Hallett | Primary and secondary saccades to goals defined by instructions[END_REF]. The task is to make an eye movement (saccade) towards or opposite the direction (left, right) of a target stimulus presented on the screen, depending on a preceding color cue. In the critical, i.e. antisaccade, condition the color cue (e.g. fixation cross in the middle of the screen) is red which indicates that a saccade in the direction opposite to the upcoming target needs to be executed, which requires the overcoming of a prepotent response tendency towards the target and a change of the motor program towards the opposite direction. This is not the case in the control, i.e. prosaccade, condition where the color cue is green, indicating that a saccade towards the target can be executed.

  ) by investigating neurochemical basis of attentional control showed that acute serotonin and dopamine depletion are able to improve attentional control in healthy individuals as behaviorally attested by a significant Stroop interference decrease. Beyond the neurochemical issue, this study clearly showed that variations of attentional control influence the performance of a cognitive test involving interference control. More recently, Laeng, Ørbo, Holmlund and Miozzo (2011), using pupillometry (i.e., the measurement of changes in pupillary diameter) also demonstrated a link between visual attention and control of interferences in a Stroop task.Laeng et al.

  g. the Stroop task (see detailed discussion below), whereas for non-linguistic interference control tasks, e.g. the Simon or Eriksen Flanker task, mostly P3 effects have been reported. As the main neural generators of the N400 have been identified the ACC and the prefrontal cortex (PFC). Moreover, in several electroencephalographical studies using a Stroop task -i.e. a cognitive control task involving a linguistic (lexical) component -, a further ERP component was found in the time window of about 550 -800 ms, that is a sustained fronto-central negative-going potential, i.e. a late sustained potential (LSP). However, the functional attribution of this component is less univocal and it has been thought to reflect the engagement of conflict resolution processes, semantic reactivation of the meaning of words following conflict resolution, or response selection. As neural generators for this component have been identified the middle or inferior frontal gyrus and the extrastriate cortex, which have been argued to respond to the presence of conflict and to underlie some aspect of conflict resolution.

Figure 2 .

 2 Figure 2. A unified neurocognitive model of the executive control time course in the

Table 1 . The functional significance attributed to the N2 effect in tasks involving cognitive control.

 1 Time window indicates the time window in which an N2 effect was observed. Included were ERP studies that found a conflict-monitoring related N2 for which source localization analyses were conducted and/or which made use of a paradigm that allows to functionally specify the function of the N2. Studies are

	presented in alphabetical order of author names. ACC, anterior cingulate cortex; IFC, Inferior-frontal cortex; ITC, Inferior-temporal cortex;
	MCC, midcingulate cortex.
	N2 effect in tasks involving cognitive control
	Reference	Paradigm	Time window	Surface topography Functional attribution	Neural generator
	Boenke et al. (2009)	Stroop	268-360	Fronto-central	Cognitive control processes involved	Medial frontal cortex,
					in conflict detection and monitoring	including ACC
	Chen et al. (2008)	Partially	240-300	Fronto-central	Conflict detection	ACC
		incongruent
		categorization task
	Chen & Melara (2009)	Simon	360-400	Central	Working memory; disruption in	-
					working memory due to Stimulus-
					Response (S-R) conflict; conflict in
					information held in working memory
	Donkers & van Boxtel	Go/Nogo	200-350	Fronto-central	Conflict monitoring
	(2004)
	Enriquez-Geppert et al.	Combined Go/Nogo	20 ms around peak in	Fronto-central	Conflict monitoring	IFC, MCC
	(2010)	-Stop-Signal task	window 200-350;
	Frings & Groh-Bordin	Negative priming	170-270 (P2/N2	Frontal, fronto-polar	Selection of previously inhibited	-
	(2007)		complex)		stimulus against incompatible
					distractors
	Gajewski et al. (2008)	Response-cueing	200-320	Fronto-central	Response selection
		task

Table 2 . The functional significance attributed to the N400 effect in tasks involving cognitive control.

 2 Time window indicates the time window in which an N400 effect was observed. Included were ERP studies that found a control-related N400 for which source localization analyses were conducted and/or which made use of a paradigm that allows to functionally specify the function of the N400. Studies are presented in alphabetical order of author names. ACC, anterior cingulate cortex; IFC, inferior frontal cortex; IPS, intraparietal sulcus; PFC,

	prefrontal cortex.
	N400 effect in tasks involving cognitive control
	Reference	Paradigm	Time window	Surface topography Functional attribution	Neural generator
	Appelbaum et al. (2009)	Stroop	450-500	Centro-parietal	Central executive control processes	ACC (posterior part), left
					(detection and/or resolution of	parietal regions
					response conflict); semantic
					incongruency
	Badzakova-Trajkov et al.	Stroop	370-480	Centro-parietal	Attentional allocation/conflict	ACC
	(2009)				identification and resolution
	Brass et al. (2005)	Task-switching	440-520	Central	Task-set updating, incongruency	IFC, IPS
					between task meanings conveyed by
					current vs preceding cue
	Bruchmann et al. (2010)	Stroop	396-576	Centro-parietal	Conflict monitoring and processing	ACC, right PFC
	Coderre et al. (2011)	Stroop	400-500	Centro-parietal	Conflict detection	ACC
	Donohue et al. (2016)	Stroop,	323-621 (Stroop)	Fronto-central	Response conflict processing	-
		Flanker	303-479 (Flanker)
	Feroz et al. (2017)	Emotional Stroop	326-426	Fronto-central	Task/stimulus (task-irrelevant	Dorsal and rostro-ventral
					emotional meaning) conflict	ACC
					processing
	Frings & Groh-Bordin	Negative priming	330-420	Left-lateralized	Enhanced semantic processing	-
	(2007)

Table 3 . The functional interpretation of the LSP (late sustained potential) effect in tasks involving cognitive control.

 3 In the column presenting surface topography, it is indicated at which sites the incongruent condition shows a positive or a negative deflection relative to the congruent condition. Time window indicates the time window in which an LSP effect was observed. Included were ERP studies that found a control-related LSP for which source localization analyses were conducted and/or which made use of a paradigm that allows to functionally specify the function of the LSP. Studies are presented in alphabetical order of author names. ACC, anterior cingulate cortex; PFC, prefrontal cortex.

	LSP effect in tasks involving cognitive control
	Reference	Paradigm	Time	Surface	Functional attribution Neural
			window	topography		generator
	Appelbaum et al.	Stroop	850-900	Positive	Processing of semantic	-
	(2009)			deflection:	meaning of words
				parieto-
				occipital
	Brass et al.	Task-	600-800	Positive	Integrating the new
	(2005)	switching		deflection:	contextual information
				Centro-	to activate the relevant
				parietal	task set in case of
				(after 680	incongruency between
				also frontal)	task meanings
					conveyed by current vs
					preceding cue
	Chen & Melara	Simon	480-520	Positive	Maintenance of current	-
	(2009)			deflection:	stimulus-response
				parietal	relations in working
					memory rather than
					conflict resolution
	Coderre et al.	Stroop	600-900	Positive	Conflict resolution or	-
	(2011)			deflection:	post-resolution
				Centro-	processes
				parietal
	Donohue et al.	Stroop,	630-1000	Positive	Allocation of attention,
	(2016)		(Stroop)	deflection:	conflict resolution
		Flanker		Centro-
			566-1000	parietal
			(Flanker)
	Feroz et al.	Emotional	626-726	Negative	Response conflict	Dorsal and
	(2017)	Stroop		deflection:	processing; also:	rostro-ventral
				fronto-	emotional arousal	ACC
				central	involved in late
					attentional processes
					that engage higher-
					order cognitive control
					to overcome
					interference
	Hanslmayr et al.	Stroop,	600-800	Negative	Engagement of central	ACC
	(2008)	Negative		deflection:	executive processes
				fronto-

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7.

Tables