Correction to: The neurocognitive underpinnings of the Simon effect: An integrative review of current research.

The original article was updated with the correct format of Fig. 2.

The neurocognitive underpinnings of the Simon effect: An integrative review of current research.

For as long as half a century the Simon task - in which participants respond to a nonspatial stimulus feature while ignoring its position - has represented a very popular tool to study a variety of cognitive functions, such as attention, cognitive control, and response preparation processes. In particular, the task generates two theoretically interesting effects: the Simon effect proper and the sequential modulations of this effect. In the present study, we review the main theoretical explanations of both kinds of effects and the available neuroscientific studies that investigated the neural underpinnings of the cognitive processes underlying the Simon effect proper and its sequential modulation using electroencephalogram (EEG) and event-related brain potentials (ERP), transcranial magnetic stimulation (TMS), and functional magnetic resonance imaging (fMRI). Then, we relate the neurophysiological findings to the main theoretical accounts and evaluate their validity and empirical plausibility, including general implications related to processing interference and cognitive control. Overall, neurophysiological research supports claims that stimulus location triggers the creation of a spatial code, which activates a spatially compatible response that, in incompatible conditions, interferes with the response based on the task instructions. Integration of stimulus-response features plays a major role in the occurrence of the Simon effect (which is manifested in the selection of the response) and its modulation by sequential congruency effects. Additional neural mechanisms are involved in supporting the correct and inhibiting the incorrect response.

Does bilingual experience influence statistical language learning?

Statistical language learning (SL) tasks measure different aspects of foreign language learning. Studies have used SL tasks to investigate whether bilingual experience confers advantages in acquiring additional languages through implicit processes. However, the results have been inconsistent, which may be related to bilingualism-related features (e.g., degree of dissimilarity between the specific language pair) and other variables such as specific processes that are targeted by the SL task. In the present study, we compared the performance of one Spanish monolingual and two bilingual (Spanish-Basque and Spanish-English) groups across three well-established SL tasks. Each task targeted a different aspect of foreign language learning; specifically, word segmentation, morphological rule generalization, and word-referent learning. In Experiment 1, we manipulated sub-lexical phonotactic patterns to vary the difficulty of three SL tasks, with the results showing no differences between the groups in word segmentation. In Experiment 2, we included non-adjacent dependencies to target affixal morphology rule learning, but again no group-related differences were found. In Experiment 3, we addressed word learning using an audio-visual SL task combining exclusive and multiple word-referent mappings, and found that bilinguals outperformed monolinguals, suggesting that bilingualism may exert influences on SL at the lexical level. This advantage might have been mediated by the high working memory demands required to perform the task. Summarizing, this study shows no evidence for a general bilingual advantage in SL, although bilinguals may outperform monolinguals under specific experimental conditions such as SL tasks that place high demands on working memory processes. In addition, the similar performance of Spanish-Basque and Spanish-English bilinguals across all three SL tasks suggests that the degree of dissimilarity between pairs of spoken languages does not modulate SL skills.

Cognitive reserve counteracts typical neural activity changes related to ageing.

Studies have shown that older adults with high Cognitive Reserve (HCR) exhibit better executive functioning than their low CR (LCR) counterparts. However, the neural processes linked to those differences are unclear. This study investigates (1) the neural processes underlying executive functions in older adults with HCR compared to older adults with LCR and (2) how executive control differences between HCR and LCR groups are modulated by increased task difficulty. We recruited 74 participants (37 in each group) with diverse CR levels, as determined by a standardised CR questionnaire. Participants performed two executive control tasks with lower and higher difficulty levels (i.e., Simon and spatial Stroop tasks, respectively) while recording the electroencephalogram. The accuracy on both tasks requiring inhibition of irrelevant information was better in the HCR than the LCR group. Also, in the task with higher difficulty level (i.e., the spatial Stroop task), event-related potential (ERP) latencies associated with inhibition (i.e., frontal N200) and updating of working memory (i.e., P300) were earlier in HCR than LCR. Moreover, the HCR, but not the LCR group, showed larger P300 amplitude in parietal than frontal regions and in the left than right hemisphere, suggesting a posterior to anterior shift of activity and loss of inter-hemispheric asymmetries in LCR participants. These results suggest that high CR counteracts neural activity changes related to ageing. Thus, high levels of CR may be related to maintenance of neural activity patterns typically observed in young adults rather than to deployment of neural compensatory mechanisms.

Age-related Changes in Cortical Excitability Linked to Decreased Attentional and Inhibitory Control.

Understanding age-related changes in cortical excitability and their relation to cognitive functions will help to improve interventions based on non-invasive brain stimulation that aim to support cognitive function in older adults. Here, we investigate the relationship between cortical excitability, executive function, and underlying neural activity in samples of healthy young and older adults. These participants performed a Simon task during electroencephalogram (EEG) recording. During the task, participants had to respond to the colour of a lateralized stimulus while ignoring its spatial location. We studied event-related brain potential correlates of attentional and inhibitory control [i.e., the posterior contralateral negativity (N2pc) and central contralateral negativity (N2cc), respectively] related to the Simon task performance. We also used transcranial magnetic stimulation (TMS) EEG coregistration. In detail, we applied single-pulse TMS during EEG recording in order to analyse global mean field power (GMFP) and TMS-evoked potentials (TEPs) as correlates of cortical excitability. We found lower GMFP amplitude within 101-200 ms in older compared to young adults. Moreover, older adults showed smaller N45 amplitude and slower P180 latency. These findings suggest cortical excitability alterations related to ageing. Older adults also exhibited longer reaction times and N2pc and N2cc latencies, indicating that it took them longer to allocate attention to the target stimulus and inhibit the tendency to respond to the attended location. Finally, in older adults, cortical excitability alterations correlated with longer reaction times and N2pc latencies. These results suggest that age-related alterations in cortical excitability represent a dysfunctional change associated with physiological ageing.

Brain Networks Modulation in Young and Old Subjects During Transcranial Direct Current Stimulation Applied on Prefrontal and Parietal Cortex.

Evidence indicates that the transcranial direct current stimulation (tDCS) has the potential to transiently modulate cognitive function, including age-related changes in brain performance. Only a small number of studies have explored the interaction between the stimulation sites on the scalp, task performance, and brain network connectivity within the frame of physiological aging. We aimed to evaluate the spread of brain activation in both young and older adults in response to anodal tDCS applied to two different scalp stimulation sites: Prefrontal cortex (PFC) and posterior parietal cortex (PPC). EEG data were recorded during tDCS stimulation and evaluated using the Small World (SW) index as a graph theory metric. Before and after tDCS, participants performed a behavioral task; a performance accuracy index was computed and correlated with the SW index. Results showed that the SW index increased during tDCS of the PPC compared to the PFC at higher EEG frequencies only in young participants. tDCS at the PPC site did not exert significant effects on the performance, while tDCS at the PFC site appeared to influence task reaction times in the same direction in both young and older participants. In conclusion, studies using tDCS to modulate functional connectivity and influence behavior can help identify suitable protocols for the aging brain.

Is there electrophysiological evidence for a bilingual advantage in neural processes related to executive functions?

Over the last two decades, a large number of studies have concluded that bilingualism enhances executive functions. However, other studies have reported no significant results. In addition, it is not clear how bilingualism might modulate specific executive control processes. Event-related potentials (ERP) are an excellent technique for identifying whether the neural correlates of executive control processes are strengthened by bilingualism, given their high temporal resolution. On the basis of previous research into the ERP correlates of executive functions, we hypothesize that specific ERP differences between monolinguals and bilinguals can be considered to indicate a bilingual advantage in executive functions. We then review the very limited number of studies that have investigated ERP differences between monolinguals and bilinguals during the performance of executive control tasks. Overall, we conclude that the existence of a bilingual advantage in neural processing related to executive functions remains uncertain and further studies are required. We highlight the utility of investigating several ERPs that have been ignored by previous studies.

Event-Related Potentials Reveal Altered Executive Control Activity in Healthy Elderly With Subjective Memory Complaints.

Several studies reported that healthy elderly with subjective memory complaints (SMC) evolve to Alzheimer's disease (AD) more frequently than elderly without subjective memory decline. In the present study, we investigated event-related potentials (ERPs) associated to executive control processes taking place during the performance of a Simon task with two irrelevant dimensions (stimulus position and direction pointed by an arrow) in healthy elderly divided in low and high SMC (LSMC, HSMC) groups. P300 was studied as a correlate of working memory. Medial frontal negativity (MFN) was studied as a correlate of conflict monitoring. Whereas the LSMC group showed interference from the stimulus position, participants with HSMC showed interference from both irrelevant dimensions. P300 latency was longer and P300 amplitude was lower when the stimulus position was incompatible with the required response but differences between both groups were not observed. MFN was not modulated in the LSMC group; however, the HSMC group showed larger MFN when the stimulus position and/or the direction pointed by the arrow were incompatible with the required response. These results suggest that participants with HSMC deployed greater conflict monitoring activity to maintain the performance when the target stimulus contained conflictive spatial information.

Interventional programmes to improve cognition during healthy and pathological ageing: Cortical modulations and evidence for brain plasticity.

A growing body of evidence suggests that healthy elderly individuals and patients with Alzheimer's disease retain an important potential for neuroplasticity. This review summarizes studies investigating the modulation of neural activity and structural brain integrity in response to interventions involving cognitive training, physical exercise and non-invasive brain stimulation in healthy elderly and cognitively impaired subjects (including patients with mild cognitive impairment (MCI) and Alzheimer's disease). Moreover, given the clinical relevance of neuroplasticity, we discuss how evidence for neuroplasticity can be inferred from the functional and structural brain changes observed after implementing these interventions. We emphasize that multimodal programmes, which combine several types of interventions, improve cognitive function to a greater extent than programmes that use a single interventional approach. We suggest specific methods for weighting the relative importance of cognitive training, physical exercise and non-invasive brain stimulation according to the functional and structural state of the brain of the targeted subject to maximize the cognitive improvements induced by multimodal programmes.

Effects of Mild Cognitive Impairment on the Event-Related Brain Potential Components Elicited in Executive Control Tasks.

We summarize here the findings of several studies in which we analyzed the event-related brain potentials (ERPs) elicited in participants with mild cognitive impairment (MCI) and in healthy controls during performance of executive tasks. The objective of these studies was to investigate the neural functioning associated with executive processes in MCI. With this aim, we recorded the brain electrical activity generated in response to stimuli in three executive control tasks (Stroop, Simon, and Go/NoGo) adapted for use with the ERP technique. We found that the latencies of the ERP components associated with the evaluation and categorization of the stimuli were longer in participants with amnestic MCI than in the paired controls, particularly those with multiple-domain amnestic MCI, and that the allocation of neural resources for attending to the stimuli was weaker in participants with amnestic MCI. The MCI participants also showed deficient functioning of the response selection and preparation processes demanded by each task.

Anodal Transcranial Direct Current Stimulation Promotes Frontal Compensatory Mechanisms in Healthy Elderly Subjects.

Recent studies have demonstrated that transcranial direct current stimulation (tDCS) is potentially useful to improve working memory. In the present study, young and elderly subjects performed a working memory task (n-back task) during an electroencephalogram recording before and after receiving anodal, cathodal, and sham tDCS over the left dorsolateral prefrontal cortex (DLPFC). We investigated modulations of behavioral performance and electrophysiological correlates of working memory processes (frontal and parietal P300 event-related potentials). A strong tendency to modulated working memory performance was observed after the application of tDCS. In detail, young, but not elderly, subjects benefited from additional practice in the absence of real tDCS, as indicated by their more accurate responses after sham tDCS. The cathodal tDCS had no effect in any group of participants. Importantly, anodal tDCS improved accuracy in elderly. Moreover, increased accuracy after anodal tDCS was correlated with a larger frontal P300 amplitude. These findings suggest that, in elderly subjects, improved working memory after anodal tDCS applied over the left DLPFC may be related to the promotion of frontal compensatory mechanisms, which are related to attentional processes.

Cognitive control activity is modulated by the magnitude of interference and pre-activation of monitoring mechanisms.

The Simon task is used to study interference from irrelevant spatial information. Interference is manifested by longer reaction times when the required response -based on non-spatial features- is spatially incompatible with stimulus position. Interference is greater when incompatible trials are preceded by compatible trials (compatible-incompatible sequence) than when they are preceded by incompatible trials (incompatible-incompatible sequence). However, the relationships between spatial attention, interference and cognitive control have not been investigated. In the present study, we distinguished three experimental conditions according to sequential effects: same mappings (SM, compatible-compatible/incompatible-incompatible sequences: low interference), opposite mappings (OM, compatible-incompatible/incompatible-compatible sequences: high interference) and unrelated mappings (UM, central-compatible/central-incompatible sequences: intermediate interference). The negativity central contralateral (N2cc, a correlate of prevention of spatial response tendencies) was larger in OM than in SM, indicating greater cognitive control for greater interference. Furthermore, N2cc was larger in UM than in SM/OM, indicating lower neural efficiency for suppressing spatial tendencies of the response after central trials. Attentional processes (negativity posterior contralateral) were also delayed in UM relative to SM/OM, suggesting attentional facilitation by similar sets of attentional shifts in successive trials. Overall, the present findings showed that cognitive control is modulated by the magnitude of interference and pre-activation of monitoring mechanisms.

Inhibition deficit in the spatial tendency of the response in multiple-domain amnestic mild cognitive impairment. An event-related potential study.

Longitudinal studies have shown that a high percentage of people with amnestic mild cognitive impairment (MCI) develop Alzheimer's disease (AD). Prodromal AD is known to involve deficits in executive control processes. In the present study, we examined such deficits by recording EEG in 13 single-domain amnestic MCI (sdaMCI), 12 multiple-domain amnestic MCI (mdaMCI) and 18 healthy elderly (control group, CG) participants while they performed a Simon task. The Simon task demands deployment of executive processes because participants have to respond to non-spatial features of a lateralized stimulus and inhibit the more automatic spatial tendency of the response. We specifically focused on the negativity central contralateral (N2cc), an event-related potential (ERP) component related to brain activity that prevents the cross-talk between direction of spatial attention and manual response preparation. The reaction time (RT) was not significantly different among the three groups of participants. The percentage of errors (PE) was higher in mdaMCI than in CG and sdaMCI participants. In addition, N2cc latency was delayed in mdaMCI (i.e., delayed implementation of mechanisms for controlling the spatial tendency of the response). The N2cc latency clearly distinguished among mdaMCI and CG/sdaMCI participants (area under curve: 0.91). Longer N2cc was therefore associated with executive control deficits, which suggests that N2cc latency is a correlate of mdaMCI.

Differences between mild cognitive impairment subtypes as indicated by event-related potential correlates of cognitive and motor processes in a Simon task.

Mild cognitive impairment (MCI) may represent a prodromal stage of Alzheimer's disease (AD), although the clinical manifestations of MCI are heterogeneous. Consequently, MCI subtypes are differentiated since amnestic decline (particularly when combined with decline on multiple cognitive domains) increases the probability of progression to AD. In the present study, event-related potential (ERP) correlates of stimulus evaluation (N2), visuospatial attention (negativity posterior-contralateral, N2pc), stimulus categorization (P3b), executive control (pre-response positivity, PP, and medial frontal negativity), and motor (lateralized readiness potential, LRP) processes were studied in 53 participants while they performed a Simon task. Participants were divided into control group (CG), multiple-domain non-amnestic MCI (mdnaMCI), single-domain amnestic MCI (sdaMCI), and multiple-domain amnesic MCI (mdaMCI). Although there were no differences in reaction times and percentage of errors in the performed Simon-type task, a differential pattern of electrophysiological correlates was observed in MCI compared to CG. Concretely, amnestic MCI (sdaMCI and mdaMCI) showed reduced motor activity (LRP amplitude; AUC: 0.84); impairment in executive control (PP amplitude; AUC: 0.80) was observed in multiple-domain MCI (mdaMCI and mdnaMCI); finally, stimulus evaluation (N2 latency; AUC: 0.86) and visuospatial attention (N2pc amplitude; AUC: 0.78) was affected in mdaMCI. Overall, results linked the poorer prognosis of the mdaMCI subtype with a greater number of differences in ERP correlates regarding CG. Therefore, the present results enable us to suggest possible ERP biomarkers for specific MCI subtypes.

Electrophysiological correlates of amnestic mild cognitive impairment in a simon task.

Amnestic mild cognitive impairment (aMCI) represents a prodromal stage of Alzheimer`s disease (AD), especially when additional cognitive domains are affected (Petersen et al., 2009). Thus, single-domain amnestic MCI (sdaMCI) and multiple-domain-amnestic MCI (mdaMCI) biomarkers are important for enabling early interventions to help slow down progression of the disease. Recording event-related potentials (ERPs) is a non-invasive and inexpensive measure of brain activity associated with cognitive processes, and it is of interest from a clinical point of view. The ERP technique may also be useful for obtaining early sdaMCI and mdaMCI biomarkers because ERPs are sensitive to impairment in processes that are not manifested at behavioral or clinical levels. In the present study, EEG activity was recorded in 25 healthy participants and 30 amnestic MCI patients (17 sdaMCI and 13 mdaMCI) while they performed a Simon task. The ERPs associated with visuospatial (N2 posterior-contralateral - N2pc -) and motor (lateralized readiness potential - LRP -) processes were examined. The N2pc amplitude was smaller in participants with mdaMCI than in healthy participants, which indicated a decline in the correlates of allocation of attentional resources to the target stimulus. In addition, N2pc amplitude proved to be a moderately good biomarker of mdaMCI subtype (0.77 sensitivity, 0.76 specificity). However, the LRP amplitude was smaller in the two MCI groups (sdaMCI and mdaMCI) than in healthy participants, revealing a reduction in the motor resources available to execute the response in sdaMCI and mdaMCI patients. Furthermore, the LRP amplitude proved to be a valid biomarker (0.80 sensitivity, 0.92 specificity) of both amnestic MCI subtypes.