Hemispheric differences in event-related potentials to stroop stimuli. Attention and color-word processing

Time Course of Reactive Brain Activities during a Stroop Color-Word Task: Evidence of Specific Facilitation and Interference Effects

The Stroop test represents a widely used task in basic and clinical research for approaching the cognitive system functioning in humans. However, a clear overview of the neurophysiological signatures associated with the different sub-domains of this task remains controversial. In the present study, we leveraged the EEG technique to explore the modulation of specific post-stimulus ERPs components during the Stroop test. Critically, to better disentangle the contribution of facilitation (i.e., faster color identification times for color-congruent Stroop words) and interference (i.e., longer color identification times for color-incongruent Stroop words) processes prompted by the Stroop test, we delivered congruent and incongruent trials in two separate experimental blocks, each including the respective neutral condition. Thanks to this methodological manipulation, we were able to clearly dissociate the two sub-processes. Electrophysiological results suggest specific markers of brain activity for the facilitation and the interference effects. Indeed, distinctive Stroop-related ERPs (i.e., the P3, the N450, and the LPC) were differently modulated in the two sub-processes. Collectively, we provide evidence of selected brain activities involved in the reactive stage of processing associated with the Stroop effect.

Distilling the distinct contralateral and ipsilateral attentional responses to lateral stimuli and the bilateral response to midline stimuli for upper and lower visual hemifield locations

A contralateral posterior negativity elicited by lateral oddballs (N2pc) and a bilateral posterior negativity elicited by vertical midline oddballs (bilateral N2) are ERP components reflecting attentional deployment that have been rarely compared. In different tasks, we explored to what extent they reflect similar underlying mechanisms of attention. We used a multiple-frame procedure to present pop-out color oddballs among distractors. A homogeneous condition contained only distractors (0 oddballs) and served as a control condition that was subtracted from oddball-present conditions to isolate attention effects. The number of oddballs and the vertical hemifield containing them (upper vs. lower) were two critical factors. For the lower hemifield, the signal amplitude increased with the number of oddballs, otherwise had similar effects and scalp distributions, suggesting the bilateral N2 acted as a bilateral N2pc and likely reflected similar underlying generators. For the upper hemifield, component amplitude also increased with the number of oddballs, but the scalp distributions were positive and more centered, suggesting inverted generators across the two vertical hemifields. An ipsilateral positivity occurred about 50 ms after a contralateral positivity, similar in magnitude, producing a biphasic contra-minus-ipsi difference wave. Previously reported smaller negative N2pc components for upper hemifield oddballs likely reflected a negative lobe artificially created by the subtraction of a lagged positive ipsilateral response. The results compel us to argue for a systematic separation of data for upper versus lower hemifields in studies of visuo-spatial attention, and the use of an experimental design permitting the separate estimation of contralateral and ipsilateral responses.

Functional definition of the N450 event-related brain potential marker of conflict processing: a numerical stroop study

Background

Several conflict processing studies aimed to dissociate neuroimaging phenomena related to stimulus and response conflict processing. However, previous studies typically did not include a paradigm-independent measure of either stimulus or response conflict. Here we have combined electro-myography (EMG) with event-related brain potentials (ERPs) in order to determine whether a particularly robust marker of conflict processing, the N450 ERP effect usually related to the activity of the Anterior Cingulate Cortex (ACC), is related to stimulus- or to response-conflict processing. EMG provided paradigm-independent measure of response conflict. In a numerical Stroop paradigm participants compared pairs of digits and pressed a button on the side where they saw the larger digit. 50% of digit-pairs were preceded by an effective cue which provided accurate information about the required response. 50% of trials were preceded by a neutral cue which did not communicate the side of response.

Results

EMG showed that response conflict was significantly larger in neutrally than in effectively cued trials. The N450 was similar when response conflict was high and when it was low.

Conclusions

We conclude that the N450 is related to stimulus or abstract, rather than to response conflict detection/resolution. Findings may enable timing ACC conflict effects.

Electrophysiological measures of conflict detection and resolution in the Stroop task

Conflict detection and resolution is crucial in a cognitive task like the Stroop task. Previous studies have identified an early negativity component (N(inc)) as a prominent marker of Stroop conflict in event-related potentials (ERPs). However, to what extent this ERP component reflects conflict detection and/or resolution is still unclear. Here, we report a Stroop task in which the stimulus onset asynchrony (SOA) of color and word stimuli presentation was manipulated in order to disentangle the roles of conflict detection and conflict resolution in generating Stroop-related ERP components. Separating the word from the color information gives us precise control over the timing of conflict. If the N(inc) is related with conflict resolution it should be absent when the word appears during response preparation, as in a long-latency positive SOA. Our data shows that the N(inc) occurs in all SOAs, even after a response has been made, supporting its role in the detection of stimulus conflict rather than conflict resolution. The use of SOA manipulation therefore allows for the examination of a wider temporal spectrum of interference in order to specify the functions of this conflict-related component. These results provide insight into the neural signatures of conflict processes, and have implications for models of cognitive control mechanisms in the brain.

Selective processing of multiple features in the human brain: effects of feature type and salience

Identifying targets in a stream of items at a given constant spatial location relies on selection of aspects such as color, shape, or texture. Such attended (target) features of a stimulus elicit a negative-going event-related brain potential (ERP), termed Selection Negativity (SN), which has been used as an index of selective feature processing. In two experiments, participants viewed a series of Gabor patches in which targets were defined as a specific combination of color, orientation, and shape. Distracters were composed of different combinations of color, orientation, and shape of the target stimulus. This design allows comparisons of items with and without specific target features. Consistent with previous ERP research, SN deflections extended between 160–300 ms. Data from the subsequent P3 component (300–450 ms post-stimulus) were also examined, and were regarded as an index of target processing. In Experiment A, predominant effects of target color on SN and P3 amplitudes were found, along with smaller ERP differences in response to variations of orientation and shape. Manipulating color to be less salient while enhancing the saliency of the orientation of the Gabor patch (Experiment B) led to delayed color selection and enhanced orientation selection. Topographical analyses suggested that the location of SN on the scalp reliably varies with the nature of the to-be-attended feature. No interference of non-target features on the SN was observed. These results suggest that target feature selection operates by means of electrocortical facilitation of feature-specific sensory processes, and that selective electrocortical facilitation is more effective when stimulus saliency is heightened.

Rapid automatized naming and immediate memory functions in Chinese Mandarin-speaking elementary readers

The purpose of this study was to evaluate rapid automatized naming skills (RAN) and immediate memory processes in 243 Chinese Mandarin-speaking elementary readers (ranging from Grade 1 to Grade 5). For RAN subtests, the mean naming time decreased monotonically with grade level in good and average readers, and a similar trajectory was found in poor readers, even though they were generally slower in rapid naming. Regardless of grouping methods (counting all participants or counting good readers only), RAN Character emerged as a significant predictor of various Chinese reading measures. Different from classical findings in English readers indicating that RAN Number was a better correlate of reading than RAN Object, RAN Object outperformed RAN Number and became a significant predictor of Chinese reading speed and spelling, suggesting that the differences in predictive power of RAN tasks may be language specific. Comparison of memory profiles for good, average, and poor readers revealed that the patterns varied depending on mode of stimulus presentation or response. Poor readers performed poorly on subtests involving a visual component and did relatively better on subtests involving verbal cues only, whereas a reversed pattern was shown in the group of good readers. The findings were interpreted to suggest that good and poor Chinese readers may be essentially different in applying visual strategies and verbal mediation during visual-verbal intra- and intermodal processing, and visual skills appear to be particularly important in reading of Chinese.

Motor conflict in Stroop tasks: direct evidence from single-trial electro-myography and electro-encephalography

Several brain imaging studies have assumed that response conflict is present in Stroop tasks. However, this has not been demonstrated directly. We examined the time-course of stimulus and response conflict resolution in a numerical Stroop task by combining single-trial electro-myography (EMG) and event-related brain potentials (ERP). EMG enabled the direct tracking of response conflict and the peak latency of the P300 ERP wave was used to index stimulus conflict. In correctly responded trials of the incongruent condition EMG detected robust incorrect response hand activation which appeared consistently in single trials. In 50-80% of the trials correct and incorrect response hand activation coincided temporally, while in 20-50% of the trials incorrect hand activation preceded correct hand activation. EMG data provides robust direct evidence for response conflict. However, congruency effects also appeared in the peak latency of the P300 wave which suggests that stimulus conflict also played a role in the Stroop paradigm. Findings are explained by the continuous flow model of information processing: Partially processed task-irrelevant stimulus information can result in stimulus conflict and can prepare incorrect response activity. A robust congruency effect appeared in the amplitude of incongruent vs. congruent ERPs between 330-400 ms, this effect may be related to the activity of the anterior cingulate cortex.

Event-related potentials dissociate facilitation and interference effects in the numerical Stroop paradigm

In the numerical Stroop paradigm (NSP) participants compare simultaneously presented Arabic digits based on either their numerical or on their physical size dimension. Responses are faster when the numerical and size dimensions are congruent with each other (facilitation), and responses are slower when the numerical and size dimensions are incongruent with each other (interference). We aimed to find out whether facilitation and interference appears during the course of perceptual or response processing. To this end, facilitation and interference effects in the amplitude of event-related brain potentials (ERPs) were examined. The onset of motor preparation was determined by monitoring the lateralized readiness potential. In numerical comparison one facilitation effect was related to perceptual processing at the level of the magnitude representation. A second facilitation effect and interference effects appeared during response processing. In size comparison facilitation and interference appeared exclusively during response processing. In both tasks, ERP interference effects were probably related to contextual analysis and to the conflict monitoring and selection for action activity of the anterior cingulate cortex. The results demonstrate that facilitation and interference effects in the NSP appear during multiple stages of processing, and that they are related to different cognitive processes. Therefore these effects should be clearly separated in studies of the NSP. A model of the processes involved in the NSP is provided and implications for studies of the NSP are drawn.

Differential dopaminergic modulation of executive control in healthy subjects

RATIONALE

Executive control (EC) has different subcomponents, e.g., response inhibition (measured, for example, by the Stroop task) and working memory (WM-measured, for example, by delayed response tasks, DRT). EC has been associated with networks involving the prefrontal cortex (PFC). Moreover, there is evidence that dopamine agonists, especially those with a D1 profile, may modulate EC, since in the PFC D1 subtype receptors are more abundant.

OBJECTIVE

This study aimed to selectively distinguish whether D1 versus D2 dopamine agonism differentially influences EC related to the inhibition of irrelevant information and WM. Because of its D1 component, we predicted that the administration of pergolide (mixed D1/D2 agonist), in comparison with bromocriptine (D2 selective agonist) and placebo, would enhance performance in both EC tasks. Using a lateralized Stroop task, we predicted a decrease in the interference effect, as well as error rates, while no increase in facilitation effects. For the DRT task, we predicted fewer error scores in the delay condition.

METHODS

Forty male healthy subjects participated in this randomized, double-blind, placebo-controlled, crossover study.

RESULTS

For the Stroop task no superiority of pergolide was found; however, with bromocriptine, decreased interference was found. No modulation of lateralization effects was shown in interference measures. Moreover, subjects on pergolide showed an absence of facilitation effects. No effects of either agonist were found for the DRT.

CONCLUSION

Our findings suggest that dopamine agonists modulate two EC tasks differently. Furthermore, there seems to be a selective modulation of different aspects of the Stroop task.

Event-related potentials to Stroop and reverse Stroop stimuli

In the Stroop task, the latency of response to a colour is either faster or slower in the presence of a congruent or incongruent colour-word (J. Exp. Psychol. 18 (1935) 643). Debate remains as to whether this effect occurs during early stimulus processing or late response competition. The present study examined the task using reaction time (RT) and event-related potentials to determine temporal differences in this processing. The 'reverse Stroop' effect (where colour interferes with processing of a colour-word) which is much less well established, was also examined. Standard Stroop interference was found as well as reverse Stroop interference. A late lateralised negativity at frontal sites was greater for Incongruent trials and also for the word-response (reverse Stroop) task, and was interpreted as semantic selection and word-rechecking effects. Late positive component latency effects generally mirrored the speed of processing of the different conditions found in RT data. Stroop effects were also found in early temporal N100 and parietal P100 components, which differentiated Congruent from Incongruent trials in the reverse Stroop but not the standard Stroop, and were interpreted as early perception of physical mismatch between the colour and word. It was concluded that Stroop stimuli are processed in parallel in a network of brain areas rather than a particular structure and that Stroop interference arises at the output stage.

Parietal lobe activation in rapid, automatized naming by adults

Three automatic naming tasks (Wiig & Nielsen, 1999) were administered to 60 normally functioning adults. The mean time required for naming 40 single-dimension (colors, forms, numbers, and letters) and 40 dual-dimension stimuli (color-form, color-number, and color-letter combinations) were compared in young (17-38 yr.) and older (40-68 yr.) men and women. Analysis of variance for the combined groups indicated significant naming-time differences for age but not for sex. There were no significant interaction effects. For men there was a significant naming time difference between age groups for forms, and for women for colors and forms. The sex-specific analyses indicated no significant differences in naming time based on age groups for color-form, color-number, or color-letter combinations. In a second study of adult subjects (n = 14), functional brain activity was measured with regional cerebral blood flow during the performance of the color, form, and color-form naming tasks. One subject was repeatedly measured during the performance of each task, whereas 13 subjects were measured during the performance of color-form naming. In comparison to normal reference values for rest and FAS verbal fluency, blood-flow measurements showed a consistent parietal-lobe activation during form and color-form naming, but only a slight activation during color naming. During all naming tasks, a significant frontal and frontotemporal flow decrease was seen in comparison to both rest and verbal fluency reference values. This functional brain activation pattern of a parietal increase and a frontotemporal decrease was consistently confirmed across subjects during the color-form naming task.

A clinical rationale for assessing rapid automatized naming in children with language disorders

Three continuous rapid naming tasks (Semel, Wiig, & Secord, 1995) were administered to 2,450 American, English-speaking, academically achieving individuals with typical language development and intellectual ability (ages 6 to 21 years) and 136 individuals with primary language disorders (LD; ages 6, 7, 9, 11, 12, 15-16). Naming time in seconds differed significantly (p < .01) between the groups for color naming (Task 1) at age 12, shape naming (Task 2) at age 6, and color-shape naming (Task 3) at ages 6, 7, 9, and 12. Naming accuracy did not differ significantly (p > .01) between groups at the majority of the age levels compared. In the normative group, naming speed increased with age in a monotonic progression. The developmental trajectory in the LD group was essentially parallel, but elevated. The percentages of individuals who failed the naming-time criteria for Task 3 (color-shape naming) differed significantly in the two groups at all ages compared (p < .05). These findings indicate that the requirements for two-dimensional, continuous naming (Task 3 color-shape naming) resulted in reduced naming speed (longer total times) and interference with fluency in language production in about half of the clinical sample.

Effects of characterological anxiety and situational arousal on the solving of a color-word interference task: hemispheric processing implications

Stroop color-word stimuli permit examination of relative hemispheric contributions to cognition. Subjects of varying trait anxiety levels underwent situational arousal manipulations. Discrete color-word stimuli were projected to the visual half-fields; motor matching responses were made. Trait anxiety affected left-hemisphere activation. Responding was faster and more accurate for moderate than low trait anxiety; at high levels, the left hemisphere became overactivated and inefficient. Situational arousal facilitated right-hemisphere performance; latencies were shorter and accuracy increased in the aroused compared with the relaxed condition. Situational arousal interacted with trait anxiety; highly trait-anxious subjects had longer latencies and decreased accuracy when relaxed than when aroused. A paradoxical effect of trait anxiety is rigidity and stereotypy of cognitive functioning, impairing ability to assume appropriate alternative cognitive modes.