Prefrontal regions play a predominant role in imposing an attentional 'set': evidence from fMRI

fMRI was used to determine whether prefrontal regions play a predominant role in imposing an attentional 'set' that drives selection of task-relevant information. While monitoring for an atypical item, individuals viewed Stroop stimuli that were either colored words or colored objects. Attentional demands were varied, being greater when the stimuli contained two distinct and incongruent sources of information about the task-relevant attribute (e.g., when attending to color, seeing the word 'blue' in red ink) as compared to only one source (e.g., seeing the word 'late' in red ink). Prefrontal but not anterior cingulate regions exhibited greater activation on incongruent than neutral trials, suggesting that prefrontal cortex has a major role in imposing an attentional 'set'. In addition, we found that prefrontal activation is most likely to occur when that attentional set is difficult to impose.     

Competition for priority in processing increases prefrontal cortex's involvement in top-down control: an event-related fMRI study of the stroop task

Prior work indicates that various aspects of task-irrelevant information (e.g. its salience, task-relatedness, emotionality) can increase the involvement of prefrontal cortex (PFC) in top-down attentional control. In light of these findings, we hypothesize that PFC's involvement increases when task-irrelevant information competes for priority in processing. In an event-related fMRI study using an oddball variant of the Stroop task, we examine the generality of this hypothesis using three manipulations designed to increase the ability of task-irrelevant information to compete for priority in processing. First, we investigated how the frequency of occurrence of task-irrelevant information affects PFC activity. Second, we examined whether conflicting color information (i.e. incongruent trials) increases activity in regions of PFC that are similar to or distinct from those sensitive to infrequently occurring task-irrelevant information. Finally, we examined the impact of the number of levels at which conflict could occur (e.g. non-response only, non-response and response). Activity in posterior-dorsolateral and posterior-inferior PFC increased for infrequently occurring task-irrelevant information, being largest when the task-irrelevant information contained conflicting color-information. In contrast, increases in mid-dorsolateral prefrontal cortex's activity were only noted when conflicting color information was present, being largest when conflict occurred at multiple levels. The anterior cingulate was primarily sensitive to the occurrence of conflict at the response level with only a small sub-region exhibiting sensitivity to non-response conflict as well. From these findings we suggest that posterior DLPFC and PIPFC are involved in biasing processing in posterior processing systems, mid-DLPFC is involved in biasing the processing of the contents of working memory, and ACC is primarily involved in response-related processes.     

Oddball and incongruity effects during Stroop task performance: a comparative fMRI study on selective attention

The aim of this fMRI study was to investigate and compare the neural mechanisms of selective attention during two different operationalizations of competition between task-relevant and task-irrelevant information: Stroop-incongruity and oddballs. For this purpose, we employed a Stroop-like oddball task in which subjects responded to the font size of presented word stimuli. Stroop-incongruity was created by (response-)incongruent word information while oddballs comprised low-frequency events in a task-irrelevant, unattended dimension. Thereby, in order to elucidate the influence of processing domain from which competition emanates, oddball conditions were created in two different attribute dimensions, color and word meaning. Either oddball condition was expected to evoke an orienting response, which participants would have to override in order to maintain adequate performance. Incongruent Stroop trials were expected to produce Stroop-interference so that subjects would have to override the predominant tendency to read and respond to word meaning. All competition conditions exhibited significantly prolonged reaction times compared to control trials, demonstrating that our experimental manipulation was indeed effective. fMRI data analyses delineated two discriminative components of competition: one component mainly related to motor preparation and another, primarily attentional component. Regarding the first, Stroop-interference increased activation mainly in regions implicated in motor control or response preparation. Regarding the second, Word-oddballs increased activation in a frontoparietal "attention network". Furthermore, Word-oddballs and Color-oddballs exhibited striking activation overlap mainly in prefrontal regions but also in posterior processing areas. Here, the data emphasized a prominent role of posterior lateral PFC in implementing top-down attentional control.     

Color Stroop and negative priming in schizophrenia: An fMRI study

Disturbances in selective attention represent a core characteristic of schizophrenia, whose neural underpinnings have yet to be fully elucidated. Consequently, we recorded brain activation using functional magnetic resonance imaging (fMRI) while 15 patients with schizophrenia and 15 age-matched controls performed a well-established measure of selective attention—the color Stroop negative priming task. We focused on two aspects of performance: overriding pre-potent responses (Stroop effect) and inhibition of prior negatively primed trials (negative priming effect). Behaviorally, controls demonstrated both significant Stroop and negative priming effects, while schizophrenic subjects only showed the Stroop effect. For the Stroop effect, fMRI indicated significantly greater activation in frontal regions–medial frontal gyrus/anterior cingulate gyrus and middle frontal gyrus for controls–but greater activation in medial parietal regions (posterior cingulate gyrus/precuneus) for patients. Negative priming elicited significant activation in right dorsolateral prefrontal cortex for both groups, but also in left dorsolateral prefrontal cortex for patients. These different patterns of fMRI activation may reflect faulty interaction in schizophrenia within networks of brain regions that are vital to selective attention.     

Examining a supramodal network for conflict processing: a systematic review and novel functional magnetic resonance imaging data for related visual and auditory stroop tasks

Cognitive control over conflicting information has been studied extensively using tasks such as the color-word Stroop, flanker, and spatial conflict task. Neuroimaging studies typically identify a fronto-parietal network engaged in conflict processing, but numerous additional regions are also reported. Ascribing putative functional roles to these regions is problematic because some may have less to do with conflict processing per se, but could be engaged in specific processes related to the chosen stimulus modality, stimulus feature, or type of conflict task. In addition, some studies contrast activation on incongruent and congruent trials, even though a neutral baseline is needed to separate the effect of inhibition from that of facilitation. In the first part of this article, we report a systematic review of 34 neuroimaging publications, which reveals that conflict-related activity is reliably reported in the anterior cingulate cortex and bilaterally in the lateral prefrontal cortex, the anterior insula, and the parietal lobe. In the second part, we further explore these candidate "conflict" regions through a novel functional magnetic resonance imaging experiment, in which the same group of subjects perform related visual and auditory Stroop tasks. By carefully controlling for the same task (Stroop), the same to-be-ignored stimulus dimension (word meaning), and by separating out inhibitory processes from those of facilitation, we attempt to minimize the potential differences between the two tasks. The results provide converging evidence that the regions identified by the systematic review are reliably engaged in conflict processing. Despite carefully matching the Stroop tasks, some regions of differential activity remained, particularly in the parietal cortex. We discuss some of the task-specific processes which might account for this finding.     

The influence of rTMS over the right dorsolateral prefrontal cortex on top-down attentional processes

Repetitive Transcranial Magnetic Stimulation (rTMS) provides a unique opportunity to study causal relationships between activity in the dorsolateral prefrontal cortex (DLPFC) and executive functioning, by modulating brain activity in SHAM controlled designs. We devised a new Stroop task paradigm in which subjects must engage in both strategic and automatic attentional processes. In the current experiment, we manipulated subjects' expectancies for incongruent stimuli. Previous research demonstrated that when subjects have a high level of expectancy that a stimulus will be incongruent, they are able to strategically adjust the relative influence of word reading on color naming. The effect of high frequency (HF) rTMS on Stroop performance of 20 right-handed healthy female volunteers was tested using a double blind within subjects design by counterbalanced crossover sham (placebo) and active rTMS over the right DLPFC. Since mood remained unchanged after rTMS, the Stroop data could be evaluated independent of mood changes. Only in the high expectancy condition, we found a decreased response time to both congruent and incongruent trials on the Stroop task performance after HF rTMS. The SHAM placebo condition yielded no effects. We conclude that high frequency stimulation over the right DLPFC has an effect on top-down attentional processes by modulating the attentional set.     

An event-related functional MRI study comparing interference effects in the Simon and Stroop tasks

The Stroop and Simon tasks typify a class of interference effects in which the introduction of task-irrelevant stimulus characteristics robustly slows reaction times. Behavioral studies have not succeeded in determining whether the neural basis for the resolution of these interference effects during successful task performance is similar or different across tasks. Event-related functional magnetic resonance imaging (fMRI) studies were obtained in 10 healthy young adults during performance of the Stroop and Simon tasks. Activation during the Stroop task replicated findings from two earlier fMRI studies. These activations were remarkably similar to those observed during the Simon task, and included anterior cingulate, supplementary motor, visual association, inferior temporal, inferior parietal, inferior frontal, and dorsolateral prefrontal cortices, as well as the caudate nuclei. The time courses of activation were also similar across tasks. Resolution of interference effects in the Simon and Stroop tasks engage similar brain regions, and with a similar time course. Therefore, despite the widely differing stimulus characteristics employed by these tasks, the neural systems that subserve successful task performance are likely to be similar as well.     

Top-down attentional control in spatially coincident stimuli enhances activity in both task-relevant and task-irrelevant regions of cortex

Models of selective attention predict that focused attention to spatially contiguous stimuli may result in enhanced activity in areas of cortex specialized for processing task-relevant and task-irrelevant information. We examined this hypothesis by localizing color-sensitive areas (CSA) and word and letter sensitive areas of cortex and then examining modulation of these regions during performance of a modified version of the Stroop task in which target and distractors are spatially coincident. We report that only the incongruent condition with the highest cognitive demand showed increased activity in CSA relative to other conditions, indicating an attentional enhancement in target processing areas. We also found an enhancement of activity in one region sensitive to word/letter processing during the most cognitively demanding incongruent condition indicating greater processing of the distractor dimension. Correlations with performance revealed that top-down modulation during the task was critical for effective filtering of irrelevant information in conflict conditions. These results support predictions made by models of selective attention and suggest an important mechanism of top-down attentional control in spatially contiguous stimuli.     

Evidence for the triadic model of adolescent brain development: Cognitive load and task-relevance of emotion differentially affect adolescents and adults

In adults, cognitive control is supported by several brain regions including the limbic system and the dorsolateral prefrontal cortex (dlPFC) when processing emotional information. However, in adolescents, some theories hypothesize a neurobiological imbalance proposing heightened sensitivity to affective material in the amygdala and striatum within a cognitive control context. Yet, direct neurobiological evidence is scarce. Twenty-four adolescents (12–16) and 28 adults (25–35) completed an emotional n-back working memory task in response to happy, angry, and neutral faces during fMRI. Importantly, participants either paid attention to the emotion (task-relevant condition) or judged the gender (task-irrelevant condition). Behaviorally, for both groups, when happy faces were task-relevant, performance improved relative to when they were task-irrelevant, while performance decrements were seen for angry faces. In the dlPFC, angry faces elicited more activation in adults during low relative to high cognitive load (2-back vs. 0-back). By contrast, happy faces elicited more activation in the amygdala in adolescents when they were task-relevant. Happy faces also generally increased nucleus accumbens activity (regardless of relevance) in adolescents relative to adults. Together, the findings are consistent with neurobiological models of adolescent brain development and identify neurodevelopmental differences in cognitive control emotion interactions.     

Prefrontal lobes and the attentional control: a neuropsychological study using modified Stroop test]

The prefrontal lobes may play an important role in attentional and cognitive control. The modified Stroop test can detect the disorder of this regulatory function in the brain-damaged patients. The Stroop conflict task was given to 35 prefrontal damaged patients (prefrontal group) and 20 subjects with temporal or parietal lesions (posterior group). The prefrontal subjects were further divided into the three subgroups, which consisted of 17 dorsolateral, 6 medial and 12 orbitofrontal patients. There were no significant differences between the prefrontal and posterior group on the Stroop test. However, the dorsolateral prefrontal damaged subgroup and the medial prefrontal damaged subgroup had significantly poor performances than the posterior group on the incongruent condition of the Stroop test. This findings indicated that the impairment of attentional control between the word reading and color naming resulted from the lesions of dorsolateral and medial part of prefrontal lobes. The cognitive control of visual attention may be supported by the neural substrates including dorsolateral prefrontal lobe, supplementary motor area and anterior cingulate cortex.     

Anterior cingulate cortex: an fMRI analysis of conflict specificity and functional differentiation

In this event-related functional magnetic resonance imaging (fMRI) study, we provide evidence that the role of the anterior cingulate cortex (ACC) in cognitive control may not be unitary, as the responses of different ACC subregions vary depending upon the nature of task-irrelevant information. More specifically, using the color-word Stroop task (congruent, incongruent, and neutral trial types), we examined the degree to which increases in neural activity within ACC are specific to conditions of conflict, as posited by the conflict monitoring theory (Botvinick et al. [1999]: Rev Neurosci 10:49-57; Carter et al. [1998]: Science 280:747-749). Although incongruent and congruent trials both involve two competing sources of color information (color word and ink color), only incongruent trials involve a direct conflict between task-relevant and task-irrelevant information. Although the anterior division of the ACC rostral zone exhibited conflict specific increases in neural activity (i.e., incongruent > congruent = neutral), the posterior division exhibited a more generalized pattern, increasing whenever the task-irrelevant information was color related, regardless of whether it was conflicting (i.e., incongruent and congruent > neutral). Our data thus suggest a possible functional differentiation within the ACC. As such, it is unlikely that the role of the ACC in cognitive control will be able to be accommodated by a single unifying theory.     

Information processing flow and neural activations in the dorsolateral prefrontal cortex in the Stroop task in schizophrenic patients. A spatially filtered MEG analysis with high temporal and spatial resolution

Using a spatially filtered magnetoencephalography analysis (synthetic aperture magnetometry), we estimated neural activations in the Stroop task in nearly real time for schizophrenic patients with/without auditory hallucinations and for normal control subjects. In addition, auditory hallucinations were examined through the information processing flow of the brain neural network, including the frontal regions. One hundred unaveraged magnetoencephalography signals during the incongruent stimulus responses were analyzed with a time window of 200 ms in steps of 50 ms. In the 25-60-Hz band, cortical regions that showed significant current source density changes were examined for each time window. The three groups showed significantly decreased current source density, corresponding to neural activation, with temporal overlap along the fundamental cognitive information processing flow: sensory input system, executive control system, motor output system. Transient neural activations in the dorsolateral prefrontal cortex were bilateral with left-side dominancy for normal controls, left-lateralized for nonhallucinators and right-lateralized for hallucinators. Our results suggest that the dysfunction in the left dorsolateral prefrontal cortex was related to auditory hallucinations, while the information processing flow was unaffected in the schizophrenic subjects in the Stroop task.     

Cortical recruitment during selective attention in multiple sclerosis: an fMRI investigation of individual differences

Recent studies with multiple sclerosis (MS) participants have provided evidence for cortical reorganization. Greater recruitment of task-related areas and additional brain regions are thought to play an adaptive role in the performance of cognitive tasks. In this study, we compared cortical circuitry recruited by MS patients and controls during a selective attention task that requires both focusing attention on task-relevant information and ignoring or inhibiting task-irrelevant information. Despite comparable behavioral performance, MS patients demonstrated increased neural recruitment of task-related areas along with additional activation of the prefrontal cortices. However, this additional activation was associated with poor behavioral performance, thereby providing evidence against compensatory brain reorganization. Future studies specifically investigating the nature of additional activation seen in MS patients in a wider variety of cognitive tasks would provide insight into the specific cognitive decline in MS.     

Trait anxiety modulates the neural efficiency of inhibitory control

An impairment of attentional control in the face of threat-related distracters is well established for high-anxious individuals. Beyond that, it has been hypothesized that high trait anxiety more generally impairs the neural efficiency of cognitive processes requiring attentional control-even in the absence of threat-related stimuli. Here, we use fMRI to show that trait anxiety indeed modulates brain activation and functional connectivities between task-relevant brain regions in an affectively neutral Stroop task. In high-anxious individuals, dorsolateral pFC showed stronger task-related activation and reduced coupling with posterior lateral frontal regions, dorsal ACC, and a word-sensitive area in the left fusiform gyrus. These results support the assumption that a general (i.e., not threat-specific) impairment of attentional control leads to reduced neural processing efficiency in anxious individuals. The increased dorsolateral pFC activation is interpreted as an attempt to compensate for suboptimal connectivity within the cortical network subserving task performance.     

Practice on conflict tasks promotes executive function of working memory in the elderly

Effects of practice on a conflict task in elderly individuals are examined with a focus on its impact on executive function in working memory. During a short-term practice period, healthy elderly participants practiced switching attention using a Stroop task that involved a conflict between a task relevant stimulus and an irrelevant stimulus. To explore neural substrates underlying practice effects, two working memory tasks were used: a focus reading span test (F-RST) and a non-focus reading span test (NF-RST); the NF-RST test demanded greater switching attention due to a conflict between the relevant task stimulus and an irrelevant task stimulus, thus requiring an attention switch from the latter to the former. Following the Stroop task practice, fMRI data showed that participants who had engaged in practice had significant increases in activation in the anterior cingulate cortex (ACC), the left inferior parietal lobule (IPL), the left dorsolateral prefrontal cortex (DLPFC) and the precuneus regions during the NF-RST. By contrast, a control group, which did not practice, showed no significant increases in these regions. Results suggest that practice on conflict tasks in elderly individuals activated regions related to conflict perceiving and attention switching regions as well as attention-maintenance regions thereby improving performance on tasks requiring a high degree of attention control of working memory.     

Activation of the caudal anterior cingulate cortex due to task‐related interference in an auditory Stroop paradigm

Successful information processing requires the focusing of attention on a certain stimulus property and the simultaneous suppression of irrelevant information. The Stroop task is a useful paradigm to study such attentional top‐down control in the presence of interference. Here, we investigated the neural correlates of an auditory Stroop task using fMRI. Subjects focused either on tone pitch (relatively high or low; phonetic task) or on the meaning of a spoken word (high/low/good; semantic task), while ignoring the other stimulus feature. We differentiated between task‐related (phonetic incongruent vs. semantic incongruent) and sensory‐level interference (phonetic incongruent vs. phonetic congruent). Task‐related interference activated similar regions as in visual Stroop tasks, including the anterior cingulate cortex (ACC) and the presupplementary motor‐area (pre‐SMA). More specifically, we observed that the very caudal/posterior part of the ACC was activated and not the dorsal/anterior region. Because identical stimuli but different task demands are compared in this contrast, it reflects conflict at a relatively high processing level. A more conventional contrast between incongruent and congruent phonetic trials was associated with a different cluster in the pre‐SMA/ACC which was observed in a large number of previous studies. Finally, functional connectivity analysis revealed that activity within the regions activated in the phonetic incongruent vs. semantic incongruent contrast was more strongly interrelated during semantically vs. phonetically incongruent trials. Taken together, we found (besides activation of regions well‐known from visual Stroop tasks) activation of the very caudal and posterior part of the ACC due to task‐related interference in an auditory Stroop task. Hum Brain Mapp, 2009. © 2009 Wiley‐Liss, Inc.     

Ignoring irrelevant stimuli in latent inhibition and Stroop paradigms: The effects of schizotypy and gender

Latent inhibition (LI), poor evidence of learning following preexposure to a task-irrelevant stimulus, reflects the ability to ignore inconsequential events. Stroop interference represents a failure to inhibit processing of a task-irrelevant word when it is incongruent with the required naming of the word's print color. The apparent commonality between the two effects is in contradiction to the literature, which indicates that LI is affected by schizotypy and schizophrenia, and perhaps gender, while Stroop interference generated by the trial-to-trial procedure is unaltered by those variables. In the present experiment, low schizotypal healthy males, but not females, exhibited LI. The same groups did not differ on Stroop interference. The results are discussed in terms of different processing requirements for task-irrelevant stimuli that are an integral part of the task-relevant target stimulus (as in Stroop) or separated from it in space (as in LI).     

Stroop performance in bipolar disorder: further evidence for abnormalities in the ventral prefrontal cortex

OBJECTIVES: Bipolar patients are impaired in Stroop task performance, a measure of selective attention. Structural and functional abnormalities in task-associated regions, in particular the prefrontal cortex (PFC), have been reported in this population. We aimed to examine the relationship between functional abnormalities, impaired task performance and the severity of depressive symptoms in bipolar patients. METHODS: Remitted bipolar patients (n = 10; all medicated), either euthymic or with subsyndromal depression, and age-matched control subjects (n = 11) viewed 10 alternating blocks of incongruent Stroop and control stimuli, naming the colour of the ink. Neural response was measured using functional magnetic resonance imaging. We computed between-group differences in neural response and within-group correlations with mood and anxiety. RESULTS: There were no significant between-group differences in task performance. During the Stroop condition, controls demonstrated greater activation of visual and dorsolateral and ventrolateral prefrontal cortical areas; bipolar patients demonstrated relative deactivation within orbital and medial prefrontal cortices. Depression scores showed a trend towards a negative correlation with the magnitude of orbitofrontal cortex deactivation in bipolar patients, whereas state anxiety correlated positively with activation of dorsolateral PFC and precuneus in controls. CONCLUSIONS: Our findings confirm previous reports of decreased ventral prefrontal activity during Stroop task performance in bipolar patients, and suggest a possible negative correlation between this and depression severity in bipolar patients. These findings further highlight the ventromedial PFC as a potential candidate for illness related dysfunction in bipolar disorder.     

Imaging informational conflict: a functional magnetic resonance imaging study of numerical stroop

We employed a parametric version of the comparison Stroop paradigm to investigate the processing of numerical magnitude and physical size under task-relevant and -irrelevant conditions to investigate two theoretical issues: (1) What is the neural fate of task-irrelevant information? (2) What is the neural basis of the resolution of the conflict between task-relevant and -irrelevant information? We show in 18 healthy adults that numerical magnitudes of numbers call for higher processing requirements than physical sizes. The enhanced activation elicited by numerical magnitudes is not modulated by task relevance, indicating autonomous processing. Moreover, the normal behavioral distance effect when the numerical dimension is task relevant and reversed distance effect when it is not show that autonomous processing fully encodes numerical magnitudes. Conflict trials elicited greater activation in bilateral inferior frontal gyri, right middle frontal gyri, and right superior frontal gyri. We postulate two sources to the conflict, namely, at cognitive and response levels.     

Deficient prefrontal attentional control in late-life generalized anxiety disorder: an fMRI investigation

Younger adults with anxiety disorders are known to show an attentional bias toward negative information. Little is known regarding the role of biased attention in anxious older adults, and even less is known about the neural substrates of any such bias. Functional magnetic resonance imaging (fMRI) was used to assess the mechanisms of attentional bias in late life by contrasting predictions of a top-down model emphasizing deficient prefrontal cortex (PFC) control and a bottom-up model emphasizing amygdalar hyperreactivity. In all, 16 older generalized anxiety disorder (GAD) patients (mean age=66 years) and 12 non-anxious controls (NACs; mean age=67 years) completed the emotional Stroop task to assess selective attention to negative words. Task-related fMRI data were concurrently acquired. Consistent with hypotheses, GAD participants were slower to identify the color of negative words relative to neutral, whereas NACs showed the opposite bias, responding more quickly to negative words. During negative words (in comparison with neutral), the NAC group showed PFC activations, coupled with deactivation of task-irrelevant emotional processing regions such as the amygdala and hippocampus. By contrast, GAD participants showed PFC decreases during negative words and no differences in amygdalar activity across word types. Across all participants, greater attentional bias toward negative words was correlated with decreased PFC recruitment. A significant positive correlation between attentional bias and amygdala activation was also present, but this relationship was mediated by PFC activity. These results are consistent with reduced prefrontal attentional control in late-life GAD. Strategies to enhance top-down attentional control may be particularly relevant in late-life GAD treatment.