Evidence for the transient nature of a neural system supporting goal-directed action

The neural dynamics of conflict adaptation induced by conflict observation: Evidence from univariate and multivariate analysis

Conflict adaptation can be expressed as greater performance (shorter response time and lower error rate) after incongruent trials when compared to congruent trials. It has been observed in designs that minimize confounding factors, i.e., feature integration, contingency learning, and temporal learning. Our current study aimed to further elucidate the temporal evolution mechanisms of conflict adaptation. To address this issue, the current study employed a combination of behavioral, univariate, and multivariate analysis (MVPA) methods in a modified color-word Stroop task, where half of the trials required button presses (DO trials), and the other half only required observation (LOOK trials). Both behavioral and the ERP results (N450 and SP) in the LOOK-DO transition trials revealed significant conflict adaptation without feature integration, contingency learning, and temporal learning, providing support for the conflict monitoring theory. Furthermore, during the LOOK trials, significant Stroop effect in the N450 and SP components were observed, indicating that conflict monitoring occurred at the stimulus level and triggered reactive control adjustments. The MVPA results decoded the congruent-incongruent and incongruent-incongruent conditions during the conflict adjustment phase but not during the conflict monitoring phase, emphasizing the unique contribution of conflict adjustment to conflict adaptation. The current research findings provided more compelling supporting evidence for the conflict monitoring theory, while also indicating that future studies should employ the present design to elucidate the specific processes of conflict adaptation.

Mapping causal links between prefrontal cortical regions and intra-individual behavioral variability

Intra-individual behavioral variability is significantly heightened by aging or neuropsychological disorders, however it is unknown which brain regions are causally linked to such variabilities. We examine response time (RT) variability in 21 macaque monkeys performing a rule-guided decision-making task. In monkeys with selective-bilateral lesions in the anterior cingulate cortex (ACC) or in the dorsolateral prefrontal cortex, cognitive flexibility is impaired, but the RT variability is significantly diminished. Bilateral lesions within the frontopolar cortex or within the mid-dorsolateral prefrontal cortex, has no significant effect on cognitive flexibility or RT variability. In monkeys with lesions in the posterior cingulate cortex, the RT variability significantly increases without any deficit in cognitive flexibility. The effect of lesions in the orbitofrontal cortex (OFC) is unique in that it leads to deficits in cognitive flexibility and a significant increase in RT variability. Our findings indicate remarkable dissociations in contribution of frontal cortical regions to behavioral variability. They suggest that the altered variability in OFC-lesioned monkeys is related to deficits in assessing and accumulating evidence to inform a rule-guided decision, whereas in ACC-lesioned monkeys it results from a non-adaptive decrease in decision threshold and consequently immature impulsive responses.

Pupillary correlates of preparatory control in the Stroop task

In three experiments, individual differences in preparatory control in the Stroop task were examined. Participants performed variants of the Stroop task while pupillary responses were examined during the preparatory interval. Variation in working memory capacity was also examined. High Stroop performers tended to demonstrate larger preparatory pupillary responses than low Stroop performers. In Experiment 2, when participants were given pre-cues indicating the congruency of the upcoming trial (MATCHING vs. CONFLICTING), high Stroop performers had larger preparatory pupillary responses for incongruent trials compared to congruent trials, whereas low Stroop performers demonstrated similar preparatory pupillary responses on both incongruent and congruent trials. These results suggest that variation in Stroop performance is partially due to individual differences in the ability to ramp up and regulate the intensity of attention allocated to preparatory control processes. Additionally, there was limited evidence that preparatory control processes partially account for the relation between working memory capacity and performance on the Stroop. Overall, these results provide evidence that individual differences in Stroop performance are partialy due to variation in preparatory control.

Attaining the recesses of the cognitive space

Existing neuropsychological tests of executive function often manifest a difficulty pinpointing cognitive deficits when these are intermittent and come in the form of omissions. We discuss the hypothesis that two partially interrelated reasons for this failure stem from relative inability of neuropsychological tests to explore the cognitive space and to explicitly take into account strategic and opportunistic resource allocation decisions, and to address the temporal aspects of both behaviour and task-related brain function in data analysis. Criteria for tasks suitable for neuropsychological assessment of executive function, as well as appropriate ways to analyse and interpret observed behavioural data are suggested. It is proposed that experimental tasks should be devised which emphasize typical rather than optimal performance, and that analyses should quantify path-dependent fluctuations in performance levels rather than averaged behaviour. Some implications for experimental neuropsychology are illustrated for the case of planning and problem-solving abilities and with particular reference to cognitive impairment in closed-head injury.

The influence of working memory capacity and lapses of attention for variation in error monitoring

In two experiments, individual differences in working memory capacity (WMC), lapses of attention, and error monitoring were examined. Participants completed multiple WMC tasks along with a version of the Stroop task. During the Stroop, pupil diameter was continuously monitored. In both experiments, error phasic pupillary responses were larger than phasic pupillary responses associated with correct incongruent and correct congruent trials. WMC and indicators of lapses of attention were correlated with error pupillary response, suggesting that high WMC and low lapse individuals had enhanced error monitoring abilities compared with low WMC and high lapse individuals. Furthermore, in Experiment 2 error awareness abilities were associated with WMC, lapses of attention, and the error phasic pupillary responses. Importantly, individual differences in the susceptibility to lapses of attention largely accounted for the relationship between WMC and error monitoring in both experiments. Collectively, these results suggest that WMC is related to error monitoring abilities, but this association is largely due to individual differences in the ability to consistently maintain task engagement and avoid lapses of attention.

The neural substrate and underlying mechanisms of executive control fluctuations in primates

Trial-by-trial alterations in response time have been linked to fluctuations of executive control and transient lapses of attention. Here, we report remarkable homologies in performance-dependent fluctuations of response time between humans and monkeys. We examined the effects of selective bilateral lesions in four frontal regions on control fluctuations in the context of a rule-shifting task. Lesions within orbitofrontal cortex (OFC), but not within superior-lateral prefrontal cortex, significantly exaggerated the performance-dependent fluctuations of control and prevented its restoration following feedback. Lesions within dorsolateral prefrontal cortex (DLPFC) or within anterior-cingulate cortex (ACC) led to instability of control and disruption of its link with monkeys' upcoming decisions. Examining the activity of DLPFC and OFC cells shed more lights on the underlying neuronal mechanisms by showing that before the start of each trial, OFC cell activity conveyed detailed information regarding the current state of executive control and the likelihood of success or failure in the future decisions. This further emphasizes the crucial role of OFC in the trial-by-trial allocation (setting) of control to the ongoing task. These findings bring insights to the neural architecture of executive control in primates and suggest that DLPFC and ACC support sustained executive control, but OFC is more involved in setting and restoring the control.

Give me just a little more time: effects of alcohol on the failure and recovery of cognitive control

Numerous externalizing behaviors, from aggression to risk taking to drug abuse, stem from impaired cognitive control, including that brought about by the acute effects of alcohol. Although research generally indicates that alcohol impairs cognitive abilities, a close examination of the literature suggests that alcohol's effects are quite variable and likely depend on a number of contextual factors. The purpose of the current study was to characterize the effects of alcohol on cognitive control in terms of neural and behavioral responses to successful and unsuccessful control attempts. Participants were randomly assigned to consume an alcohol (0.80 g/kg ETOH), placebo, or nonalcoholic control beverage prior to completing a cognitive control (flanker) task while event-related brain potentials were recorded. Alcohol reduced the amplitude of the error-related negativity on error trials and increased the posterror compatibility effect in response time. Of particular interest, neural indices of conflict monitoring and performance adjustment (frontal slow wave) were attenuated by alcohol, but only on trials following errors. These functions had recovered, however, by 2 trials after an error. These findings suggest that alcohol's effects on cognitive control are best characterized as impaired (or delayed) recovery following control failures. Implications of these findings for understanding alcohol's effects on behavioral undercontrol are discussed.

The neural dynamics of conflict adaptation within a look-to-do transition

BACKGROUND

For optimal performance in conflict situations, conflict adaptation (conflict detection and adjustment) is necessary. However, the neural dynamics of conflict adaptation is still unclear.

METHODS

In the present study, behavioral and electroencephalography (EEG) data were recorded from seventeen healthy participants during performance of a color-word Stroop task with a novel look-to-do transition. Within this transition, participants looked at the Stroop stimuli but no responses were required in the 'look' trials; or made manual responses to the Stroop stimuli in the 'do' trials.

RESULTS

In the 'look' trials, the amplitude modulation of N450 occurred exclusively in the right-frontal region. Subsequently, the amplitude modulation of sustained potential (SP) emerged in the posterior parietal and right-frontal regions. A significantly positive correlation between the modulation of reconfiguration in the 'look' trials and the behavioral conflict adaptation in the 'do' trials was observed. Specially, a stronger information flow from right-frontal region to posterior parietal region in the beta band was observed for incongruent condition than for congruent condition. In the 'do' trials, the conflict of 'look' trials enhanced the amplitude modulations of N450 in the right-frontal and posterior parietal regions, but decreased the amplitude modulations of SP in these regions. Uniquely, a stronger information flow from centro-parietal region to right-frontal region in the theta band was observed for iI condition than for cI condition.

CONCLUSION

All these findings showed that top-down conflict adaptation is implemented by: (1) enhancing the sensitivity to conflict detection and the adaptation to conflict resolution; (2) modulating the effective connectivity between parietal region and right-frontal region.

Neural correlates of conceptual knowledge for actions

The neural correlates of conceptual knowledge for actions are not well understood. To begin to address this knowledge gap, we tested the hypothesis that the retrieval of conceptual knowledge for actions depends on neural systems located in higher-order association cortices of left premotor/prefrontal, parietal, and posterior middle temporal regions. The investigation used the lesion method and involved 90 subjects with damage to various regions of the left or right hemisphere. The experimental tasks measured retrieval of knowledge for actions, in a nonverbal format: Subjects evaluated attributes of pictured actions, and compared and matched pictures of actions. In support of our hypothesis, we found that the regions of highest lesion overlap in subjects with impaired retrieval of conceptual knowledge for actions were in the left premotor/prefrontal sector, the left parietal region, and in the white matter underneath the left posterior middle temporal region. These sites are partially distinct from those identified previously as being important for the retrieval of words for actions. We propose that a key function of the sites is to operate as two-way intermediaries between perception and concept retrieval, to promote the retrieval of the multidimensional aspects of knowledge that are necessary and sufficient for the mental representation of a concept of a given action.

On the role of conflict and control in social cognition: event-related brain potential investigations

Numerous social-cognitive models posit that social behavior largely is driven by links between constructs in long-term memory that automatically become activated when relevant stimuli are encountered. Various response biases have been understood in terms of the influence of such "implicit" processes on behavior. This article reviews event-related potential (ERP) studies investigating the role played by cognitive control and conflict resolution processes in social-cognitive phenomena typically deemed automatic. Neurocognitive responses associated with response activation and conflict often are sensitive to the same stimulus manipulations that produce differential behavioral responses on social-cognitive tasks and that often are attributed to the role of automatic associations. Findings are discussed in the context of an overarching social cognitive neuroscience model in which physiological data are used to constrain social-cognitive theories.

Tracking the Temporal Dynamics of Updating Cognitive Control: An Examination of Error Processing

In 2 experiments, we used event-related brain potentials (ERPs) to examine the temporal dynamics of neural processes related to adjustments of cognitive control following errors in the counting Stroop task. The ERPs elicited by errors revealed the error-related negativity (ERN) and error positivity consistent with a large body of literature. In addition, errors were associated with a frontal slow wave between 200 and 2000 ms after the response that was consistent with the activity of neural generators in the lateral frontal cortex. The ERN and frontal slow wave were correlated with posterror slowing of response time and positive affect (i.e., happiness and calmness) during task performance. These data are consistent with the idea that interactions between anterior cingulate cortex and lateral frontal cortex support adjustments of cognitive control and that this neural network is sensitive to the influence of affect experienced during task performance.

The missing link between action and cognition

The study of the neural correlates of motor behaviour at the systems level has received increasing consideration in recent years. One emerging observation from this research is that neural regions typically associated with cognitive operations may also be recruited during the performance of motor tasks. This apparent convergence between action and cognition - domains that have most often been studied in isolation - becomes especially apparent when examining new complex motor skills such as those involving sequencing or coordination, and when taking into account external (environment-related) factors such as feedback availability and internal (performer-related) factors such as pathology. Neurally, overlap between action and cognition is prominent in frontal lobe areas linked to response selection and monitoring. Complex motor tasks are particularly suited to reveal the crucial link between action and cognition and the generic brain areas at the interface between these domains.

Automatic and controlled processes and the development of addictive behaviors in adolescents: a review and a model

This paper presents a review and a model of the development of addictive behaviors in (human) adolescents, with a focus on alcohol. The model proposes that addictive behaviors develop as the result of an imbalance between two systems: an appetitive, approach-oriented system that becomes sensitized with repeated alcohol use and a regulatory executive system that is not fully developed and that is compromised by exposure to alcohol. Self-regulation critically depends on two factors: ability and motivation to regulate the appetitive response tendency. The motivational aspect is often still weak in heavy drinking adolescents, who typically do not recognize their drinking as problematic. Motivation to regulate use often develops only years later, after the individual has encountered serious alcohol-related problems. Unfortunately, at that point behavioral change becomes harder due to several neurocognitive adaptations that result from heavy drinking. As we document, there is preliminary support for the central elements of the model (appetitive motivation vs. self-regulation), but there is a paucity of research directly addressing these mechanisms in human adolescents. Further, we emphasize that adolescent alcohol use primarily takes place in a social context, and that therefore studies should not solely focus on intra-individual factors predicting substance use and misuse but also on interpersonal social factors. Finally, we discuss implications of the model for interventions.

Performance-related activity in medial rostral prefrontal cortex (area 10) during low-demand tasks

Neuroimaging studies have frequently observed relatively high activity in medial rostral prefrontal cortex (PFC) during rest or baseline conditions. Some accounts have attributed this high activity to the occurrence of unconstrained stimulus-independent and task-unrelated thought processes during baseline conditions. Here, the authors investigated the alternative possibility that medial rostral PFC supports attention toward the external environment during low-demand conditions. Participants performed a baseline simple reaction time (RT) task, along with 3 other tasks that differed in the requirement to attend to external stimuli versus stimulus-independent thought. Medial rostral PFC activation was observed in the baseline task and in a condition requiring strong engagement with external stimuli, relative to 2 conditions with a greater requirement for stimulus-independent thought. An important finding was that activity in this region was associated with faster RTs in the baseline task, ruling out an explanation in terms of task-unrelated thought processes during this condition. Thus, at least under certain circumstances, medial rostral PFC appears to support attention toward the external environment, facilitating performance in situations that do not require extensive processing of experimental stimuli.

Working memory capacity and the top-down control of visual search: Exploring the boundaries of "executive attention"

The executive attention theory of working memory capacity (WMC) proposes that measures of WMC broadly predict higher order cognitive abilities because they tap important and general attention capabilities (R. W. Engle & M. J. Kane, 2004). Previous research demonstrated WMC-related differences in attention tasks that required restraint of habitual responses or constraint of conscious focus. To further specify the executive attention construct, the present experiments sought boundary conditions of the WMC-attention relation. Three experiments correlated individual differences in WMC, as measured by complex span tasks, and executive control of visual search. In feature-absence search, conjunction search, and spatial configuration search, WMC was unrelated to search slopes, although they were large and reliably measured. Even in a search task designed to require the volitional movement of attention (J. M. Wolfe, G. A. Alvarez, & T. S. Horowitz, 2000), WMC was irrelevant to performance. Thus, WMC is not associated with all demanding or controlled attention processes, which poses problems for some general theories of WMC.

Temporal dissociation of components of cognitive control dysfunction in severe TBI: ERPs and the cued-Stroop task

Cognitive control comprises two essential interactive component processes: a regulative component supporting the activation and implementation of control and an evaluative component that monitors the need for regulative control and signals when adjustments in control are necessary. Survivors of severe traumatic brain injury (TBI) experience cognitive control impairments, but the specific nature of these impairments is poorly characterized. Using event-related potentials (ERPs) acquired in the context of a trial-by-trial task-switching version of the Stroop task we temporally dissociated the regulative and evaluative processes in order to shed light on the potential roles of these components in TBI-related cognitive control impairment. Behaviorally, TBI patients showed a specific performance deficit suggestive of a failure to implement cognitive control in the service of processing conflict information. ERP findings showed that TBI patients were impaired in both the implementation of control and subsequent detection and processing of conflict. TBI patients were also impaired on a measure of working memory capacity, a measure that correlated with the ability to implement regulative control and overcome conflict. These findings suggest that patients with predominantly chronic severe TBI patients are impaired on both regulative and evaluative components of cognitive control, and may have implications for the design and evaluation of behavioral and pharmacological remediation strategies.

Stereotype activation and control of race bias: Cognitive control of inhibition and its impairment by alcohol

Two experiments tested the hypothesis that alcohol increases race-biased responding via impairment of self-regulatory cognitive control. Participants consumed either a placebo or alcohol and then made speeded responses to stereotypic trait words presented after White and Black face primes while behavioral and event-related brain potential (ERP) data were recorded. Alcohol did not affect stereotype activation in either experiment. Experiment 2 showed that alcohol significantly impaired the ability to inhibit race-biased responses but did not reliably influence control of counterstereotypic responses. This disinhibition appears driven by impairment of regulative cognitive control, as indexed by amplitude of the negative slow wave ERP component. These findings suggest that controlling racial bias can be a function of effective implementation of basic self-regulatory processes in addition to the motivational processes identified in other research.

Strategic control and medial frontal negativity: Beyond errors and response conflict

Errors in timed choice tasks typically produce an error-related negativity (ERN) in the event-related potential (ERP). The error specificity of the ERN has been challenged by studies showing a correct response negativity (CRN). Forty-five participants engaged in a flanker task in which both compatibility between flankers and target and the probability of compatible flankers were manipulated. Correct responses elicited a CRN, the amplitude of which increased with the degree of mismatch between the presence of conflict and conflict probability, even on low-conflict (compatible) trials. The fronto-central N2 component was larger on high-conflict (incompatible) correct response trials. However, in contrast to some recent accounts, this N2 was largest for highly probable stimuli. These findings suggest revision to models of the effects of conflict on response-related negativity to account for strategic adjustments made in preparation for the response.

Event-based prospective memory following severe closed-head injury

Twenty-four severe closed-head injury (CHI) participants and 24 controls completed event-based prospective memory tasks concurrently with an ongoing working memory task. The event cue was either integrated with the ongoing working memory task (focal cue) or peripheral to it. Prospective remembering was poorer for the CHI group in both the focal- and peripheral-cue conditions. The groups did not differ on the ongoing task. The peripheral cue and the integrated focal cue also did not differ in ability to trigger prospective remembering. The results suggest that, even with highly salient event cues, severe CHI participants (> 1 year postinjury) are more likely than controls to exhibit prospective memory failures. The data revealed a link between CHI participants' prospective memory failures and momentary lapses of intention.

The Effects of Aging on Controlled Attention and Conflict Processing in the Stroop Task

Recent computational modeling and behavioral work indicate that age-related declines in the ability to represent task context may contribute to disruptions of working memory and selective attention in older adults. However, it is unclear whether age-related declines in context processing arise from a disruption of the encoding or maintenance of task context and how age-related declines in context processing interact with mechanisms supporting conflict detection and resolution processes contributing to efficient selection of task-relevant information. This study examines the effects of aging on the neural correlates of context and conflict processing in the Stroop task using event-related brain potentials (ERPs). Age-related differences in the time course of modulations of the ERPs associated with encoding (P3) and maintaining (slow wave) task context were observed. There were also age-related differences in the N450, conflict SP, and ERN associated with conflict processing that interacted with task context. These data indicate that aging is associated with declines in the efficiency of those neural mechanisms supporting both context and conflict processing, and that the effects of aging are not pervasive but rather interact with task context.

Neural mechanisms of transient and sustained cognitive control during task switching

A hybrid blocked and event-related functional magnetic resonance imaging (fMRI) study decomposed brain activity during task switching into sustained and transient components. Contrasting task-switching blocks against single-task blocks revealed sustained activation in right anterior prefrontal cortex (PFC). Contrasting task-switch trials against task-repeat and single-task trials revealed activation in left lateral PFC and left superior parietal cortex. In both sets of regions, activation dynamics were strongly modulated by trial-by-trial fluctuations in response speed. In addition, right anterior PFC activity selectively covaried with the magnitude of mixing cost (i.e., task-repeat versus single-task trial performance), and left superior parietal activity selectively covaried with the magnitude of the switching cost (i.e., task-switch versus task-repeat trial performance). These results indicate a functional double dissociation in brain regions supporting different components of cognitive control during task switching and suggest that both sustained and transient control processes mediate the behavioral performance costs of task switching.

Alcohol and cognitive control: implications for regulation of behavior during response conflict

Alcohol intoxication often leads to dysregulated behavior in contexts characterized by conflict between prepotent response tendencies and incompatible alternative responses. Recent research has identified 2 components of an anterior executive attention system that are essential for adaptive behavior when response conflict exists. Event-related potential (ERP) measures of evaluative and regulative cognitive control were collected to determine if impaired executive attention was responsible for observed behavior deficits when intoxicated. Intoxicated participants displayed task performance deficits on incongruent color-naming trials relative to sober controls. Alcohol did not affect P3 magnitude/latency, indicating that timing and integrity of stimulus evaluation remained intact. In contrast, alcohol did reduce frontal components of ERP that index evaluative and regulative cognitive control processes.

Neural correlates of cognitive control and conflict detection in the Stroop and digit-location tasks

In two experiments, event-related brain potentials were used to examine the neural correlates of cognitive control and conflict processing in the Stroop and digit-location tasks. The relevant dimension was cued on a trial-by-trial basis before stimulus onset. This method allows one to dissociate modulations of the event-related brain potentials (ERPs) associated with cognitive control from modulations of the ERPs associated with conflict processing that are temporally confounded in the standard Stroop task. Cognitive control was associated with a slow wave over the occipital-parietal region that differentiated goal-compatible from goal-incompatible responses in both the Stroop and digit-location tasks. Conflict processing was associated with a N450, in the Stroop task, and a conflict sustained potential (SP) in both tasks. The amplitude of the N450 was similar for color naming and word reading trials. The character of the conflict SP varied over the frontal and central-parietal regions. Over the left lateral frontal region the conflict SP was similar in amplitude across type of response in the Stroop and digit-location tasks. Over the right lateral frontal region the conflict SP was greater in amplitude for the less dominant stimulus dimension (i.e. color or digit). The findings of these experiments clearly demonstrate that different neural systems support cognitive control and conflict processing, with some degree of hemispheric specialization observed for conflict processing.

Working-memory capacity and the control of attention: the contributions of goal neglect, response competition, and task set to Stroop interference

Individual differences in working-memory (WM) capacity predicted performance on the Stroop task in 5 experiments, indicating the importance of executive control and goal maintenance to selective attention. When the Stroop task encouraged goal neglect by including large numbers of congruent trials (RED presented in red), low WM individuals committed more errors than did high WM individuals on the rare incongruent trials (BLUE in red) that required maintaining access to the "ignore-the-word" goal for accurate responding. In contrast, in tasks with no or few congruent trials, or in high-congruency tasks that followed low-congruency tasks, WM predicted response-time interference. WM was related to latency, not accuracy, in contexts that reinforced the task goal and so minimized the difficulty of actively maintaining it. The data and a literature review suggest that Stroop interference is jointly determined by 2 mechanisms, goal maintenance and competition resolution, and that the dominance of each depends on WM capacity, as well as the task set induced by current and previous contexts.

The role of prefrontal cortex in working-memory capacity, executive attention, and general fluid intelligence: an individual-differences perspective

We provide an "executive-attention" framework for organizing the cognitive neuroscience research on the constructs of working-memory capacity (WMC), general fluid intelligence, and prefrontal cortex (PFC) function. Rather than provide a novel theory of PFC function, we synthesize a wealth of single-cell, brain-imaging, and neuropsychological research through the lens of our theory of normal individual differences in WMC and attention control (Engle, Kane, & Tuholski, 1999; Engle, Tuholski, Laughlin, & Conway, 1999). Our critical review confirms the prevalent view that dorsolateral PFC circuitry is critical to executive-attention functions. Moreover, although the dorsolateral PFC is but one critical structure in a network of anterior and posterior "attention control" areas, it does have a unique executive-attention role in actively maintaining access to stimulus representations and goals in interference-rich contexts. Our review suggests the utility of an executive-attention framework for guiding future research on both PFC function and cognitive control.

Lapses of intention and performance variability reveal age-related increases in fluctuations of executive control

We examine the hypothesis that the efficiency of executive control processes is less stable over time in older than younger adults. An age-related decrease in the efficiency of executive control should result in an increase in performance variability in task conditions requiring the recruitment of executive control processes and not in task conditions requiring minimal involvement of executive control. Performance variability was similar for younger and older adults in task conditions requiring minimal executive control and greater for older than younger adults in task conditions requiring executive control. These and other data are consistent with the proposal that aging is associated with a decrease in the stability of executive control over time.

Event-related neural activity associated with habit and recollection

The neural activity associated with conscious recollection and habit was examined using event-related brain potentials. In a training phase, participants learned A-B, A-C word associations in which the probability of specific responses was varied. Once a habit was established, participants studied and were tested on a series of short lists consisting of word pairs seen in training. The process-dissociation procedure was used to estimate the contribution of habit and recollection to memory performance. Habit estimates reflected the probability with which information was presented in training but recollection estimates did not show this effect. Recollection was associated with sustained negativity over the parieto-occipital region, which was opposite in polarity over the frontal regions. Indices of habit strength were associated with a sustained positivity over left fronto-temporal regions and a sustained negativity over right fronto-central regions. Partial-least squares analyses revealed two significant latent variables that distinguished recollection and habit, consistent with the distinction between consciously controlled and automatic influences of memory.

Effects of task context and fluctuations of attention on neural activity supporting performance of the stroop task

The influence of task context and transient fluctuations in attentional control on neural processes supporting performance of the Stroop task was investigated using event-related brain potentials. Task context was manipulated by varying the proportion of congruent and incongruent trials across different blocks of trials, and fluctuations of attentional control were considered by examining differences between trials eliciting faster and slower responses. The amplitudes of the N450, thought to reflect the suppression of a conceptual level processing system, and a temporo-parietal slow wave, thought to index the processing of color information, were greater when trials were mostly congruent in comparison to when trials were mostly incongruent. These findings indicate that the neural systems supporting inhibition and color processing are modulated by task demands. For the N450 the effect of task context interacted with the efficiency of attentional control being present for those trials eliciting faster responses and not for those trials eliciting slower responses. This finding is consistent with those from a growing number of studies indicating that the neural systems supporting attentional control are transient in nature, tending to fluctuate in efficiency over time.