Robert West

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.

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.

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.

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.

In defense of the frontal lobe hypothesis of cognitive aging.

Over the past decade a number of reviewers have argued that the pattern of spared and impaired cognitive functions observed in the cognitive aging literature can be understood within the context of a neuropsychologically constrained model that proposes that the prefrontal cortex is more vulnerable to the effects of normal aging than other cortical regions (Dempster, 1992; Hartley, 1993; Moscovitch & Winocur, 1992; West, 1996). While this model has served as a valuable heuristic in my own thinking about cognitive aging, the current review of Greenwood and theoretical work emerging from other laboratories argues that the “frontal lobe hypothesis” (FLH) no longer provides a useful or accurate framework within which to understand brain behavior relationships in cognitive aging. While I agree with several of the ideas developed in the review of Greenwood (this issue) I believe the continued utility of the FLH can be demonstrated by considering two questions: (1) What does the FLH predict about cognitive aging and is there support for these predictions? and (2) Has the time come to move beyond a general specification of the FLH? In answering these questions I will provide one or two findings from the empirical literature. These are intended to be illustrative, not exhaustive.

Neural correlates of conflict processing

In this study we examined the neural correlates of conflict processing in the Stroop, counting, and digit-location tasks using event-related brain potentials (ERPs). The behavioral data revealed robust interference in response time and accuracy for all tasks. The interference effect for response time was greater in the Stroop task than the other tasks; in contrast, the interference effect for response accuracy was greater in the counting tasks than the other tasks. The N450 and sustained potential (SP) were elicited in each task. Partial least-squares (PLS) analysis was used to examine the structural relationships between the ERPs, task design, and behavior. TaskPLS analysis revealed that the N450 and SP were associated with a single latent variable leading to the suggestion that a common set of neural generators was recruited during conflict processing across the tasks and that there were differences between ERPs related to early processing across the three tasks. BehavioralPLS analysis revealed that the amplitude of the SP was positively correlated with response time and accuracy, indicating that this modulation of the ERPs may be related to response selection rather than to conflict resolution.

Neural activity associated with the realization of a delayed intention

This study examines neural activity associated with the realization of a delayed intention within the context of the noticing+search model of prospective memory (PM) using event-related brain potentials (ERPs). The noticing+search model proposes that PM is supported by two related processes, noticing (the detection of a PM cue in the environment) and search (the retrieval of an intention from memory). In two experiments participants performed a PM task that permitted the dissociation of the noticing and search processes. Noticing was associated a phasic negativity over the occipital-parietal region (N320) and search was associated with a sustained modulation (slow-wave) reflecting a negativity over the right frontal region and a broadly distributed positivity over the parietal region. The amplitude of the N320 was greater when the PM cue was associated with an intention than when the cue was irrelevant to task performance, leading to the proposal that noticing may be accomplished through the attentional modulation of neural systems which support processing of the defining features of the PM cue. The topography of the slow-wave resembled that of modulations of the ERP associated with the recollection of information in studies of retrospective memory leading to the suggestion that similar neural processes may support the recovery of information from memory in both prospective and retrospective memory tasks.

Adjustments of cognitive control in younger and older adults.

The effects of aging on behavioral and electrophysiological indices of adjustments of cognitive control were examined in two experiments. Specifically, we considered the effects of aging of patterns of response time and modulations of the event-related brain potentials (ERPs) on sequential trial effects, error-related slowing, and local switch costs in two modified Stroop tasks. The behavioral data reveal that sequential trial effects were observed when color, but not word, identification was required and that these effects were similar in younger and older adults; that the degree of error-related slowing was similar for younger and older adults in Experiment 1 and greater in older than younger adults in Experiment 2; that local switch costs in response time were similar in younger and older adults and that the requirement to switch between color and word identification resulted in a dramatic increase in the number of color intrusion errors in older adults in Experiment 2. The ERP data revealed that endogenously generated adjustments of cognitive control were associated with a parietal slow wave that mirrored the behavioral data and was similar in amplitude in younger and older adults and an anterior frontal slow wave that was absent in older adults. The ERP data also revealed that an enhancement of the P3 component and a frontal slow wave that differentiated color switch trials from color non-switch and word trials in younger adults were attenuated in older adults.

Neural correlates of the formation and realization of delayed intentions

Prospective memory (PM) can be thought of as the ability to successfully form and later realize intentions that must be delayed over some period of time. In this study, event-related brain potentials were used to explore the neural activity associated with the formation and realization of an intention. Greater negativity over the frontal-polar region was associated with intention formation trials in which the intention was later realized. On PM cue trials, an N300 was associated with the detection of a cue. For PM cue trials, a late positive complex was observed that might have reflected the retrieval of an intention from memory, and a frontal slow wave was observed that might have reflected the activity of a neural system that supported disengagement from the ongoing activity when the cue was detected.

Neurophysiological evidence for disturbances of conflict processing in patients with schizophrenia.

Deficits in cognition are a hallmark of schizophrenia. In the present study, the authors investigated the effects of schizophrenia on the neural correlates of conflict processing in a single-trial version of the Stroop task by using event-related brain potentials. Relative to matched controls, patients with schizophrenia showed increased Stroop interference in response time, but this effect was eliminated when the effect of response slowing was controlled. In controls, conflict processing was associated with a negative wave peaking between 400 and 500 ms (N450) and conflict sustained potential (SP) peaking between 600 and 800 ms after stimulus onset. In patients with schizophrenia, the amplitude of the N450 was significantly attenuated and the conflict SP was absent. These results provide evidence for the existence of altered neural processes associated with conflict processing that may be associated with dysfunction of the anterior cingulate and prefrontal cortex in patients with schizophrenia.