Biomedical subjects

Albert Kok

Publications and source records attributed to Albert Kok.

13 recordsLinked to original sources

PubMed2006

Selective attention to spatial and non-spatial visual stimuli is affected differentially by age: effects on event-related brain potentials and performance data.

To assess selective attention processes in young and old adults, behavioral and event-related potential (ERP) measures were recorded. Streams of visual stimuli were presented from left or right locations (Experiment 1) or from a central location and comprising two different spatial frequencies (Experiment 2). In both experiments, results were compared in visual-only and visual+auditory stimulus context conditions. Participants were forced to respond fast in both experiments, while maintaining high accuracy. In Experiment 1, no behavioral effects of aging were found; however, an enlargement of the N1 component in the older age group suggested that older adults initial selection process was larger than that of young adults. A late frontal effect following the P300 elicited by attended non-targets was larger in the visual+auditory condition than in the visual-only condition in the old age group. This effect was interpreted as reflecting a memory update of the relevant target location. In Experiment 2, older adults made relatively more errors in the visual+auditory condition than in visual-only condition, more so than the young adults. Older adults' ERP data were also characterized by an enlargement of the occipital selection negativity, compared to the young age group. In contrast to experiment 1, no late frontal post-P3 effect could be found, suggesting that the memory trace of the relevant stimulus feature was updated less frequently, explaining the reduction in response accuracy in the visual+auditory stimulus context conditions.

Acoustic Stimulation↗

PubMed2006

The control of attention and actions: current research and future developments.

In this introductory article of the special issue of Brain Research, we first present an overview of some general questions relating to cognitive control. For instance, one of the questions that remain to be answered is what control mechanisms and their neural underpinnings really 'do', beyond what is done by more basic 'computational' or data processing systems in the brain. We then briefly describe the four major issues addressed in the separate articles of this issue, namely attentional orienting, task set switching, performance and error monitoring, and response inhibition. In conclusion, we focus on some new methodological directions and illustrate how the study of cognitive control may be augmented by relatively newly emerging theoretical and empirical perspectives.

Animals↗

PubMed2006

Probability effects in the stop-signal paradigm: the insula and the significance of failed inhibition.

In the present randomized, mixed-trial event-related fMRI study, we examined the neural mechanisms underlying inhibitory control using a stop-signal paradigm in which stop-signal frequency was manipulated parametrically across blocks. As hypothesized, presenting stop signals less frequently was accompanied by a stronger set to respond to the go stimuli as subjects were faster in responding to go stimuli on no stop-signal trials and made more commission errors (i.e., were less successful in inhibiting the go response) on stop-signal trials. When response inhibition was successful, having to inhibit responses more frequently compared to less frequently was associated with greater activation in occipital areas. This presumably reflects enhanced visual attention to the stop signal. When response inhibition failed, greater activity was observed in bilateral insula when stop signals were presented less compared to more frequently. The insula may thus play a role in processing the significance of inhibitory failure.

Adult↗

PubMed2006

Caffeine improves anticipatory processes in task switching.

We studied the effects of moderate amounts of caffeine on task switching and task maintenance using mixed-task (AABB) blocks, in which participants alternated predictably between two tasks, and single-task (AAAA, BBBB) blocks. Switch costs refer to longer reaction times (RT) on task switch trials (e.g. AB) compared to task-repeat trials (e.g. BB); mixing costs refer to longer RTs in task-repeat trials compared to single-task trials. In a double-blind, within-subjects experiment, two caffeine doses (3 and 5mg/kg body weight) and a placebo were administered to 18 coffee drinkers. Both caffeine doses reduced switch costs compared to placebo. Event-related brain potentials revealed a negative deflection developing within the preparatory interval, which was larger for switch than for repeat trials. Caffeine increased this switch-related difference. These results suggest that coffee consumption improves task-switching performance by enhancing anticipatory processing such as task set updating, presumably through the neurochemical effects of caffeine on the dopamine system.

Adolescent↗

PubMed2005

Spatio-temporal dynamics of top-down control: directing attention to location and/or color as revealed by ERPs and source modeling.

This study investigated the nature and dynamics of the top-down control mechanisms that afford attentional selection using event-related potentials (ERPs) and dipole-source modeling. Subjects performed a task in which they were cued to direct attention to color, location, a conjunction of color and location or no specific feature on a trial-by-trial basis. Overall, similar ERP patterns were observed for directing attention to color and location, suggesting that spatial and non-spatial attention rely to a great extent on similar control mechanisms. The earliest attention-directing effect, at 340 ms, was localized to ventral posterior cortex and may reflect processes by which the cue is linked to its associated feature. Only late in the cue-target interval, differences in ERP were observed between directing attention to color and location. These originated from anterior and ventral posterior areas and may represent differences in, respectively, maintenance and perceptual biasing processes. The ventral posterior sources estimated for these late effects of directing attention to location and color were located posterior to those estimated for the modulatory effects of, respectively, spatial and non-spatial attention. This suggests that the precise neural populations involved in perceptual biasing and attentional modulation may differ. Conjunction cues initially elicited less posterior positivity than color and location cues, but evoked greater central positivity from 540 ms on. This central effect may reflect feature integration or ongoing processes related to cue-symbol translation. These results extend our understanding of the spatio-temporal dynamics of top-down attentional control.

Adult↗

PubMed2005

The orienting of visuospatial attention: an event-related brain potential study.

This study investigated the electrophysiological correlates of shifting, maintaining, and relaxing the focus of attention, using a symbolic cuing task. Cues and imperative stimuli were presented in rapid succession, and the ADJAR procedure was used to remove the contribution of event-related potential (ERP) activity associated with the imperative stimulus from the cue-related ERP waveforms. Initial analyses, comparing left and right attention-directing cues, replicated previous findings of early directing attention negativity (EDAN) and anterior directing attention negativity (ADAN) effects. To isolate ERP activity that is common to leftward and rightward attention shifts, the combined ERP activity elicited by attention-directing cues was compared to the ERP activity elicited by non-informative cues. This analysis revealed a strong and broadly distributed early positivity followed by a sustained central negativity, possibly reflecting the controlled orienting and subsequent maintenance of attentional focus. Finally, imperative stimuli preceded by non-informative cues were characterized by an enhanced posterior P2 effect, with a scalp distribution indicative of generators in visual areas. This result suggests a relatively late (re)activation in visual areas associated with the processing of stimuli that had not been cued in advance.

Adolescent↗

PubMed2005

Generating spatial and nonspatial attentional control: An ERP study.

The present study used event-related potentials and dipole source modeling to investigate dimension specificity in attentional control. Subjects performed cued attention tasks in which the task-relevant information (a) was always the same, (b) varied between features within the same dimension, or (c) varied between features of two different dimensions. Thus, both demands on control processes involved in generating an attentional set and the dimension (color or location) of the task-relevant feature were varied. Attentional control was associated with a dorsal posterior positivity starting at 260 ms postcue, which was stronger over left posterior scalp regions from 580 ms onward, especially when color was task relevant. This positivity likely reflects generic processes involved in the generation of an attentional set that were followed in time by dimension-specific processes related to the persistence of the task-relevant information in working memory.

Adolescent↗

PubMed2004

Caffeine strengthens action monitoring: evidence from the error-related negativity.

The medial frontal cortex, especially the anterior cingulate cortex (ACC), is involved in action monitoring. We studied the role of moderate amounts of caffeine in action monitoring as expressed by the error-related negativity (ERN), an event-related brain component that reflects ACC activity. In a double-blind, placebo-controlled, within-subjects experiment, two caffeine doses (3 and 5 mg/kg body weight) and a placebo were administered to habitual coffee drinkers. Compared with placebo, both caffeine doses enlarged the ERN. Amplitudes of the P2 and P3 components were not affected by caffeine. Thus, the enlarged ERN after caffeine reflects a specific effect on action monitoring. We conclude that consumption of a few cups of coffee strengthens central information processing, specifically the monitoring of ongoing cognitive processes for signs of erroneous outcomes. Brain areas related to action monitoring such as the ACC presumably mediate these caffeine effects.

Adolescent↗

PubMed2004

ERP components associated with successful and unsuccessful stopping in a stop-signal task.

The primary aim of this study was to examine how response inhibition is reflected in components of the event-related potential (ERP), using the stop-signal paradigm as a tool to manipulate response inhibition processes. Stop signals elicited a sequence of N2/P3 components that partly overlapped with ERP components elicited by the reaction stimulus. N2/P3 components were more pronounced on stop-signal trials than on no-stop-signal trials. At Cz, the stop-signal P3 peaked earlier on successful than on unsuccessful stop trials. This finding extends the horse race model by demonstrating that the internal response to the stop signal (as reflected in stop-signal P3) is not constant, but terminates at different moments in time on successful and unsuccessful stop trials. In addition, topographical distributions and dipole analysis of high density EEG recordings indicated that different cortical generators were involved in P3s elicited on successful and unsuccessful stop-signal trials. The latter results suggest that P3 on successful stop-signal trials not only reflects stop-signal processing per se, but also efficiency of inhibitory control.

Adolescent↗

PubMed2003

Attention as a characteristic of nonclinical dissociation: an event-related potential study.

Individual differences in dissociative style (which is generally considered a risk factor for dissociative pathology) were studied in a nonclinical sample. It was hypothesized that high-dissociative participants would show enhanced attentional abilities toward both relevant and irrelevant stimulus features. In the experiment, threatening and affectively neutral words were classified on their affective valence and the presence of the letter A. To facilitate the full deployment of dissociative abilities, a feature (i.e., negative valence) was included that would automatically attract attention but not interfere with the processing of the central feature. Both the behavioral measures (i.e., reaction time) and the central neural measures (i.e., event-related potentials) showed that the ability to direct attention to the central feature was enhanced in the high dissociators. The high dissociators, moreover, showed evidence of directing attention to both affective valence and the letter A in the letter detection task. It is concluded that dissociative style does not correspond to a damaged or disturbed function but to an enhanced ability to direct and divide attention.

Adult↗

PubMed2003

Effects of mixed versus blocked design on stimulus evaluation: combining underadditive effects.

According to the asynchronous discrete coding model of Miller, two manipulations should display underadditive effects on reaction time if they slow down noncontingent stages associated with the processing of two separable dimensions of a stimulus. Underadditive effects are also predicted by a dual route model when a task variable is factorially varied with design type (mixed vs blocked). Interpretations of both underadditive effects and their combination were evaluated. Intact and degraded stimuli were presented to 18 young adults either in a single block (mixed) or in separate blocks (blocked). Spatial stimulus-response (S-R) compatibility was manipulated in all conditions. Stimulus degradation and S-R compatibility interacted underadditively, but only in blocked presentations. Both interpretations of underadditive effects were supported. Eye-movement registrations provided additional support for the alternative routes model.

Adolescent↗

PubMed2002

A computational account of altered error processing in older age: dopamine and the error-related negativity.

When participants commit errors or receive feedback signaling that they have made an error, a negative brain potential is elicited. According to Holroyd and Coles's (in press) neurocomputational model of error processing, this error-related negativity (ERN) is elicited when the brain first detects that the consequences of an action are worse than expected. To study age-related changes in error processing, we obtained performance and ERN measures of younger and high-functioning older adults. Experiment 1 demonstrated reduced ERN amplitudes in older adults in the context of otherwise intact brain potentials. This result could not be attributed to uncertainty about the required response in older adults. Experiment 2 revealed impaired performance and reduced response- and feedback-related ERNs of older adults in a probabilistic learning task. These age changes could be simulated by manipulation of a single parameter of the neurocomputational model, this manipulation corresponding to weakened phasic activity of the mesencephalic dopamine system.

Adolescent↗

PubMed2002

Intermodal spatial attention differs between vision and audition: an event-related potential analysis.

Subjects were required to attend to a combination of stimulus modality (vision or audition) and location (left or right). Intermodal attention was measured by comparing event-related potentials (ERPs) to visual and auditory stimuli when the modality was relevant or irrelevant, while intramodal (spatial) attention was measured by comparing ERPs to visual and auditory stimuli presented at relevant and irrelevant spatial locations. Intramodal spatial attention was expressed differently in visual and auditory ERPs. When vision was relevant, spatial attention showed a contralateral enhancement of posterior N1 and P2 components and enhancement of parietal P3. When audition was relevant, spatial attention showed a biphasic fronto-central negativity, starting after around 100 ms. The same effects were also present in ERPs to stimuli that were presented in the irrelevant modality. Thus, spatial attention was not completely modality specific. Intermodal attention effects were also expressed differently in vision and audition. Taken together, the obtained ERP patterns of the present study show that stimulus attributes such as modality and location are processed differently in vision and audition.

Adolescent↗