A. Wijers - Academia.edu (original) (raw)

Papers by A. Wijers

Research paper thumbnail of Sentence comprehension and word repetition: A positron emission tomography investigation

Psychophysiology, Oct 31, 1999

Using positron emission tomography, visual presentation of sentences was shown to cause increased... more Using positron emission tomography, visual presentation of sentences was shown to cause increased regional cerebral blood flow relative to word lists in the left lateral anterior superior and middle temporal gyri, attributable to cognitive processes that occur during sentence comprehension in addition to those carried out during word comprehension. Additional comparisons showed that repeating words~in a different context, when subjects did not attempt to learn the initial lists! led to significant patterns of both increased blood flow~left putamen and right caudate! and decreased blood flow~left posterior temporal lobe!. Increases are argued to reflect retrieval of memory traces, whereas decreases reflect diminished necessity for processing of input. A decrease in the left inferior parietal lobe was attributable to other factors.

Research paper thumbnail of Stressen of relaxen: neurochemische aspecten van bedrijvigheid en rust

Research paper thumbnail of Selective Attention and Error Processing in an Illusory Conjunction Task

Journal of Psychophysiology, Mar 6, 2015

Research paper thumbnail of Localizing components of a complex task: sentence processing and working memory

Neuroreport, Sep 13, 1998

Research paper thumbnail of Processing of visual semantic information to concrete words: temporal dynamics and neural mechanisms indicated by event-related brain potentials

Http Dx Doi Org 10 1080 02643290442000338, Jan 5, 2007

Event-related brain potentials were used to study the retrieval of visual semantic information to... more Event-related brain potentials were used to study the retrieval of visual semantic information to concrete words, and to investigate possible structural overlap between visual object working memory and concreteness effects in word processing. Subjects performed an object working memory task that involved 5 s retention of simple 4-angled polygons (load 1), complex 10-angled polygons (load 2), and a no-load baseline condition. During the polygon retention interval subjects were presented with a lexical decision task to auditory presented concrete (imageable) and abstract (nonimageable) words, and pseudowords. ERP results are consistent with the use of object working memory for the visualisation of concrete words. Our data indicate a two-step processing model of visual semantics in which visual descriptive information of concrete words is first encoded in semantic memory (indicated by an anterior N400 and posterior occipital positivity), and is subsequently visualised via the network for object working memory (reflected by a left frontal positive slow wave and a bilateral occipital slow wave negativity). Results are discussed in the light of contemporary models of semantic memory.

Research paper thumbnail of Localizing cognitive components of a complex task: sentence processing and working memory

Research paper thumbnail of Selective Visual Attention: Selective Cuing, Selective Cognitive Processing, and Selective Response Processing

Cognitive Electrophysiology, 1994

Research paper thumbnail of Re: "The effect of cortisol on emotional responses depends on order of cortisol and placebo administration in a within-subject design" by Wirth et al

Psychoneuroendocrinology, 2011

Re: ''The effect of cortisol on emotional responses depends on order of cortisol and placebo admi... more Re: ''The effect of cortisol on emotional responses depends on order of cortisol and placebo administration in a within-subject design'' by Wirth et al.

Research paper thumbnail of Selective attention to a facial feature with and without facial context: an ERP-study

International journal of psychophysiology : official journal of the International Organization of Psychophysiology, 2002

The present experiment addressed the question whether selectively attending to a facial feature (... more The present experiment addressed the question whether selectively attending to a facial feature (mouth shape) would benefit from the presence of a correct facial context. Subjects attended selectively to one of two possible mouth shapes belonging to photographs of a face with a happy or sad expression, respectively. These mouths were presented randomly either in isolation, embedded in the original photos, or in an exchanged facial context. The ERP effect of attending mouth shape was a lateral posterior negativity, anterior positivity with an onset latency of 160-200 ms; this effect was completely unaffected by the type of facial context. When the mouth shape and the facial context conflicted, this resulted in a medial parieto-occipital positivity with an onset latency of 180 ms, independent of the relevance of the mouth shape. Finally, there was a late (onset at approx. 400 ms) expression (happy vs. sad) effect, which was strongly lateralized to the right posterior hemisphere and wa...

Research paper thumbnail of Visual semantic features are activated during the processing of concrete words: event-related potential evidence for perceptual semantic priming

Brain research. Cognitive brain research, 2000

There has been conflicting evidence to date regarding the existence of non-strategic semantic pri... more There has been conflicting evidence to date regarding the existence of non-strategic semantic priming based on semantic similarity, and in particular on visual-perceptual semantic features (e.g., button-coin: words refer to objects with the same global shape). Both event-related potential (ERP) and reaction time (RT) measures were employed to investigate visual-perceptual semantic priming in a word-pair lexical decision task designed to minimise the contribution of conscious strategic processing. While no RT priming effect was observed, a robust priming effect was obtained on the N400 component of the ERP. This result shows that semantic priming, as indexed by the N400 component, can be supported by nonassociative visual-perceptual semantic relations. The data are consistent with perceptual form information being accessed during the processing of concrete words, and provide support for models of semantic representation which incorporate semantic features and form information.

Research paper thumbnail of Chapter 9 Brain potential analysis of selective attention

Handbook of Perception and Action, 1996

Research paper thumbnail of Visual spatial attention to stimuli presented on the vertical and horizontal meridian: An ERP study

Psychophysiology, 1994

In the first experiment, 48 subjects carried out a visual spatial attention task. Stimuli were pr... more In the first experiment, 48 subjects carried out a visual spatial attention task. Stimuli were presented at the vertical meridian, either above or below a fixation dot, and the subjects were instructed to attend to one of these stimulus positions and ignore the other position. In three different conditions, the distances between stimulus positions and fixation were 0.5 degrees, 0.9 degrees, and 1.3 degrees. Subjects searched for the presence of prememorized target letters at the attended location: memory load was one or four items in different conditions. The P1/N1 enhancement typically found on the horizontal dimension was not observed on the vertical dimension. Instead, a positive shift of the attended compared with the unattended stimuli was found, which was most prominent at anterior electrodes. This positivity showed effects of the distance manipulation. The N2b-P3a effects of attention and the effect of memory load (search negativity) normally present in this kind of selective search task were also found. Reaction times were faster when attention was directed above fixation than when it was directed below fixation. The event-related potential data suggested that this difference could be attributed to a more efficient neglecting of irrelevant stimuli presented below fixation. In Experiment 2, we examined whether the absence of the P1/N1 enhancement as the result of spatial attention in Experiment 1 could be attributed to (a) the presentation of stimuli along the vertical meridian instead of along the horizontal meridian, (b) the use of midline electrodes instead of lateralized electrodes, and (c) the relatively small spatial separation between the relevant and irrelevant stimuli. Twelve subjects searched for the presence of a single target letter at an attended position in three different conditions. In two of the conditions the letters were presented to the left or right of fixation. The distance between fixation and the stimulus positions was 1.3 degrees in one of these conditions and 3 degrees in the other condition. In the third condition, the stimuli were presented at 3 degrees above or below fixation. In all three conditions effects similar to those in Experiment 1 were observed. In addition, in all three conditions an enhancement of the P1 and N1 components was found at two lateral occipitotemporal electrodes.

Research paper thumbnail of Attention to Color: An Analysis of Selection, Controlled Search, and Motor Activation, Using Event-Related Potentials

Psychophysiology, 1989

In this study the organization of information processing in a selective search task was examined ... more In this study the organization of information processing in a selective search task was examined by analyzing event-related potentials. This task consisted of searching for target letters in a relevant (attended) color. The ERPs revealed two different effects of attention: an early occipital negativity (+/- 150 ms) reflecting feature-specific attention, and a later, central N2b component (+/- 240 ms) reflecting covert orienting of attention. A later, prolonged negativity (search-related negativity) (+/- 300 ms), maximal at Cz, was related to controlled search to letters in the attended color. Detection of relevant targets resulted in a parietal P3b component. Depending on stimulus presentation conditions an earlier response to both attended and unattended targets was found: an N2 component (+/- 250 ms). In these same conditions, C'3-C'4 asymmetries (Corrected Motor Asymmetries--CMA) suggested motor activation at +/- 300 ms, in the same time range as search-related negativity. It was argued that N2 and CMA suggest the existence of a preattentive target detection system, operating in parallel with a slower serial attentive system, as reflected by N2b and search negativity.

Research paper thumbnail of Acute cortisol administration reduces subjective fatigue in healthy women

Psychophysiology, 2006

Treatment with cortisol has been found to decrease fatigue and increase feelings of vigor in both... more Treatment with cortisol has been found to decrease fatigue and increase feelings of vigor in both patients and healthy male subjects. We obtained self-reports of mood before 35 mg cortisol or placebo ingestion, 70 min later, and after the healthy female subjects performed cognitive tasks for 1 h in a double-blind within-subject study. Cortisol decreased fatigue, increased vigor, and tended to decrease tension. Effects on fatigue were largest after task performance, when fatigue had increased, suggesting that improvement of fatigue by cortisol is observed when subjects are fatigued. This is the first study to demonstrate improvements in fatigue in healthy female subjects; this is particularly relevant because of the high prevalence of hypocortisolimic fatigue syndromes in women and recent evidence that many psychiatric disorders may involve stress-induced hypocortisolemia that is responsive to cortisol replacement.

Research paper thumbnail of Sentence comprehension and word repetition: A positron emission tomography investigation

Psychophysiology, 1999

Using positron emission tomography, visual presentation of sentences was shown to cause increased... more Using positron emission tomography, visual presentation of sentences was shown to cause increased regional cerebral blood flow relative to word lists in the left lateral anterior superior and middle temporal gyri, attributable to cognitive processes that occur during sentence comprehension in addition to those carried out during word comprehension. Additional comparisons showed that repeating words~in a different context, when subjects did not attempt to learn the initial lists! led to significant patterns of both increased blood flow~left putamen and right caudate! and decreased blood flow~left posterior temporal lobe!. Increases are argued to reflect retrieval of memory traces, whereas decreases reflect diminished necessity for processing of input. A decrease in the left inferior parietal lobe was attributable to other factors.

Research paper thumbnail of Brain Potentials During Selective Attention, Memory Search, and Mental Rotation

Psychophysiology, 1989

Event-related potentials were measured in a task that combined the classic selective attention pa... more Event-related potentials were measured in a task that combined the classic selective attention paradigm with memory search and mental rotation paradigms. Subjects were required to attend to stimulus letters in one color and to ignore stimuli in a different color. Within the attended category subjects searched for target letters from a prememorized set (the memory set) and indicated whether they were presented normally or in mirror-image. Letters in all stimulus categories were presented randomly in either their upright position or rotated over 60 degrees, 120 degrees, or 180 degrees. The event-related potentials showed that the earliest effect of attending to color was a positivity at the anterior electrodes and a negativity at Oz (onset about 150 ms), followed by the enhancement of a central N2 component (N2b, onset about 220 ms). The early effect is thought to reflect selective processing of elementary stimulus features, the later N2b the covert orienting of attention. A later, prolonged central negativity elicited by attended stimuli covaried with the duration of the memory search process. Target detection resulted in a parietal P3b component, and also in an earlier negativity (in the range approximately 200-300 ms) to both attended and unattended targets, suggesting an early preattentive target classification. There were two effects of the rotation of stimuli. First there was an early occipital effect (about 200-300 ms), irrespective of attention and stimulus categories. It was argued that this effect reflected the preattentive identification of the orientation of the stimulus letters. A later (onset about 350-400 ms) parietal effect consisted of an increase in negativity as a function of the angle over which letters were rotated. This prolonged negativity had a later onset latency than the search-related negativity, and was restricted to the event-related potentials to target letters in the attended input channel. It was suggested that this component is the manifestation of a mental rotation process. It was argued that rotation-related negativity and memory search-related activity reflect operations in distinct working memory subsystems.

Research paper thumbnail of An ERP study of visual spatial attention and letter target detection for isoluminant and nonisoluminant stimuli

Research paper thumbnail of Working memory processes show different degrees of lateralization: Evidence from event-related potentials

Psychophysiology, 2001

This study aimed to identify different processes in working memory, using event-related potential... more This study aimed to identify different processes in working memory, using event-related potentials~ERPs! and response times. Abstract polygons were presented for memorization and subsequent recall in a delayed matching-to-sample paradigm. Two polygons were presented bilaterally for memorization and a cue indicated whether one~and if so, which one of the two! or both polygons had to be memorized. A subsequent test figure was presented unilaterally to the left or right visual field and had to be compared with the memorized figure~s!. ERP results suggested that memorization takes place in a visual buffer in contralateral posterior brain areas, whereas identification of the test stimulus as a target appears to be mainly a left hemispheric process. Increased response times were found for nontarget test stimuli as compared to targets, and for target test stimuli that were presented contralaterally with respect to the location of the memorized stimulus. In addition, response times were slower when two figures were memorized than when only one was memorized.

Research paper thumbnail of Free recall of pleasant words from recency positions is especially sensitive to acute administration of cortisol

Psychoneuroendocrinology, 2004

In a recent study we investigated the acute effects of cortisol administration in healthy male vo... more In a recent study we investigated the acute effects of cortisol administration in healthy male volunteers on free recall of pleasant, unpleasant, and neutral nouns using a between-subjects double-blind placebo-controlled design. The volunteers were administered 10 mg of hydrocortisone or placebo between 9:00 and 10:30. Two hours after administration of cortisol a decline in recall of neutral and pleasant words was found, while recall of unpleasant words did not change. These results are consistent with a possible inhibitory influence of cortisol on a prefrontal dopaminergic mechanism involved in approach and positivity bias. In this paper we first explain why this interpretation would predict recall of pleasant words from recency positions to be especially sensitive to cortisol administration. Comparing primacy and recency recall of pleasant and unpleasant words, there proved to be a selective decline in recall of pleasant recency words. These results did not appear to stem from differences in recall strategies between our groups of volunteers.

Research paper thumbnail of Implicit sub-unit learning in a serial reaction time task in humans

Neuroscience Letters, 2001

In the present serial reaction time task experiment (SRT), a fixed 12-item sequence was practiced... more In the present serial reaction time task experiment (SRT), a fixed 12-item sequence was practiced in order to evaluate the effect on response times to 3-item sub sequences (triplets) in a subsequent random sequence. Subjects were visually cued to press one out of four keys with a corresponding right-hand finger. The occurrence of implicit sequence knowledge was evidenced by the increase in mean response time when the transition was made from the final 12-item sequence block to the subsequent random block. In the stimulus-set applied, a total of 36 triplets could be constructed, of which 24 triplets were encountered only during the random blocks (random-only triplet set) (RO-set), whereas 12 triplets were also part of the sequence used in the sequence blocks (sequence-also triplet set) (SA-set). Approximately 35% of the triplets that comprised the two random blocks were also presented in the sequence blocks. There was no difference in mean response times between the triplet sets in the random block that preceded the sequence blocks. In the final random block, however, the SA-set induced significantly faster responses as compared with the RO-set. We argue that stimulus response associations within the SA-set are responsible for the difference in response times between the two triplet sets in the final random block.

Research paper thumbnail of Sentence comprehension and word repetition: A positron emission tomography investigation

Psychophysiology, Oct 31, 1999

Using positron emission tomography, visual presentation of sentences was shown to cause increased... more Using positron emission tomography, visual presentation of sentences was shown to cause increased regional cerebral blood flow relative to word lists in the left lateral anterior superior and middle temporal gyri, attributable to cognitive processes that occur during sentence comprehension in addition to those carried out during word comprehension. Additional comparisons showed that repeating words~in a different context, when subjects did not attempt to learn the initial lists! led to significant patterns of both increased blood flow~left putamen and right caudate! and decreased blood flow~left posterior temporal lobe!. Increases are argued to reflect retrieval of memory traces, whereas decreases reflect diminished necessity for processing of input. A decrease in the left inferior parietal lobe was attributable to other factors.

Research paper thumbnail of Stressen of relaxen: neurochemische aspecten van bedrijvigheid en rust

Research paper thumbnail of Selective Attention and Error Processing in an Illusory Conjunction Task

Journal of Psychophysiology, Mar 6, 2015

Research paper thumbnail of Localizing components of a complex task: sentence processing and working memory

Neuroreport, Sep 13, 1998

Research paper thumbnail of Processing of visual semantic information to concrete words: temporal dynamics and neural mechanisms indicated by event-related brain potentials

Http Dx Doi Org 10 1080 02643290442000338, Jan 5, 2007

Event-related brain potentials were used to study the retrieval of visual semantic information to... more Event-related brain potentials were used to study the retrieval of visual semantic information to concrete words, and to investigate possible structural overlap between visual object working memory and concreteness effects in word processing. Subjects performed an object working memory task that involved 5 s retention of simple 4-angled polygons (load 1), complex 10-angled polygons (load 2), and a no-load baseline condition. During the polygon retention interval subjects were presented with a lexical decision task to auditory presented concrete (imageable) and abstract (nonimageable) words, and pseudowords. ERP results are consistent with the use of object working memory for the visualisation of concrete words. Our data indicate a two-step processing model of visual semantics in which visual descriptive information of concrete words is first encoded in semantic memory (indicated by an anterior N400 and posterior occipital positivity), and is subsequently visualised via the network for object working memory (reflected by a left frontal positive slow wave and a bilateral occipital slow wave negativity). Results are discussed in the light of contemporary models of semantic memory.

Research paper thumbnail of Localizing cognitive components of a complex task: sentence processing and working memory

Research paper thumbnail of Selective Visual Attention: Selective Cuing, Selective Cognitive Processing, and Selective Response Processing

Cognitive Electrophysiology, 1994

Research paper thumbnail of Re: "The effect of cortisol on emotional responses depends on order of cortisol and placebo administration in a within-subject design" by Wirth et al

Psychoneuroendocrinology, 2011

Re: ''The effect of cortisol on emotional responses depends on order of cortisol and placebo admi... more Re: ''The effect of cortisol on emotional responses depends on order of cortisol and placebo administration in a within-subject design'' by Wirth et al.

Research paper thumbnail of Selective attention to a facial feature with and without facial context: an ERP-study

International journal of psychophysiology : official journal of the International Organization of Psychophysiology, 2002

The present experiment addressed the question whether selectively attending to a facial feature (... more The present experiment addressed the question whether selectively attending to a facial feature (mouth shape) would benefit from the presence of a correct facial context. Subjects attended selectively to one of two possible mouth shapes belonging to photographs of a face with a happy or sad expression, respectively. These mouths were presented randomly either in isolation, embedded in the original photos, or in an exchanged facial context. The ERP effect of attending mouth shape was a lateral posterior negativity, anterior positivity with an onset latency of 160-200 ms; this effect was completely unaffected by the type of facial context. When the mouth shape and the facial context conflicted, this resulted in a medial parieto-occipital positivity with an onset latency of 180 ms, independent of the relevance of the mouth shape. Finally, there was a late (onset at approx. 400 ms) expression (happy vs. sad) effect, which was strongly lateralized to the right posterior hemisphere and wa...

Research paper thumbnail of Visual semantic features are activated during the processing of concrete words: event-related potential evidence for perceptual semantic priming

Brain research. Cognitive brain research, 2000

There has been conflicting evidence to date regarding the existence of non-strategic semantic pri... more There has been conflicting evidence to date regarding the existence of non-strategic semantic priming based on semantic similarity, and in particular on visual-perceptual semantic features (e.g., button-coin: words refer to objects with the same global shape). Both event-related potential (ERP) and reaction time (RT) measures were employed to investigate visual-perceptual semantic priming in a word-pair lexical decision task designed to minimise the contribution of conscious strategic processing. While no RT priming effect was observed, a robust priming effect was obtained on the N400 component of the ERP. This result shows that semantic priming, as indexed by the N400 component, can be supported by nonassociative visual-perceptual semantic relations. The data are consistent with perceptual form information being accessed during the processing of concrete words, and provide support for models of semantic representation which incorporate semantic features and form information.

Research paper thumbnail of Chapter 9 Brain potential analysis of selective attention

Handbook of Perception and Action, 1996

Research paper thumbnail of Visual spatial attention to stimuli presented on the vertical and horizontal meridian: An ERP study

Psychophysiology, 1994

In the first experiment, 48 subjects carried out a visual spatial attention task. Stimuli were pr... more In the first experiment, 48 subjects carried out a visual spatial attention task. Stimuli were presented at the vertical meridian, either above or below a fixation dot, and the subjects were instructed to attend to one of these stimulus positions and ignore the other position. In three different conditions, the distances between stimulus positions and fixation were 0.5 degrees, 0.9 degrees, and 1.3 degrees. Subjects searched for the presence of prememorized target letters at the attended location: memory load was one or four items in different conditions. The P1/N1 enhancement typically found on the horizontal dimension was not observed on the vertical dimension. Instead, a positive shift of the attended compared with the unattended stimuli was found, which was most prominent at anterior electrodes. This positivity showed effects of the distance manipulation. The N2b-P3a effects of attention and the effect of memory load (search negativity) normally present in this kind of selective search task were also found. Reaction times were faster when attention was directed above fixation than when it was directed below fixation. The event-related potential data suggested that this difference could be attributed to a more efficient neglecting of irrelevant stimuli presented below fixation. In Experiment 2, we examined whether the absence of the P1/N1 enhancement as the result of spatial attention in Experiment 1 could be attributed to (a) the presentation of stimuli along the vertical meridian instead of along the horizontal meridian, (b) the use of midline electrodes instead of lateralized electrodes, and (c) the relatively small spatial separation between the relevant and irrelevant stimuli. Twelve subjects searched for the presence of a single target letter at an attended position in three different conditions. In two of the conditions the letters were presented to the left or right of fixation. The distance between fixation and the stimulus positions was 1.3 degrees in one of these conditions and 3 degrees in the other condition. In the third condition, the stimuli were presented at 3 degrees above or below fixation. In all three conditions effects similar to those in Experiment 1 were observed. In addition, in all three conditions an enhancement of the P1 and N1 components was found at two lateral occipitotemporal electrodes.

Research paper thumbnail of Attention to Color: An Analysis of Selection, Controlled Search, and Motor Activation, Using Event-Related Potentials

Psychophysiology, 1989

In this study the organization of information processing in a selective search task was examined ... more In this study the organization of information processing in a selective search task was examined by analyzing event-related potentials. This task consisted of searching for target letters in a relevant (attended) color. The ERPs revealed two different effects of attention: an early occipital negativity (+/- 150 ms) reflecting feature-specific attention, and a later, central N2b component (+/- 240 ms) reflecting covert orienting of attention. A later, prolonged negativity (search-related negativity) (+/- 300 ms), maximal at Cz, was related to controlled search to letters in the attended color. Detection of relevant targets resulted in a parietal P3b component. Depending on stimulus presentation conditions an earlier response to both attended and unattended targets was found: an N2 component (+/- 250 ms). In these same conditions, C'3-C'4 asymmetries (Corrected Motor Asymmetries--CMA) suggested motor activation at +/- 300 ms, in the same time range as search-related negativity. It was argued that N2 and CMA suggest the existence of a preattentive target detection system, operating in parallel with a slower serial attentive system, as reflected by N2b and search negativity.

Research paper thumbnail of Acute cortisol administration reduces subjective fatigue in healthy women

Psychophysiology, 2006

Treatment with cortisol has been found to decrease fatigue and increase feelings of vigor in both... more Treatment with cortisol has been found to decrease fatigue and increase feelings of vigor in both patients and healthy male subjects. We obtained self-reports of mood before 35 mg cortisol or placebo ingestion, 70 min later, and after the healthy female subjects performed cognitive tasks for 1 h in a double-blind within-subject study. Cortisol decreased fatigue, increased vigor, and tended to decrease tension. Effects on fatigue were largest after task performance, when fatigue had increased, suggesting that improvement of fatigue by cortisol is observed when subjects are fatigued. This is the first study to demonstrate improvements in fatigue in healthy female subjects; this is particularly relevant because of the high prevalence of hypocortisolimic fatigue syndromes in women and recent evidence that many psychiatric disorders may involve stress-induced hypocortisolemia that is responsive to cortisol replacement.

Research paper thumbnail of Sentence comprehension and word repetition: A positron emission tomography investigation

Psychophysiology, 1999

Using positron emission tomography, visual presentation of sentences was shown to cause increased... more Using positron emission tomography, visual presentation of sentences was shown to cause increased regional cerebral blood flow relative to word lists in the left lateral anterior superior and middle temporal gyri, attributable to cognitive processes that occur during sentence comprehension in addition to those carried out during word comprehension. Additional comparisons showed that repeating words~in a different context, when subjects did not attempt to learn the initial lists! led to significant patterns of both increased blood flow~left putamen and right caudate! and decreased blood flow~left posterior temporal lobe!. Increases are argued to reflect retrieval of memory traces, whereas decreases reflect diminished necessity for processing of input. A decrease in the left inferior parietal lobe was attributable to other factors.

Research paper thumbnail of Brain Potentials During Selective Attention, Memory Search, and Mental Rotation

Psychophysiology, 1989

Event-related potentials were measured in a task that combined the classic selective attention pa... more Event-related potentials were measured in a task that combined the classic selective attention paradigm with memory search and mental rotation paradigms. Subjects were required to attend to stimulus letters in one color and to ignore stimuli in a different color. Within the attended category subjects searched for target letters from a prememorized set (the memory set) and indicated whether they were presented normally or in mirror-image. Letters in all stimulus categories were presented randomly in either their upright position or rotated over 60 degrees, 120 degrees, or 180 degrees. The event-related potentials showed that the earliest effect of attending to color was a positivity at the anterior electrodes and a negativity at Oz (onset about 150 ms), followed by the enhancement of a central N2 component (N2b, onset about 220 ms). The early effect is thought to reflect selective processing of elementary stimulus features, the later N2b the covert orienting of attention. A later, prolonged central negativity elicited by attended stimuli covaried with the duration of the memory search process. Target detection resulted in a parietal P3b component, and also in an earlier negativity (in the range approximately 200-300 ms) to both attended and unattended targets, suggesting an early preattentive target classification. There were two effects of the rotation of stimuli. First there was an early occipital effect (about 200-300 ms), irrespective of attention and stimulus categories. It was argued that this effect reflected the preattentive identification of the orientation of the stimulus letters. A later (onset about 350-400 ms) parietal effect consisted of an increase in negativity as a function of the angle over which letters were rotated. This prolonged negativity had a later onset latency than the search-related negativity, and was restricted to the event-related potentials to target letters in the attended input channel. It was suggested that this component is the manifestation of a mental rotation process. It was argued that rotation-related negativity and memory search-related activity reflect operations in distinct working memory subsystems.

Research paper thumbnail of An ERP study of visual spatial attention and letter target detection for isoluminant and nonisoluminant stimuli

Research paper thumbnail of Working memory processes show different degrees of lateralization: Evidence from event-related potentials

Psychophysiology, 2001

This study aimed to identify different processes in working memory, using event-related potential... more This study aimed to identify different processes in working memory, using event-related potentials~ERPs! and response times. Abstract polygons were presented for memorization and subsequent recall in a delayed matching-to-sample paradigm. Two polygons were presented bilaterally for memorization and a cue indicated whether one~and if so, which one of the two! or both polygons had to be memorized. A subsequent test figure was presented unilaterally to the left or right visual field and had to be compared with the memorized figure~s!. ERP results suggested that memorization takes place in a visual buffer in contralateral posterior brain areas, whereas identification of the test stimulus as a target appears to be mainly a left hemispheric process. Increased response times were found for nontarget test stimuli as compared to targets, and for target test stimuli that were presented contralaterally with respect to the location of the memorized stimulus. In addition, response times were slower when two figures were memorized than when only one was memorized.

Research paper thumbnail of Free recall of pleasant words from recency positions is especially sensitive to acute administration of cortisol

Psychoneuroendocrinology, 2004

In a recent study we investigated the acute effects of cortisol administration in healthy male vo... more In a recent study we investigated the acute effects of cortisol administration in healthy male volunteers on free recall of pleasant, unpleasant, and neutral nouns using a between-subjects double-blind placebo-controlled design. The volunteers were administered 10 mg of hydrocortisone or placebo between 9:00 and 10:30. Two hours after administration of cortisol a decline in recall of neutral and pleasant words was found, while recall of unpleasant words did not change. These results are consistent with a possible inhibitory influence of cortisol on a prefrontal dopaminergic mechanism involved in approach and positivity bias. In this paper we first explain why this interpretation would predict recall of pleasant words from recency positions to be especially sensitive to cortisol administration. Comparing primacy and recency recall of pleasant and unpleasant words, there proved to be a selective decline in recall of pleasant recency words. These results did not appear to stem from differences in recall strategies between our groups of volunteers.

Research paper thumbnail of Implicit sub-unit learning in a serial reaction time task in humans

Neuroscience Letters, 2001

In the present serial reaction time task experiment (SRT), a fixed 12-item sequence was practiced... more In the present serial reaction time task experiment (SRT), a fixed 12-item sequence was practiced in order to evaluate the effect on response times to 3-item sub sequences (triplets) in a subsequent random sequence. Subjects were visually cued to press one out of four keys with a corresponding right-hand finger. The occurrence of implicit sequence knowledge was evidenced by the increase in mean response time when the transition was made from the final 12-item sequence block to the subsequent random block. In the stimulus-set applied, a total of 36 triplets could be constructed, of which 24 triplets were encountered only during the random blocks (random-only triplet set) (RO-set), whereas 12 triplets were also part of the sequence used in the sequence blocks (sequence-also triplet set) (SA-set). Approximately 35% of the triplets that comprised the two random blocks were also presented in the sequence blocks. There was no difference in mean response times between the triplet sets in the random block that preceded the sequence blocks. In the final random block, however, the SA-set induced significantly faster responses as compared with the RO-set. We argue that stimulus response associations within the SA-set are responsible for the difference in response times between the two triplet sets in the final random block.