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Peter Klaver

Publications and source records attributed to Peter Klaver.

18 recordsLinked to original sources
PubMed2006

Functional dissociations in top-down control dependent neural repetition priming.

Little is known about the neural mechanisms underlying top-down control of repetition priming. Here, we use functional brain imaging to investigate these mechanisms. Study and repetition tasks used a natural/man-made forced choice task. In the study phase subjects were required to respond to either pictures or words that were presented superimposed on each other. In the repetition phase only words were presented that were new, previously attended or ignored, or picture names that were derived from previously attended or ignored pictures. Relative to new words we found repetition priming for previously attended words. Previously ignored words showed a reduced priming effect, and there was no significant priming for pictures repeated as picture names. Brain imaging data showed that neural priming of words in the left prefrontal cortex (LIPFC) and left fusiform gyrus (LOTC) was affected by attention, semantic compatibility of superimposed stimuli during study and cross-modal priming. Neural priming reduced for words in the LIPFC and for words and pictures in the LOTC if stimuli were previously ignored. Previously ignored words that were semantically incompatible with a superimposed picture during study induce increased neural priming compared to semantically compatible ignored words (LIPFC) and decreased neural priming of previously attended pictures (LOTC). In summary, top-down control induces dissociable effects on neural priming by attention, cross-modal priming and semantic compatibility in a way that was not evident from behavioral results.

Adult↗
PubMed2006

Rhinal-hippocampal coupling during declarative memory formation: dependence on item characteristics.

Lesion and imaging studies have demonstrated that encoding and retrieval of declarative memories, i.e. consciously accessible events and facts, depend on operations within the rhinal cortex and the hippocampus, two substructures of the medial temporal lobe. Analysis of intracranially recorded EEG in presurgical epilepsy patients revealed that successful memory formation is accompanied within one second by a transient enhancement and later decrease of Rhinal-hippocampal phase synchronization in the gamma range, as well as enhanced connectivity in the low-frequency range. In these studies, words with a high frequency of occurrence were used as stimulus material. Here, we re-examined these effects in another group of 10 presurgical epilepsy patients, this time not only for high-frequency, but also for low-frequency words. For successfully memorized compared to later forgotten high-frequency words we again observed an early phase coupling and later decoupling within the gamma range, as well as enhanced coupling within the sub-gamma range. However, for remembered as compared to forgotten low-frequency words clear synchronization increases were only observed for the delta band, but not for the gamma band. Our data suggest, that broadband Rhinal-hippocampal coupling including the gamma range only occurs, when significant semantic associations are processed within rhinal cortex, as is the case for high-frequency words.

Adult↗
PubMed2005

Word imageability affects the hippocampus in recognition memory.

Concrete words, whose meanings are readily imagined, are better remembered than abstract words. However, the neural correlates of this effect are poorly understood. Here, we investigated the effect of imageability on brain activity in the medial temporal lobe (MTL) processes underlying recognition memory. We recorded event-related potentials (ERPs) via depth electrodes from within the MTL in 14 patients with drug-resistant epilepsy. Patients performed a continuous word recognition task with words of high and low imageability (controlled for word frequency). Behaviorally, recognition performance was better for high, compared to low, imageable words. Two ERP components associated with recognition memory, the AMTL-N400 and the hippocampal late negative component, showed an old/new effect, but only the hippocampal P600 showed a main effect of imageability. We suggest that the hippocampal effect of imageability in recognition memory may be associated with conceptual or pictorial information processing of concrete words.

Adult↗
PubMed2004

Phase-locking characteristics of limbic P3 responses in hippocampal sclerosis.

Amplitudes of the P3 recorded invasively from the medial temporal lobe (MTL-P3) have been reported to be reduced on the side of a mediotemporal epileptogenic focus. This reduction has been attributed to the massive cell loss within the hippocampus associated with hippocampal sclerosis. It has remained unclear how functional connectivity between the hippocampus and rhinal cortex, as well as within the hippocampus, is altered in hippocampal sclerosis. To investigate this issue, we analyzed to what extent stimulus-related phase-locking and power changes within the low-frequency range (2-30 Hz) and within the gamma band (32-48 Hz), as well as rhinal-hippocampal phase synchronization contribute to the averaged MTL-P3 potentials. Event-related responses were recorded via bilateral depth electrodes in epilepsy patients with unilateral hippocampal sclerosis, who performed a visual oddball experiment. On the contralateral (nonsclerotic) side, successful target detection was associated with an increase of power and phase locking of hippocampal activity in both the low-frequency range and in the gamma range. Besides, there were rhinal-hippocampal synchronization enhancements in the theta and gamma range. On the ipsilateral (sclerotic) side, the event-related power increase in the low-frequency range had almost disappeared, a finding likely to be explained by the loss of principle neurons. However, low-frequency phase-locking, rhinal-hippocampal synchronization, as well as event-related power changes in the gamma range persisted ipsilaterally, although there were differences in temporal and spectral characteristics. These findings support the hypothesis that functional connectivity between hippocampus and rhinal cortex, as well as intrahippocampal connectivity, are partially preserved in hippocampal sclerosis.

Adult↗
PubMed2004

Process dissociation between contextual retrieval and item recognition.

We employed a source memory task in an event related fMRI study to dissociate MTL processes associated with either contextual retrieval or item recognition. To introduce context during study, stimuli (photographs of buildings and natural landscapes) were transformed into one of four single-color-scales: red, blue, yellow, or green. In the subsequent old/new recognition memory test, all stimuli were presented as gray scale photographs, and old-responses were followed by a four-alternative source judgment referring to the color in which the stimulus was presented during study. Our results suggest a clear-cut process dissociation within the human MTL. While an activity increase accompanies successful retrieval of contextual information, an activity decrease provides a familiarity signal that is sufficient for successful item recognition.

Adult↗
PubMed2004

Using visual advance information: an event-related functional MRI study.

Our event-related functional MRI (efMRI) study investigates whether visual advance information (AI) affects rather perceptual or central response-related processing areas. Twelve subjects were required to make a go/no-go decision to a conjunction of a specific color and motion direction. The stimuli were preceded by a cue, providing 100% valid advance information about motion direction. Partial and full advance information (PAI and FAI) predicted possible targets, respectively, certain nontargets, neutral cues (NAI) gave no prediction. The time between cue and stimulus (stimulus onset asynchrony, SOA) was varied. A response benefit was found after PAI as compared with NAI. The benefit was small with a short SOA (150 ms), increased with intermediate SOA (450 ms) and sustained with long SOA (750 ms). Perceptual and central processing areas were more active with increasing SOA, but only central response-related processing areas were selectively modulated by cue information. In particular, supplementary motor area and bilateral inferior parietal lobe were more active with PAI than with NAI. If comparing NAI with FAI, more errors were made and activity was larger in central processing areas. Our results suggest that, depending on the processing time, cues providing perceptual information modulate central response-related processes.

Adult↗
PubMed2004

A new approach in clinical neuropsychology to the assessment of spatial working memory: the block suppression test.

The Block Suppression Paradigm developed by Beblo, Klaver, Grubich, Wachowius, and Herrmann (1999) is based on the Corsi Block tapping test and requires that a subject reproduces every 2nd block in a given sequence. Results from two studies of a standardized version, the Block Suppression Test (BST), are presented here. In Study 1 the BST was administered to 48 healthy subjects along with a battery of comprehensive neuropsychological tests. The reliability of the BST proved satisfactory under psychometric analysis, while Principal Component Analysis (PCA) confirmed its validity. In Study 2 the BST was administered to a clinical sample of 31 brain-damaged patients to demonstrate its clinical practicability.

Adult↗
PubMed2004

Temporal and cerebellar brain regions that support both declarative memory formation and retrieval.

Using event-related fMRI, we scanned young healthy subjects while they memorized real-world photographs and subsequently tried to recognize them within a series of new photographs. We confirmed that activity in the medial temporal lobe (MTL) and inferior prefrontal cortex correlates with declarative memory formation as defined by the subsequent memory effect, stronger responses to subsequently remembered than forgotten items. Additionally, we confirmed that activity in specific regions within the parietal lobe, anterior prefrontal cortex, anterior cingulate and cerebellum correlate with recognition memory as measured by the conventional old/new effect, stronger responses for recognized old items (hits) than correctly identified new items (correct rejections). To obtain a purer measure of recognition success, we introduced two recognition effects by comparing brain responses to hits and old items misclassified as new (misses). The positive recognition effect (hits > misses) revealed prefrontal, parietal and cerebellar contributions to recognition, and in line with electrophysiological findings, the negative recognition effect (hits < misses) revealed an anterior medial temporal contribution. Finally, by inclusive masking, we identified temporal and cerebellar brain areas that support both declarative memory formation and retrieval. For matching operations during recognition, these areas may re-use representations formed and stored locally during encoding.

Adult↗
PubMed2004

Neural bases of cognitive ERPs: more than phase reset.

Up to now, two conflicting theories have tried to explain the genesis of averaged event-related potentials (ERPs): Whereas one hypothesis claims that ERPs originate from an event-related activation of neural assemblies distinct from background dynamics, the other hypothesis states that ERPs are produced by phase resetting of ongoing oscillatory activity. So far, this question has only been addressed for early ERP components. Late ERP components, however, are generally thought to represent superimposed activities of several anatomically distinct brain areas. Thus, the question of which mechanism underlies the genesis of late ERP components cannot be easily answered based on scalp recordings. In contrast, two well-investigated late ERP components recorded invasively from within the human medial temporal lobe (MTL) in epilepsy patients, the so-called MTL-P300 and the anterior MTL-N400 (AMTL-N400), are based on single source activity. Hence, we investigated whether the MTL-P300 and the AMTL-N400 are based on an event-related activity increase, a phase reset of ongoing oscillatory activity or both. ERPs were recorded from the hippocampus and rhinal cortex in subjects performing a visual oddball paradigm and a visual word recognition paradigm. With wavelet techniques, stimulus-related phase-locking and power changes were analyzed in a frequency range covering 2 to 48 Hz. We found that the MTL-P300 is accompanied by both phase reset and power increase and that both effects overlap partly in time. In contrast, the AMTL-N400 is initially associated with phase locking without power increase and only later during the course of the AMTL-N400 we observed an additional power increase. In conclusion, both aspects, event-related activation of neural assemblies and phase resetting of ongoing activity seem to be involved in the generation of late ERP components as recorded in cognitive tasks. Therefore, separate analysis of event-related power and phase-locking changes might reveal specific insights into the mechanisms underlying different cognitive functions.

Adult↗
PubMed2003

Menstrual cycle-dependent neural plasticity in the adult human brain is hormone, task, and region specific.

In rodents, cyclically fluctuating levels of gonadal steroid hormones modulate neural plasticity by altering synaptic transmission and synaptogenesis. Alterations of mood and cognition observed during the menstrual cycle suggest that steroid-related plasticity also occurs in humans. Cycle phase-dependent differences in cognitive performance have almost exclusively been found in tasks probing lateralized neuronal domains, i.e., cognitive domains such as language, which are predominantly executed by one hemisphere. To search for neural correlates of hormonally mediated neural plasticity in humans, we thus conducted a functional magnetic resonance imaging study measuring brain activity related to a semantic decision task in the language domain. This was contrasted with a letter-matching task in the perceptual domain, in which we expected no steroid hormone-mediated effect. We investigated 12 young healthy women in a counterbalanced repeated-measure design during low-steroid menstruation and high-steroid midluteal phase. Steroid serum levels correlated with the volume and lateralization of particular brain activations related to the semantic task but not with brain activity related to the perceptual task. More specifically, bilateral superior temporal recruitment correlated positively with progesterone and medial superior frontal recruitment with both progesterone and estradiol serum levels, whereas activations in inferior and middle frontal cortex were unaffected by steroid levels. In contrast to these specific interactions, testosterone levels correlated nonselectively with overall activation levels by neural and/or vascular factor(s). In conclusion, our data demonstrate steroid hormone responsivity in the adult human brain by revealing neural plasticity in the language domain, which appears hormone, task, and region specific.

Adult↗
PubMed2003

Is synchronized neuronal gamma activity relevant for selective attention?

Today, much evidence exists that sensory feature binding is accomplished by phase synchronization of induced neuronal gamma activity (30-80 Hz). Recent studies furthermore suggest that phase synchronization of induced gamma activity may represent a general mechanism enabling transient associations of neural assemblies and thus may play a central role in cortical information processing. Here, we describe findings indicating that synchronized gamma activity is moreover specifically involved in selective attention. While feature binding appears to depend primarily on induced gamma synchronization, attentional processes seem to involve both induced and evoked gamma oscillations. Yet it is still an open question, as to which top-down and bottom-up processes are associated with attentional modulation of gamma activity. A possible mechanism to project influences from attentional control structures to areas concerned with stimulus representation and vice versa, may be neuronal synchronization and the resulting firing rate changes of coincidence-detecting neurons in target areas.

Animals↗
PubMed2003

Rhinal-hippocampal theta coherence during declarative memory formation: interaction with gamma synchronization?

The hippocampus and the rhinal cortex, two substructures of the medial temporal lobe, together play a crucial role in human declarative memory formation. To investigate in detail the mechanism connecting these two structures transiently during memory formation we recorded depth EEG in epilepsy patients from within the hippocampus and the rhinal cortex. During this recording, patients performed a single-trial word list-learning paradigm with a free recall memory test following a distraction task. Rhinal-hippocampal EEG coherence and spectral power at both locations in the time interval up to 2 s after onset of word presentation were analysed in the frequency range 1-19 Hz. Successful as opposed to unsuccessful memory formation was associated with a general rhinal-hippocampal coherence enhancement, but without alterations in spectral power. Coherence increases in the theta range were correlated with the previously reported memory-related changes in rhinal-hippocampal gamma phase synchronization. This correlation may suggest an interaction of the two mechanisms during declarative memory formation. While theta coherence might be associated with slowly modulated coupling related to an encoding state, rhinal-hippocampal gamma synchronization may be more closely related to actual memory processes by enabling fast coupling and decoupling of the two structures.

Adult↗
PubMed2002

Human declarative memory formation: segregating rhinal and hippocampal contributions.

The medial temporal lobe (MTL) is the core structure of the declarative memory system, but which specific operation is performed by anatomically defined MTL substructures? One hypothesis proposes that the hippocampus carries out an exclusively mnemonic operation during declarative memory formation that is insensitive to content, whereas the rhinal cortex carries out an operation supporting memory formation indirectly. To explore the interaction between a salient item feature and memory formation, we contrasted neural correlates of memory formation of high- and low-frequency words. Event-related potentials (ERPs) were recorded via depth electrodes from within the MTL in nine epilepsy patients while they memorized single words. To assess memory formation, ERPs to words subsequently recalled in a free recall test were contrasted with ERPs to forgotten words. More high- than low-frequency words were remembered. High-frequency words led to distinct ERP subsequent memory effects in rhinal cortex and hippocampus. Low-frequency words, however, were only associated with the hippocampal ERP effect. The anatomically restricted interaction between word frequency and memory formation might indicate a semantically affected operation in the parahippocampal region supporting memory formation indirectly. By contrast, the missing interaction in hippocampal recordings might suggest a direct correlate of declarative memory formation that is insensitive to item properties.

Adult↗
PubMed2002

Suppression of EEG gamma activity may cause the attentional blink.

The attentional blink (AB) is an impairment of attention, which occurs when subjects have to report a target stimulus (T2) following a previous target (T1) with a short delay (up to 600 ms). Theories explaining the AB assume that processing of T2 is more vulnerable to decay or substitution, as long as attention is allocated to T1. Existing models of the AB, however, do not account for the fact that T2 detection accuracy reaches the minimum when T2 is presented after about 300 ms and not immediately following T1. Therefore, a new model is suggested, which is based on chronometrical considerations together with recent neurophysiological findings concerning the relation between the P3 event-related potential and the AB, the interaction between P3 and gamma oscillations, and the significance of the early evoked gamma band response. We hypothesize that suppression of the early gamma response to T2, accompanying the P3 related to T1, causes the AB.

Attention↗
PubMed2002

Human scalp recorded sigma activity is modulated by slow EEG oscillations during deep sleep.

The EEG during deep sleep exhibits a distinct cortically generated slow oscillation of around and below 1 Hz which can be distinguished from other delta (0.5-3.5 Hz) activity. Intracranial studies showed that this slow oscillation triggers and groups cortical network firing. In the present study, we examined whether the phases of the slow oscillation during sleep stage 4 are correlated with the magnitude of sigma (12-16 Hz) and gamma (> 20 Hz) scalp activity. For this purpose, 10-min segments of uninterrupted stage 4 sleep EEG from 9 subjects were analyzed by applying wavelet techniques. We found that scalp recorded sigma, but not gamma, activity is modulated by the phases of the slow oscillation during deep sleep. Enhancement of sigma activity was observed to be triggered by the peak of the surface positive slow wave component, whereas reduction of sigma activity started around the peak of the negative component.

Adult↗
PubMed2002

Covariation of spectral and nonlinear EEG measures with alpha biofeedback.

This study investigated how different spectral and nonlinear EEG measures covaried with alpha power during auditory alpha biofeedback training, performed by 13 healthy subjects. We found a significant positive correlation of alpha power with the largest Lyapunov-exponent, pointing to an increased dynamical instability of the EEG accompanying alpha enhancement. Alpha power amplification, moreover, was significantly correlated with a decrease of spectral entropy within the alpha range. This outcome reflects a sharpening of the alpha peak during biofeedback training. The fact that the sharpening effect clearly preceded the increase of alpha amplitude could be exploited in future biofeedback settings.

Adult↗
PubMed2002

The interaction of rhinal cortex and hippocampus in human declarative memory formation.

Human declarative memory formation crucially depends on processes within the medial temporal lobe (MTL). These processes can be monitored in real-time by recordings from depth electrodes implanted in the MTL of patients with epilepsy who undergo presurgical evaluation. In our studies, patients performed a word memorization task during depth EEG recording. Afterwards, the difference between event-related potentials (ERPs) corresponding to subsequently remembered versus forgotten words was analyzed. These kind of studies revealed that successful memory encoding is characterized by an early process generated by the rhinal cortex within 300 ms following stimulus onset. This rhinal process precedes a hippocampal process, which starts about 200 ms later. Further investigation revealed that the rhinal process seems to be a correlate of semantic preprocessing which supports memory formation, whereas the hippocampal process appears to be a correlate of an exclusively mnemonic operation. These studies yielded only indirect evidence for an interaction of rhinal cortex and hippocampus. Direct evidence for a memory related cooperation between both structures, however, has been found in a study analyzing so called gamma activity, EEG oscillations of around 40 Hz. This investigation showed that successful as opposed to unsuccessful memory formation is accompanied by an initial enhancement of rhinal-hippocampal phase synchronization, which is followed by a later desynchronization. Present knowledge about the function of phase synchronized gamma activity suggests that this phase coupling and decoupling initiates and later terminates communication between the two MTL structures. Phase synchronized rhinal-hippocampal gamma activity may, moreover, accomplish Hebbian synaptic modifications and thus provide an initial step of declarative memory formation on the synaptic level.

Cortical Synchronization↗