Biomedical subjects

Wolfgang Klimesch

Publications and source records attributed to Wolfgang Klimesch.

At least 19 recordsLinked to original sources

PubMed2006

P1 and traveling alpha waves: evidence for evoked oscillations.

The hypothesis is tested whether the P1 of the event-related potential (ERP) component behaves like an evoked, traveling alpha wave. This hypothesis is based on different kinds of evidence showing, e.g., that-after undergoing phase reorganization-frequencies in the broad alpha range become synchronized (aligned) in absolute phase and contribute significantly to the generation of the P1. We investigated data from a Stroop task in which subjects had to respond only to the color and ignore the meaning of the presented words. Analyzing topographical phase relationships expressed in terms of traveling speed (with respect to Pz as trailing site) revealed that a systematic posterior to anterior traveling pattern appeared only in the broad time window of the P1-N1 complex and in the extended alpha frequency range. The obtained findings are consistent with the oscillatory ERP model and suggest that the P1 component may be considered a manifestation of an evoked, traveling alpha wave. We assume that the P1 reflects a top-down process in a sense that traveling alpha waves control or "gate" the direction of information processing in the brain.

Adult↗

PubMed2006

Differential effects of the stimulus sequence on CNV and P300.

During the presentation of stimulus sequences in oddball paradigms, participants tend to implicitly evaluate the conditional probability of target occurrence. It is not sure, however, if subjective estimation of conditional probabilities modulates target expectancy and target processing in the same manner. In the present experiment, the amplitudes of CNV and P300 were studied separately to compare preparatory and decision mechanisms and their sensitivity to variations in target probability. Amplitudes of both components were measured at different positions in the stimulus sequence, which modulates target probability as a function of distance from the preceding target. Results showed shorter RTs with increased probability of target occurrence (that is, with longer distance to the previous target). CNV amplitude was low when target probability was zero and maximal when the occurrence of a target was possible, regardless of its probability. Conversely, a gradual increase with augmenting probability was observed for P300 amplitude. Thus, preparatory activity as reflected by the CNV showed an all or nothing response to variations in probability, whereas action closure mechanisms, indexed by P300 components, were found to be more sensitive to subtle differences in stimulus expectancy.

Adult↗

PubMed2006

EEG alpha oscillations: the inhibition-timing hypothesis.

The traditional belief is that the event-related alpha response can solely be described in terms of suppression or event-related desynchronization (ERD). Recent research, however, has shown that under certain conditions alpha responds reliably with an increase in amplitudes (event-related synchronization or ERS). ERS is elicited in situations, where subjects withhold or control the execution of a response and is obtained over sites that probably are under, or exert top-down control. Thus, we assume that alpha ERS reflects top-down, inhibitory control processes. This assumption leads over to the timing aspect of our hypothesis. By the very nature of an oscillation, rhythmic amplitude changes reflect rhythmic changes in excitation of a population of neurons. Thus, the time and direction of a change - described by phase - is functionally related to the timing of neuronal activation processes. A variety of findings supports this view and shows, e.g., that alpha phase coherence increases between task-relevant sites and that phase lag lies within a time range that is consistent with neuronal transmission speed. Another implication is that phase reset will be a powerful mechanism for the event-related timing of cortical processes. Empirical evidence suggests that the extent of phase locking is a functionally sensitive measure that is related to cognitive performance. Our general conclusion is that alpha ERS plays an active role for the inhibitory control and timing of cortical processing whereas ERD reflects the gradual release of inhibition associated with the emergence of complex spreading activation processes.

Animals↗

PubMed2006

Alpha phase reset contributes to the generation of ERPs.

An unresolved question in electroencephalogram (EEG) research is whether event-related potentials (ERPs) are generated by phase-reset or evoked response. We analyzed data of a visual feature detection task and will show 1) phase concentration in the alpha frequency range, 2) ongoing alpha activity prior to stimulus onset, 3) evoked alpha oscillation in the ERP, 4) lack of power increase during phase concentration, 5) decrease in amplitude variance during early evoked components preceding a decrease in power, and 6) the same cortical sources for induced prestimulus power and evoked poststimulus power. Because none of these data provide unequivocal evidence for phase reset, we additionally tested the basic assumption of the evoked model, which is the additivity of the evoked response on the basis of a simulation approach. Our findings suggest that nonadditive processes-typical for a phase reset-are involved in the generation of the ERP. Thus, together with the other findings this study provides unequivocal evidence for phase resetting in the human EEG.

Adult↗

PubMed2006

Upper alpha ERD and absolute power: their meaning for memory performance.

A variety of studies have shown that EEG alpha activity in the upper frequency range is associated with different types of cognitive processes, memory performance, perceptual performance and intelligence, but in strikingly different ways. For semantic memory performance we have found that resting or reference power is positively associated with performance, whereas during actual processing of the task, small power--reflected by a large extent of event-related desynchronization (ERD)--is related to good performance. We also have shown that the induction of large alpha reference power by neurofeedback training or repetitive transcranial magnetic stimulation (rTMS) at individual alpha frequency mimicked exactly the situation which is typical for good memory performance under normal situations: increased alpha reference power is associated with large ERD and good performance. Recent studies have demonstrated that this relationship holds true only for memory and not perceptual tasks that require the identification of simple visual stimuli under difficult conditions. In contrast to good memory performance, good perceptual performance is related to small pre-stimulus alpha power and a small ERD. We interpret this finding in terms of cortical inhibition vs. activation preceding task performance by assuming that large rhythmic alpha activity reflects inhibition. We assume that small reference alpha enhances perceptual performance because the cortex is activated and prepared to process the stimulus, whereas memory performance is enhanced if the cortex is deactivated before a task is performed because in typical memory tasks selective processing can start only after the to-be-remembered item or cue is presented. We also suggest that conflicting results about alpha ERD and the neural efficiency hypothesis (which assumes that highly intelligent exhibit a small ERD) can also be interpreted in terms of inhibition. Only if an intelligence test actually requires the activation of (semantic) memory, a large (because task specific) ERD can be observed. If other processing systems are required, the semantic memory system may even become suppressed, which is reflected by alpha event-related synchronization (ERS) or at least a largely decreased ERD.

Alpha Rhythm↗

PubMed2006

Gender-dependent differences in sensation seeking and social interaction are correlated with saliva testosterone titre in adolescents.

OBJECTIVES: In the present study, we tested the hypothesis that gender - dependent differences in novelty seeking, leadership, and sympathy might correlate with testosterone titre. Since several studies report that the impact of testosterone on personality traits is more visible under emotional challenging situations, we harvested saliva testosterone upon an anticipated stressor (academic examination) and under basal conditions. SETTING AND DESIGN: 19 female and 23 male adolescents (17 to 19 years of age) completed standardized questionnaires on sensation seeking, anxiety, and social interaction. Two weeks later, they had to write an anticipated, rigorous examination in mathematics in their school. Before and after the examination, saliva had been harvested from each subject and testosterone titre has been estimated. METHODS: Saliva testosterone was quantified using a luminescence immunoassay (LIA). Each subject completed questionnaires on sensations seeking according to Zuckerman (SSS - V), anxiety (STAI), and social interactions. RESULTS: Both genders showed an increase in their testosterone titre shortly after examination or announcement of test scores. A Spearman correlation coefficient indicated a significant correlation between testosterone titre and sensation seeking subscales in female but not in male subjects. Analysis of social interactions revealed that peers regarded male subjects, who had high testosterone titres, as leaders but not as likeable individuals, whereas they regarded female subjects, who had high testosterone titres, not as leaders but as highly likeable individuals. CONCLUSION: Theses findings strongly suggest that testosterone has gender specific effects on novelty seeking, dominance, and sympathy.

Adolescent↗

PubMed2005

Distinguishing the evoked response from phase reset: a comment to Mäkinen et al.

Mäkinen et al. [Mäkinen, V., Tiitinen, H., May, P., 2005. Auditory event-related responses are generated independently of ongoing brain activity. Neuroimage 24, 961-968] suggest the use of amplitude variance to distinguish the evoked response from phase reset. Because their data do not exhibit a drop in amplitude variance, they conclude that ERPs are generated by 'processes separate from and additive to ongoing brain activity.' We argue that this conclusion is premature because of unrealistic assumptions about the processes underlying an event-related modulation of oscillations. A realistic phase reset model has to consider at least two parameters, degree of phase reset (or 'phase concentration') and amplitude change (event-related increase or decrease in amplitude). With simulated data, we show that a variable increase in amplitude size increases amplitude variance and masks the influence of a phase reset. On the other hand, an event-related decrease in amplitude without a phase reset leads to a sharp drop in variance. Furthermore, simulation of a frequency-specific phase reset shows a drop in variance that may be too small to be detected empirically. Thus, we conclude that amplitude variance is not capable of distinguishing the evoked response from phase reset.

Brain↗

PubMed2005

EEG alpha synchronization and functional coupling during top-down processing in a working memory task.

Electroencephalogram (EEG) alpha (around 10 Hz) is the dominant rhythm in the human brain during conditions of mental inactivity. High amplitudes as observed during rest usually diminish during cognitive effort. During retention of information in working memory, however, power increase of alpha oscillations can be observed. This alpha synchronization has been interpreted as cortical idling or active inhibition. The present study provides evidence that during top-down processing in a working memory task, alpha power increases at prefrontal but decreases at occipital electrode sites, thereby reaching a state in which alpha power and frequency become very similar over large distances. Two experimental conditions were compared. In the first, visuospatial information only had to be retained in memory whereas the second condition additionally demanded manipulation of the information. During the second condition, stronger alpha synchronization at prefrontal sites and larger occipital alpha suppression was observed as compared to that for pure retention. This effect was accompanied by assimilation of prefrontal and occipital alpha frequency, stronger functional coupling between prefrontal and occipital brain areas, and alpha latency shifts from prefrontal cortex to primary visual areas, possibly indicating the control of posterior cortical activation by anterior brain areas. An increase of prefrontal EEG alpha amplitudes, which is accompanied by a decrease at posterior sites, thus may not be interpreted in terms of idling or "global" inhibition but may enable a tight functional coupling between prefrontal cortical areas, and thereby allows the control of the execution of processes in primary visual brain regions.

Adult↗

PubMed2005

Increasing individual upper alpha power by neurofeedback improves cognitive performance in human subjects.

The hypothesis was tested of whether neurofeedback training (NFT)--applied in order to increase upper alpha but decrease theta power--is capable of increasing cognitive performance. A mental rotation task was performed before and after upper alpha and theta NFT. Only those subjects who were able to increase their upper alpha power (responders) performed better on mental rotations after NFT. Training success (extent of NFT-induced increase in upper alpha power) was positively correlated with the improvement in cognitive performance. Furthermore, the EEG of NFT responders showed a significant increase in reference upper alpha power (i.e. in a time interval preceding mental rotation). This is in line with studies showing that increased upper alpha power in a prestimulus (reference) interval is related to good cognitive performance.

Adult↗

PubMed2005

Fronto-parietal EEG coherence in theta and upper alpha reflect central executive functions of working memory.

For human working memory the neural correlates of the phonological loop and the visuospatial sketch pad are well explored. In contrast, less is known about central executive processes. Neuroimaging studies suggest that central executive processes are related to a complex fronto-parietal network. In the present study we investigate the question whether varying demands on central executive processes are reflected by differences in coherent activity between and within a fronto-parietal network. We calculated coherence during a visuospatial working memory task. Under an easy executive condition subjects had to mentally imagine previously studied abstract patterns, whereas in the difficult condition, subjects had to mentally manipulate these patterns. The results indicate the involvement of prefrontal areas in executive functions reflected by a decrease of anterior upper alpha short-range connectivity and a parallel increase of fronto-parietal long-range coherence mirroring activation of a fronto-parietal network.

Adult↗

PubMed2005

The functional significance of theta and upper alpha oscillations.

Recent findings are reviewed indicating that upper alpha oscillations - when analyzed with appropriate time/ frequency resolution - show a similar physiological reactivity as theta in working memory tasks. Comparable to theta, a load dependent increase in power can be observed during retention and increased evoked activity during retrieval. During retrieval attempts theta behaves like a traveling wave spreading from anterior to posterior sites. During actual retrieval, however, evoked upper alpha becomes transiently nested in theta. We suggest that theta reflects working memory functions whereas upper alpha may be important for the reactivation of long-term memory codes in short-term memory.

Alpha Rhythm↗

PubMed2005

Alpha phase synchronization predicts P1 and N1 latency and amplitude size.

The oscillatory phase-resetting model predicts that event-related potential (ERP) components are generated by a superposition of evoked oscillations with different frequencies. We investigate this question in a memory task in which human subjects had to retrieve a verbal label in response to the presentation of a picture. The results show that (i) evoked oscillations in the delta, theta, alpha and beta range undergo a significant phase resetting and (ii) become synchronized in absolute phase during small time windows that (iii) coincide with the latencies of the P1-N1 complex. Our conclusion is that the latencies of ERP components can be predicted at least in part by phase synchronization between frequencies.

Adult↗

PubMed2004

Sleep spindles and their significance for declarative memory consolidation.

STUDY OBJECTIVES: Functional significance of stage 2 sleep spindle activity for declarative memory consolidation. DESIGN: Randomized, within-subject, multicenter. SETTING: Weekly sleep laboratory visits, actigraphy, and sleep diary (4 weeks). PARTICIPANTS: Twenty-four healthy subjects (12 men) aged between 20 and 30 years. INTERVENTIONS: Declarative memory task or nonlearning control task before sleep. MEASUREMENT AND RESULTS: This study measured spindle activity during stage 2 sleep following a (declarative) word-pair association task as compared to a control task. Participants performed a cued recall in the evening after learning (160 word pairs) as well as in the subsequent morning after 8 hours of undisturbed sleep with full polysomnography. Overnight change in the number of recalled words, but not absolute memory performance, correlated significantly with increased spindle activity during the experimental night (r24 = .63, P < .01). Time spent in each sleep stage could not account for this relationship. CONCLUSION: A growing body of evidence supports the active role of sleep for information reprocessing. Whereas past research focused mainly on the distinct rapid eye movement and slow-wave sleep, these results indicate that increased sleep stage 2 spindle activity is related to an increase in recall performance and, thus, may reflect memory consolidation.

Adult↗

PubMed2004

Visual discrimination performance is related to decreased alpha amplitude but increased phase locking.

This study investigated the question whether good and bad performance in a visual discrimination task is related to resting alpha power in a different way as it is known from memory tasks. The results show that good perceptual but not memory performance is related to low alpha amplitudes. In addition, we found that large phase resetting in the alpha band, and enhanced early components in the ERP are related to good performance in the discrimination task. The conclusion of this study is that in contrast to memory performance which is related to large resting alpha activity low alpha amplitudes are an indicator for good perceptual performance.

Adult↗

PubMed2004

Theta coupling in the human electroencephalogram during a working memory task.

The role of coupling between prefrontal and temporo-parietal brain areas within the theta frequency range of the human electroencephalogram was explored in a working memory task. During encoding of visual information higher theta amplitudes were observed in the right compared to the left hemisphere. Retrieval of visuospatial and verbal information elicited a more bilateral activation pattern. These effects were accompanied by theta coupling between dorsolateral prefrontal and right posterior temporal electrode sites during encoding. During retrieval prefrontal and bilateral temporo-parietal brain areas were coupled. These results support the idea of working memory functions being dependent on distributed prefrontal-temporal networks.

Acoustic Stimulation↗

PubMed2004

Phase-locked alpha and theta oscillations generate the P1-N1 complex and are related to memory performance.

An oscillatory phase resetting model is presented and data are reported which indicate that early components of the event-related potential are due to the superposition of evoked oscillations. The following hypotheses were tested and could be confirmed: (i) theta and alpha show a significant increase in phase locking during the time window of the P1 and N1 as compared to a prestimulus reference, (ii) the dynamics of event-related changes in evoked theta and alpha power obey the same principles as are known from event-related de-/synchronization research, and (iii) latency measures of the P1-N1 complex are negatively correlated with individual alpha frequency. In addition, we have found that theta phase locking is larger during encoding than recognition and that good memory performers show a larger increase in theta and alpha phase locking during recognition in the time window of the N1. Our general conclusion is that the P1-N1 complex is generated primarily by evoked alpha and theta oscillations reflecting the synchronous activation of a working- and semantic memory system.

Adult↗

PubMed2004

Alpha power dependent light stimulation: dynamics of event-related (de)synchronization in human electroencephalogram.

The desynchronization and resynchronization of alpha oscillations was studied in 10 normal subjects after visual stimulation of both eyes under two experimental conditions, "eyes opened" and "eyes closed". The electroencephalogram (EEG) was recorded bipolarly over the occipital area and sampled at 200 Hz. The data was processed in real time and evaluated online. In accordance with the alpha power in the 7 to 13 Hz band, short red light flashes of 10 ms duration were presented at intervals of at least 2 s. This stimulation resulted in an event-related desynchronization (ERD) followed by resynchronization. Trials were controlled for artifacts, averaged offline, and the amount of event-related (de)synchronization was calculated. The event-related desynchronization was significantly larger in the eyes open paradigm. In addition, the latencies of event-related desynchronization and resynchronization maxima were larger in the eyes open paradigm compared to the eyes closed one.

Alpha Rhythm↗

PubMed2003

Enhancing cognitive performance with repetitive transcranial magnetic stimulation at human individual alpha frequency.

We applied rapid-rate repetitive transcranial magnetic stimulation (rTMS) at individual alpha frequency (IAF) to improve cognitive performance by influencing the dynamics of alpha desynchronization. Previous research indicates that a large upper alpha power in a reference interval preceding a task is related to both large suppression of upper alpha power during the task and good performance. Here, we tested the hypothesis that rTMS at individual upper alpha frequency (IAF + 1 Hz) can enhance alpha power in the reference interval, and can thus improve task performance. Repetitive TMS was delivered to the mesial frontal (Fz) and right parietal (P6) cortex, and as sham condition with 90 degrees-tilted coil (P6 position). The behavioural effect was assessed in a mental rotation task. Further control conditions were rTMS at a lower IAF (IAF - 3 Hz) and at 20 Hz. The results indicate that rTMS at IAF + 1 Hz can enhance task performance and, concomitantly, the extent of task-related alpha desynchronization. This provides further evidence for the functional relevance of oscillatory neuronal activity in the alpha band for the implementation of cognitive performance.

Adolescent↗