Biomedical subjects

Simon Hanslmayr

Publications and source records attributed to Simon Hanslmayr.

9 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

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↗

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↗

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↗