Roman Freunberger

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

Electroencephalogram (EEG) α (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 α oscillations can be observed. This α 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, α power increases at prefrontal but decreases at occipital electrode sites, thereby reaching a state in which α 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 α synchronization at prefrontal sites and larger occipital α suppression was observed as compared to that for pure retention. This effect was accompanied by assimilation of prefrontal and occipital α frequency, stronger functional coupling between prefrontal and occipital brain areas, and α 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 α 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. Hum Brain Mapp, 2005.

Are event-related potential components generated by phase resetting of brain oscillations? A critical discussion.

The event-related potential (ERP) is one of the most popular measures in human cognitive neuroscience. During the last few years there has been a debate about the neural fundamentals of ERPs. Two models have been proposed: The evoked model states that additive evoked responses which are completely independent of ongoing background electroencephalogram generate the ERP. On the other hand the phase reset model suggests a resetting of ongoing brain oscillations to be the neural generator of ERPs. Here, evidence for either of the two models is presented and validated, and their possible impact on cognitive neuroscience is discussed. In addition, future prospects on this field of research are presented.

A short review of slow phase synchronization and memory: evidence for control processes in different memory systems?

An integrative theoretical approach about memory related oscillations is presented. The basic assumptions are that memory related oscillations are probably confined to theta and upper alpha and that other frequencies particularly in the gamma range are important for memory primarily because they become coupled to lower frequencies and/or because they play a specific role for a high precision timing of neural events (including phenomena such as LTP or LTD). In contrast to previous studies, where we related theta and upper alpha to a variety of different memory processes, we suggest here that these oscillations are associated with top-down control processes in two large storage systems, working memory (WM) and long-term memory (LTM). These systems may have their own types of top-down processes that control access to and/or manipulation of stored information. The hypothesis, suggested here is that theta and upper alpha reflect these processes which can be best studied when analyzing phase.

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.

Oscillatory mechanisms of process binding in memory

A central topic in cognitive neuroscience is the question, which processes underlie large scale communication within and between different neural networks. The basic assumption is that oscillatory phase synchronization plays an important role for process binding--the transient linking of different cognitive processes--which may be considered a special type of large scale communication. We investigate this question for memory processes on the basis of different types of oscillatory synchronization mechanisms. The reviewed findings suggest that theta and alpha phase coupling (and phase reorganization) reflect control processes in two large memory systems, a working memory and a complex knowledge system that comprises semantic long-term memory. It is suggested that alpha phase synchronization may be interpreted in terms of processes that coordinate top-down control (a process guided by expectancy to focus on relevant search areas) and access to memory traces (a process leading to the activation of a memory trace). An analogous interpretation is suggested for theta oscillations and the controlled access to episodic memories.

Visual P2 component is related to theta phase-locking

In this study we investigated the hypothesis whether P2-related differences tested in a visual priming paradigm are associated with theta phase-locking. We recorded the EEG from 31 electrodes and calculated phase-locking index and total power differences for frequencies between 2 and 20 Hz. ERPs (event-related potentials) were analyzed for P1, N1 and P2 components. P2 showed strongest task-related amplitude differences between congruent and incongruent targets. A source analyses was performed for the P2 component using sLoreta that revealed local generators of the P2 in parieto-occipital regions. Phase-locking analyses showed specific effects in the theta range (4-6 Hz) appearing in time windows at around the P2 component. We draw the conclusion that phase-locked theta reflect top-down regulation processes mediating information between memory systems and is in part involved in the modulation of the P2 component.

Dissociation between phase-locked and nonphase-locked alpha oscillations in a working memory task

The functions of human alpha oscillations (∼10 Hz) were related to cognitive processes such as memory and top‐down control. Recent models suggest that alpha phase serves as a mechanism especially relevant for the timing of neural activity, whereas alpha amplitude is important for the inhibition of task‐irrelevant brain areas. This study investigates directly the influence of top‐down modulation on phase‐locked and nonphase‐locked alpha rhythms. We conducted an EEG experiment where subjects performed a working memory task. In the encoding phase of the task subjects had to learn presented pictures of nonliving objects that could later be asked to be retrieved. We varied the top‐down modulation by including cues indicating either to remember or to forget (not to remember) the next following item. Spectral analyses showed that nonremember cues elicited pronounced alpha amplitude increase compared to remember cues. Furthermore, phase‐locking in low frequencies, especially in the alpha range (7–12 Hz), was stronger for remember as opposed to not‐to‐remember items. In conclusion, we propose that alpha amplitude reflects top‐down modulated inhibition and that alpha phase is important for the exact timing of neural activity and can be related to binding processes. Hum Brain Mapp, 2009.

Brain oscillatory correlates of working memory constraints.

It has been claimed that the coordination of neuronal oscillations differing in frequency is relevant for cognition. However, the validity of this claim has scarcely been investigated. Recent studies revealed that cross-frequency phase coupling and modulations of alpha-power dissociate between retention of relevant and suppression of irrelevant information in visual working memory (WM). We summarize these important results, and discuss possible implications for understanding the neural mechanisms of WM constraints.

Alpha oscillations and early stages of visual encoding

For a long time alpha oscillations have been functionally linked to the processing of visual information. Here we propose an new theory about the functional meaning of alpha. The central idea is that synchronized alpha reflects a basic processing mode that controls access to information stored in a complex long-term memory system, which we term knowledge system in order to emphasize that it comprises not only declarative memories but any kind of knowledge comprising also procedural information. Based on this theoretical background, we assume that during early stages of perception, alpha “directs the flow of information” to those neural structures which represent information that is relevant for encoding. The physiological function of alpha is interpreted in terms of inhibition. We assume that alpha enables access to stored information by inhibiting task-irrelevant neuronal structures and by timing cortical activity in task relevant neuronal structures. We discuss a variety findings showing that evoked alpha and phase locking reflect successful encoding of global stimulus features in an early post-stimulus interval of about 0–150 ms.

Alpha phase coupling reflects object recognition.

In the present study, we investigate the role of upper alpha oscillations for semantic access and retrieval processes. In each of a series of trials, subjects were presented trains of distorted pictures (with decreasing levels of degradation), and were asked to respond as quickly as possible when they recognize the meaning of the picture. The results show that during the time-window of picture recognition, upper alpha power decreased but inter-areal phase synchronization increased as compared to meaningless control pictures. We assume that synchronous alpha oscillations--particularly during a decrease in alpha power--reflect topographically specific neural network activity that is related to the access of semantic information in LTM.