Marcel C. M. Bastiaansen

Neuronal Dynamics Underlying High- and Low-Frequency EEG Oscillations Contribute Independently to the Human BOLD Signal

Work on animals indicates that BOLD is preferentially sensitive to local field potentials, and that it correlates most strongly with gamma band neuronal synchronization. Here we investigate how the BOLD signal in humans performing a cognitive task is related to neuronal synchronization across different frequency bands. We simultaneously recorded EEG and BOLD while subjects engaged in a visual attention task known to induce sustained changes in neuronal synchronization across a wide range of frequencies. Trial-by-trial BOLD fluctuations correlated positively with trial-by-trial fluctuations in high-EEG gamma power (60-80 Hz) and negatively with alpha and beta power. Gamma power on the one hand, and alpha and beta power on the other hand, independently contributed to explaining BOLD variance. These results indicate that the BOLD-gamma coupling observed in animals can be extrapolated to humans performing a task and that neuronal dynamics underlying high- and low-frequency synchronization contribute independently to the BOLD signal.

Trial-by-trial coupling between EEG and BOLD identifies networks related to alpha and theta EEG power increases during working memory maintenance

PET and fMRI experiments have previously shown that several brain regions in the frontal and parietal lobe are involved in working memory maintenance. MEG and EEG experiments have shown parametric increases with load for oscillatory activity in posterior alpha and frontal theta power. In the current study we investigated whether the areas found with fMRI can be associated with these alpha and theta effects by measuring simultaneous EEG and fMRI during a modified Sternberg task This allowed us to correlate EEG at the single trial level with the fMRI BOLD signal by forming a regressor based on single trial alpha and theta power estimates. We observed a right posterior, parametric alpha power increase, which was functionally related to decreases in BOLD in the primary visual cortex and in the posterior part of the right middle temporal gyrus. We relate this finding to the inhibition of neuronal activity that may interfere with WM maintenance. An observed parametric increase in frontal theta power was correlated to a decrease in BOLD in regions that together form the default mode network. We did not observe correlations between oscillatory EEG phenomena and BOLD in the traditional WM areas. In conclusion, the study shows that simultaneous EEG-fMRI recordings can be successfully used to identify the emergence of functional networks in the brain during the execution of a cognitive task.

Event-Induced Theta Responses as a Window on the Dynamics of Memory

An important, but often ignored distinction in the analysis of EEG signals is that between evoked activity and induced activity. Whereas evoked activity reflects the summation of transient post-synaptic potentials triggered by an event, induced activity, which is mainly oscillatory in nature, is thought to reflect changes in parameters controlling dynamic interactions within and between brain structures. We hypothesize that induced activity may yield information about the dynamics of cell assembly formation, activation and subsequent uncoupling, which may play a prominent role in different types of memory operations. We then describe a number of analysis tools that can be used to study the reactivity of induced rhythmic activity, both in terms of amplitude changes and of phase variability. We briefly discuss how alpha, gamma and theta rhythms are thought to be generated, paying special attention to the hypothesis that the theta rhythm reflects dynamic interactions between the hippocampal system and the neocortex. This hypothesis would imply that studying the reactivity of scalp-recorded theta may provide a window on the contribution of the hippocampus to memory functions. We review studies investigating the reactivity of scalp-recorded theta in paradigms engaging episodic memory, spatial memory and working memory. In addition, we review studies that relate theta reactivity to processes at the interface of memory and language. Despite many unknowns, the experimental evidence largely supports the hypothesis that theta activity plays a functional role in cell assembly formation, a process which may constitute the neural basis of memory formation and retrieval. The available data provide only highly indirect support for the hypothesis that scalp-recorded theta yields information about hippocampal functioning. It is concluded that studying induced rhythmic activity holds promise as an additional important way to study brain function.

Oscillatory neuronal dynamics during language comprehension

Language comprehension involves two basic operations: the retrieval of lexical information (such as phonologic, syntactic, and semantic information) from long-term memory, and the unification of this information into a coherent representation of the overall utterance. Neuroimaging studies using hemodynamic measures such as PET and fMRI have provided detailed information on which areas of the brain are involved in these language-related memory and unification operations. However, much less is known about the dynamics of the brains language network. This chapter presents a literature review of the oscillatory neuronal dynamics of EEG and MEG data that can be observed during language comprehension tasks. From a detailed review of this (rapidly growing) literature the following picture emerges: memory retrieval operations are mostly accompanied by increased neuronal synchronization in the theta frequency range (4-7 Hz). Unification operations, in contrast, induce high-frequency neuronal synchronization in the beta (12-30 Hz) and gamma (above 30 Hz) frequency bands. A desynchronization in the (upper) alpha frequency band is found for those studies that use secondary tasks, and seems to correspond with attentional processes, and with the behavioral consequences of the language comprehension process. We conclude that it is possible to capture the dynamics of the brains language network by a careful analysis of the event-related changes in power and coherence of EEG and MEG data in a wide range of frequencies, in combination with subtle experimental manipulations in a range of language comprehension tasks. It appears then that neuronal synchrony is a mechanism by which the brain integrates the different types of information about language (such as phonological, orthographic, semantic, and syntactic information) represented in different brain areas.

Tangential derivative mapping of axial MEG applied to event-related desynchronization research.

OBJECTIVES A problem with the topographic mapping of MEG data recorded with axial gradiometers is that field extrema are measured at sensors located at either side of a neuronal generator instead of at sensors directly above the source. This is problematic for the computation of event-related desynchronization (ERD) on MEG data, since ERD relies on a correspondence between the signal maximum and the location of the neuronal generator. METHODS We present a new method based on computing spatial derivatives of the MEG data. The limitations of this method were investigated by means of forward simulations, and the method was applied to a 150-channel MEG dataset. RESULTS The simulations showed that the method has some limitations. (1) Fewer channels reduce accuracy and amplitude. (2) It is less suitable for deep or very extended sources. (3) Multiple sources can only be distinguished if they are not too close to each other. Applying the method in the calculation of ERD on experimental data led to a considerable improvement of the ERD maps. CONCLUSIONS The proposed method offers a significant advantage over raw MEG signals, both for the topographic mapping of MEG and for the analysis of rhythmic MEG activity by means of ERD.

EEG theta and gamma responses to semantic violations in online sentence processing

We explore the nature of the oscillatory dynamics in the EEG of subjects reading sentences that contain a semantic violation. More specifically, we examine whether increases in theta ( approximately 3-7 Hz) and gamma (around 40 Hz) band power occur in response to sentences that were either semantically correct or contained a semantically incongruent word (semantic violation). ERP results indicated a classical N400 effect. A wavelet-based time-frequency analysis revealed a theta band power increase during an interval of 300-800 ms after critical word onset, at temporal electrodes bilaterally for both sentence conditions, and over midfrontal areas for the semantic violations only. In the gamma frequency band, a predominantly frontal power increase was observed during the processing of correct sentences. This effect was absent following semantic violations. These results provide a characterization of the oscillatory brain dynamics, and notably of both theta and gamma oscillations, that occur during language comprehension.

I see what you mean: Theta power increases are involved in the retrieval of lexical semantic information

An influential hypothesis regarding the neural basis of the mental lexicon is that semantic representations are neurally implemented as distributed networks carrying sensory, motor and/or more abstract functional information. This work investigates whether the semantic properties of words partly determine the topography of such networks. Subjects performed a visual lexical decision task while their EEG was recorded. We compared the EEG responses to nouns with either visual semantic properties (VIS, referring to colors and shapes) or with auditory semantic properties (AUD, referring to sounds). A time-frequency analysis of the EEG revealed power increases in the theta (4-7Hz) and lower-beta (13-18Hz) frequency bands, and an early power increase and subsequent decrease for the alpha (8-12Hz) band. In the theta band we observed a double dissociation: temporal electrodes showed larger theta power increases in the AUD condition, while occipital leads showed larger theta responses in the VIS condition. The results support the notion that semantic representations are stored in functional networks with a topography that reflects the semantic properties of the stored items, and provide further evidence that oscillatory brain dynamics in the theta frequency range are functionally related to the retrieval of lexical semantic information.

Anticipatory attention: an event-related desynchronization approach

This paper addresses the question of whether anticipatory attention--i.e. attention directed towards an upcoming stimulus in order to facilitate its processing--is realized at the neurophysiological level by a pre-stimulus desynchronization of the sensory cortex corresponding to the modality of the anticipated stimulus, reflecting the opening of a thalamocortical gate in the relevant sensory modality. It is argued that a technique called Event-Related Desynchronization (ERD) of rhythmic 10-Hz activity is well suited to study the thalamocortical processes that are thought to mediate anticipatory attention. In a series of experiments, ERD was computed on EEG and MEG data, recorded while subjects performed a time estimation task and were informed about the quality of their time estimation by stimuli providing Knowledge of Results (KR). The modality of the KR stimuli (auditory, visual, or somatosensory) was manipulated both within and between experiments. The results indicate to varying degrees that preceding the presentation of the KR stimuli, ERD is present over the sensory cortex, which corresponds to the modality of the KR stimulus. The general pattern of results supports the notion that a thalamocortical gating mechanism forms the neurophysiological basis of anticipatory attention. Furthermore, the results support the notion that Event-Related Potential (ERP) and ERD measures reflect fundamentally different neurophysiological processes.

Event-related alpha and theta responses in a visuo-spatial working memory task

OBJECTIVE To explore the reactivity of the theta and alpha rhythms during visuo-spatial working memory. METHODS One hundred and seventy-four subjects performed a delayed response task. They had to remember the spatial location of a target stimulus on a computer screen for a 1 or a 4s retention interval. The target either remained visible throughout the entire interval (sensory trials) or disappeared after 150ms (memory trials). Changes in induced band power (IBP) in the electroencephalogram (EEG) were analyzed in 4 narrow, individually adjusted frequency bands between 4 and 12Hz. RESULTS After presentation of the target stimulus, a phasic power increase was found, irrespective of condition and delay interval, in the lower (roughly, 4-8Hz) frequency bands, with a posterior maximum. During the retention interval, sustained occipital-parietal alpha power increase and frontal theta power decrease were found. Most importantly, the memory trials showed larger IBP decreases in the theta band over frontal electrodes than the sensory trials. CONCLUSIONS The phasic power increase following target onset is interpreted to reflect encoding of the target location. The sustained theta decrease, which is larger for memory trials, is tentatively interpreted to reflect visuo-spatial working memory processes.

Event-related theta power increases in the human EEG during online sentence processing

By analyzing event-related changes in induced band power in narrow frequency bands of the human electroencephalograph, the present paper explores a possible functional role of the alpha and theta rhythms during the processing of words and of sentences. The results show a phasic power increase in the theta frequency range, together with a phasic power decrease in the alpha frequency range, following the presentation of words in a sentence. These effects may be related to word processing, either lexical or in relation to sentence context. Most importantly, there is a slow and highly frequency-specific increase in theta power as a sentence unfolds, possibly related to the formation of an episodic memory trace, or to incremental verbal working memory load.