Elsevier

Clinical Neurophysiology

Volume 110, Issue 11, 1 November 1999, Pages 1842-1857
Clinical Neurophysiology

Invited review
Event-related EEG/MEG synchronization and desynchronization: basic principles

https://doi.org/10.1016/S1388-2457(99)00141-8Get rights and content

Abstract

An internally or externally paced event results not only in the generation of an event-related potential (ERP) but also in a change in the ongoing EEG/MEG in form of an event-related desynchronization (ERD) or event-related synchronization (ERS). The ERP on the one side and the ERD/ERS on the other side are different responses of neuronal structures in the brain. While the former is phase-locked, the latter is not phase-locked to the event. The most important difference between both phenomena is that the ERD/ERS is highly frequency band-specific, whereby either the same or different locations on the scalp can display ERD and ERS simultaneously. Quantification of ERD/ERS in time and space is demonstrated on data from a number of movement experiments.

Section snippets

Event-related potentials/fields vs. event-related EEG/MEG changes (ERD/ERS)

Several kinds of events, the most notably being sensory stimuli, can induce time-locked changes in the activity of neuronal populations that are generally called event-related potentials (ERPs). In order to detect such ERPs, averaging techniques are commonly used. The basic assumption is that the evoked activity, or signal of interest, has a more or less fixed time-delay to the stimulus, while the ongoing EEG/MEG activity behaves as additive noise. The averaging procedure will enhance the

Frequency-specificity of brain oscillations

In general, the frequency of brain oscillations is negatively correlated with their amplitude, which means that the amplitude of fluctuations decreases with increasing frequency. For example, the Rolandic mu rhythm with a frequency between 8 and 13 Hz has a larger amplitude than the central beta rhythm with frequencies around 20 Hz. The beta rhythm again has a larger amplitude than oscillations around 40 Hz. Because the amplitude of oscillations is proportional to the number of synchronously

Time course of ERD/ERS

One of the basic features of ERD/ERS measurements is that the EEG/MEG power within identified frequency bands is displayed relative (as percentage) to the power of the same EEG/MEG derivations recorded during the reference or baseline period a few seconds before the event occurs. Because event-related changes in ongoing EEG/MEG need time to develop and to recover, especially when alpha band rhythms are involved, the interval between two consecutive events should last at least some seconds. In

Determination of subject-specific frequency bands

One of the most important questions in ERD/ERS analysis is how to determine the upper and the lower limits of the bandpass filter. The following methods can be used:

  • 1.

    detection of the most reactive frequency band based on the comparison of two short-term power spectra;

  • 2.

    continuous wavelet transform (CWT);

  • 3.

    definition of frequency bands relative to the spectral peak frequency.

ERD in memory and movement tasks

The alpha band rhythms demonstrate a relatively widespread desynchronization (ERD) in perceptual, judgement and memory tasks (van Winsum et al., 1984, Sergeant et al., 1987, Pfurtscheller and Klimesch, 1992, Klimesch et al., 1992, Klimesch et al., 1993, Klimesch et al., 1994, Aftanas et al., 1996, Sterman et al., 1996). An increase of task complexity or attention results in an increased magnitude of ERD (Boiten et al., 1992, Dujardin et al., 1993). It has to be kept in mind, however, that the

Simultaneous occurrence of ERD and ERS in the alpha and lower beta bands

A visual input results not only in a desynchronization of occipital alpha rhythms but also in an enhancement or synchronization of central mu rhythms. Brechet and Lecasble (1965) reported on an enhanced mu rhythm during flicker stimulation, Koshino and Niedermeyer (1975) on enhanced (synchronized) Rolandic rhythms during pattern vision and Pfurtscheller (1992) on a central alpha power increase in a reading task. The opposite, an enhancement of occipital alpha rhythms and desynchronization of

Post-movement beta ERS

One interesting oscillating brain signal, with a relatively good signal-to-noise ratio in the human scalp EEG, is the post-movement beta ERS. These induced beta oscillations are found in the first second after termination of a voluntary movement, when the Rolandic mu rhythm still displays a desynchronized pattern of low amplitude. This low amplitude activity, with a focus around the corresponding sensorimotor representation area, results in embedded beta oscillations with a good signal-to-noise

Interpretation of ERD and ERS in the alpha and lower beta band

Increased cellular excitability in thalamo-cortical systems results in a low amplitude desynchronized EEG (Steriade and Llinas, 1988). Therefore, ERD can be interpreted as an electrophysiological correlate of activated cortical areas involved in processing of sensory or cognitive information or production of motor behavior (Pfurtscheller, 1992). An increased and/or more widespread ERD could be the result of the involvement of a larger neural network or more cell assemblies in information

ERS in the gamma band

In addition to oscillations in the alpha and lower beta bands, induced oscillations are also found in the frequency band around 40 Hz. Such oscillations were reported with visual stimulation (Gray et al., 1988) and in a movement task (Murthy and Fetz, 1992, Rougeul et al., 1979;Pfurtscheller et al., 1993) and may be related to a binding of sensory information and sensorimotor integration, respectively. Oscillations in the alpha and lower beta range would be too slow to serve as carrier signals

ERD/ERS in neurological disorders

The quantification of movement-related desynchronization can improve the diagnosis of functional deficits in patients with cerebrovascular disorders and Parkinson's disease (PD). It was shown that there is a high correlation between morphological and functional findings in cerebrovascular disorders. The ERD is reduced or abolished over the affected hemisphere. Based on ERD measurements during voluntary hand movement it was, for example, possible to differentiate between superficial and deep

Acknowledgements

The investigation has been supported in part by the Fonds zur Foerderung der Wissenschaftlichen Forschung (projects P11 571MED and P12 407MED) and the Ludwig Boltzmann Institute of Medical Informatics and Neuroinformatics.

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