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