Alpha-band Oscillations

== Two types of event-related amplitude responses

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Alpha-band activity is particularly interesting because it is the only frequency domain (with the exception of slow beta, a frequency higher than alpha, up to approximately 20 Hz) that responds to a stimulus and/or task demands either with a decrease or increase in amplitude/power, termed event-related desynchronization and synchronization, or ERD and ERS, respectively. The response of other frequencies (particularly in the delta, theta and gamma frequency range) is ‘simply’ characterized by ERS (see [4,5]).

This is interesting because one implicit assumption in EEG research is that oscillations have an impact on information processing that is proportional to their magnitude. In this sense, ERS, but not ERD, would have to be considered the ‘active’ task-relevant alpha-band response. This, however, is in contradiction with the classical view (e.g., [6]) which holds that suppression of alpha-band activity (ERD) is the ‘typical’ event-related alpha response. This view comes from the basic observation that alpha amplitudes are large when the eyes are closed and become suppressed when eyes are opened. It has been known since the early days of EEG research that this type of alpha suppression is not due to visual stimulation because it can also be observed in a completely darkened room [7,8].

ERD can be observed in response to a variety of different tasks, and it was assumed that it reflects cortical activation, or more precisely, cortical excitation. More recently, however, it was recognized that under certain task demands ERS (instead of ERD) can be observed.