In addition to eliciting an ERP, an endogenous or exogenous event can also result in frequency-specific changes to ongoing EEG oscillations that are not phase locked to the stimulus and so cannot be extracted by trial averaging. (Pfurtscheller and Lopes da Silva, 1999). Spectral analysis techniques, such as time-frequency analysis, are applied to the event-related trials to quantify such changes. In such event-related oscillation (ERO) analyses, the dynamics of the power of frequency-specific oscillations are quantified and these spatiotemporal dynamics are examined as they relate to task-specific sensory, motor and/or cognitive processes. Event-related decreases (increases) of power in specific frequency bands are considered to be due to decreases (increases) in synchrony of the underlying neuronal assemblies. These synchrony changes are thought to result from event-related changes in the oscillatory control parameters of these neuronal networks. Thus, a relative decrease in band power is often referred to as event-related desynchronization (ERD) and a relative power increase as an event-related synchronization (ERS). An event-related increase in non-phase-locked frequency-specific activity is also often referred to as an induced oscillation (Pfurtscheller and Lopes da Silva, 1999).
