Event-related oscillations over the spectral range of the EEG (1-50 Hz) have been suggested to underlie a number of different cognitive processes. For instance, event-related alpha oscillations have been attributed to attentional resources, semantic memory, and stimulus processing (Klimesch et al., 1994, 1997a, b; Basar et al., 1997), whereas, beta and gamma oscillations have been associated with sensory integrative processes (Schurmann et al., 1997; Basar et al., 2001a, b). Oscillations in the delta and theta frequency ranges have been associated with signal detection, decision-making, conscious awareness, recognition memory and episodic retrieval (Klimesch et al., 1994, 2001; Doppelmayr et al., 1998; Gevins et al., 1998; Basar et al., 1999, 2001c, d; Schurmann et al., 2001). It has suggested that high frequency oscillations (above 30 Hz) reflect synchronization of neuronal ensembles that are interacting over short distances in response to primarily sensory processes (Bressler & Freeman, 1980; Ohl et al., 2003), whereas, lower frequency oscillations (1-4 Hz) are generated by synchronization of ensembles interacting at longer distances during higher cognitive processing (see Lubar, 1997; Kopell et al., 2000).
