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 (Basar et al., 1997; Klimesch et al., 1994, 1997a, 1997b), whereas, beta and gamma oscillations have been associated with sensory integrative processes (Basar et al., 2001a, 2001b; Schurmann et al., 1997). Oscillations in the delta and theta frequency ranges have been associated with signal detection, decision-making, conscious awareness, recognition memory and episodic retrieval (Basar et al., 1999c, 2001c, 2001d; Doppelmayr et al., 1998; Gevins et al., 1998; Klimesch et al., 1994, 2001; Schurmann et al., 2001). It has been 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 and 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 (Kopell et al., 2000; Lubar, 1997).
