Until recently, ERPs were the basic electrophysiological indices of cognition that provided valuable insights into human brain processes. Substantial literature now indicates that some ERP features may arise from changes in the dynamics of ongoing EEG rhythms/oscillations of different frequency bands that reflect ongoing sensory and/or cognitive processes (Basar 1999; Basar et al. 1999; Buzsaki 2006). In other words, the EEG oscillations that are measured in a resting state become organized, amplified, and/or coupled during mental activity, or the network activity induced by an event or stimulus may trigger specific oscillatory responses, thus giving rise to an “evoked” (strongly locked to the stimulus/event) or “induced” (weakly associated with to the stimulus/event) rhythmicity (Basar 1980; Basar 1999; Makeig et al. 2002). These EROs can influence the timing of neural activity and coordinate synchronous activity in groups of active neurons (Fries 2005). Thus, EROs represent a basic mechanism of neural communication, providing links to associative and integrative brain functions. High-frequency (i.e., β, γ) EROs are implicated in short range communication, whereas low frequencies (i.e., δ, θ, and α) EROs are involved in longer-range communication between brain areas (von Stein and Sarnthein 2000).
