Event-related oscillations over the spectral range of the EEG (generally between 1-50 Hz, although responses at lower and higher frequencies have also been reported [80-82], 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 [83-88], whereas, beta and gamma oscillations have been suggested to be involved with sensory integrative processes [89-91]. Oscillations in the delta and theta frequency ranges have been associated with signal detection, decision-making, conscious awareness, recognition memory and episodic retrieval [29, 79, 84, 92-96]. 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 [97-98], whereas, lower frequency oscillations (1-4 Hz) are generated by synchronization of ensembles interacting at longer distances during higher cognitive processing [99-100].
