The importance of analyzing event-related phase-locked and non-phase-locked θ oscillations separately has been raised by a number of researchers (Bastiaansen and Hagoort, 2003; Klimesch et al., 1998). Deiber et al. (2007) investigated phase-locked and non-phase-locked θ power separately during a visual oddball detection task, and reported evoked θ activity phase-locked to the stimulus and localized to the parieto-occipital region. In parallel, induced θ oscillations in the form of an ERS over the frontal region was found. The induced oscillations were modulated by task demand, where increased working memory load and / or attentional demand resulted in a larger frontal ERS. These results concur with the general finding in task-related studies that show increased levels of θ ERS with increasing task demands (Klimesch, 1996, 1999; Pfurtscheller, 1992). A number of studies have reported increasing levels of θ synchronization as a function of working memory load (Gevins et al., 1997; Jensen and Tesche, 2002; Krause et al., 2000; McEvoy et al., 2001). Missonnier et al. (2006) investigated the relative contribution of attentional demand and working memory on frontal θ ERS in a
