The importance of analyzing event-related phase-locked and non-phase-locked theta activity separately has been highlighted by a number of researchers (Bastiaansen and Hagoort, 2003; Klimesch et al., 1998). A study by Deiber et al. (2007) reported findings on a visual oddball detection task similar to ours, in which evoked and induced oscillatory activity was examined separately. They reported evoked theta activity phase-locked to the visual stimulus and localized to the parieto-occipital region and in parallel, induced theta activity in the form of an ERS over the frontal region. This induced theta activity was found to be modulated by task load, where increased attentional demand and working memory load resulted in a larger frontal ERS. The evoked parieto-occipital activity did not show modulation to task load. Preliminary analysis on the non-phase-locked activity of our data has revealed a frontally-focused ERS, which is larger in the target compared to non-target stimuli. We are in the process of analyzing these induced oscillations further to investigate their utility as biomarkers for alcoholism and whether they provide independent group discrimination to the ERP amplitude measures.
