How much does phase resetting contribute to event-related EEG abnormalities in schizophrenia?

Objective: Patients suffering from schizophrenia demonstrate impaired low frequency electrophysiological responses to stimuli, but it remains unclear whether these abnormalities arise from phase resetting of ongoing oscillations, new phase-locked (evoked) activity or non-phase-locked (induced) activity. Our goal is to clarify the contribution of each of these three processes to the impairment of neural activity during information processing in schizophrenia, by using statistics that do not confound increases in the mean post-stimulus signal with phase resetting. Methods: Thirty-four male schizophrenia patients and 34 healthy matched controls performed an auditory oddball task. We applied the analysis procedure developed by Martinez-Montes et al. [18] based on complex-valued wavelet transform to event-related signal elicited by target stimuli. Results: The largest abnormalities were found for phase-locked delta (1–4 Hz) and non-phase-locked theta (4–8 Hz). Delta phase resetting was moderately impaired and related to symptoms of disorganization. It also predicted evoked theta signal. Conclusion: The substantial reduction of both evoked and induced oscillatory activity in schizophrenia indicates diminished recruitment of brain circuits engaged not only in stimulus-locked perceptual processing but also in more extensive processing less tightly time locked to the stimulus. Although reduced phase resetting makes a lesser contribution, it indicates a deficit in the ability to harness ongoing electrical activity.