Sensory-Evoked Gamma Oscillations in Chronic Schizophrenia

Abstract

Background: The early visual-evoked gamma oscillation (VGO) elicited by Gestalt stimuli is reduced in schizophrenia patients compared to healthy individuals, but it is unknown whether this effect is specific to these particular stimuli and task. In contrast, the early auditory-evoked gamma oscillation (AGO) was reported to be unaffected in a sample of unmedicated, mostly first-episode schizophrenics (Gallinat et al, 2004, Clin Neurophysiol), but it is unknown whether this oscillation is abnormal in chronic, medicated patients. We investigated these issues by examining the VGO and AGO in chronic schizophrenics (SZ) and matched healthy controls (HC). Methods: Subjects (21 HC, 23 SZ) performed visual and auditory oddball tasks. Visual stimuli were letters, and auditory stimuli were simple tones. Event-related spectral measures (phase locking factor and evoked power) were computed on Morlet wavelet-transformed single epochs from the standard trials. Results: VGO phase locking at occipital electrodes was reduced in SZ compared to HC. In contrast, AGO phase locking and evoked power did not differ between groups. Conclusions: The VGO deficit may be a general phenomenon in schizophrenia, while the AGO evoked by simple tone stimuli does not appear to be abnormal in chronic, medicated schizophrenics.