Sensory-evoked gamma oscillations in chronic schizophrenia.

BACKGROUND The early visual-evoked gamma oscillation (VGO) elicited by Gestalt stimuli is reduced in schizophrenia patients compared with 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, 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 schizophrenic (SZ) and matched healthy control (HC) subjects. 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 with 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, whereas the AGO evoked by simple tone stimuli does not appear to be abnormal in chronic, medicated schizophrenia patients.