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20. Abnormal Auditory Gamma Band Response to Auditory Stimuli in First Episode Psychosis
Introduction: Schizophrenia patients have abnormal gamma band auditory steady state activity (~40Hz) extracted from the electroencephalogram (EEG). This may be indicative of a cortical circuit dysfunction inherent to the disease. We examined the auditory steady state gamma band response to auditory stimuli in first episode psychosis (FEP), to see whether these abnormalities are present in the early course of the disease.
Methods: Participants included 20 FEP and 20 healthy control participants matched for age, sex, and socioeconomic status. During EEG recording, clicks played at 40Hz (1ms click, 500ms duration trains, 1000ms SOA) were presented at three different loudness levels (75 dB, 80 dB, 85 dB SPL). The stimuli were presented once while participants attended and once while they ignored the sounds. Steady state auditory evoked potential (SSAEP) was measured using Morlet wavelets from the evoked response between 50ms and 550ms from the stimulus onset, within the frequency range of 39Hz to 41Hz.
Results: Within the control group, there was a significant difference in SSAEP amplitude between attended versus ignored sounds (p < 0.05). Within the FEP group, there was no significant difference between conditions (p = .357). Moreover, there was a near significant interaction effect of group (control or FEP) on the difference between conditions (p = 0.062). There was no significant effect of loudness on the SSAEP amplitude.
Conclusion: FEP participants showed an abnormal lack of modulation of SSAEP amplitude between attend and ignore conditions. These abnormalities are present not only in chronic patients, but also during the early course of schizophrenia.