Gamma-Band Auditory Steady-State Responses Are Impaired in First Episode Psychosis

Abstract

Background: In chronic schizophrenia and chronic bipolar disorder, gamma band (30–100 Hz) auditory steady-state EEG responses (ASSRs) are reduced in power and phase locking, likely reflecting neural circuit dysfunction. Here we examined whether gamma ASSR deficits are also present at first hospitalization for psychosis. Methods: Subjects were 16 first episode schizophrenia patients (SZ), 16 first episode affective disorder patients (AFF) (13 with bipolar disorder), and 33 healthy controls (HC). Stimuli were 20, 30, and 40 Hz binaural click trains. ASSR phase locking and evoked power were analyzed using the Morlet wavelet transform. Results: At 40 Hz stimulation, SZ and AFF had significantly reduced phase locking compared to HC. This deficit was more pronounced over the left hemisphere in SZ. Evoked power at 40 Hz was also reduced in the patients compared to HC. At 30 Hz stimulation phase locking and evoked power were reduced in both patient groups. The 20 Hz ASSR did not differ between groups, but phase locking and evoked power of the 40 Hz harmonic of the 20 Hz ASSR were reduced in both SZ and AFF. Phase locking of this 40 Hz harmonic was correlated with total positive symptoms in SZ. Conclusions: The gamma ASSR deficit is present at first hospitalization for both schizophrenia and affective disorder, but shows a left hemisphere bias in first hospitalized SZ. Some of the neural circuitry abnormalities underlying the gamma ASSR deficit may be common to psychoses in general, while others may be specific to particular disorders.