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 electroencephalogram 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 control subjects (HC). Stimuli were 20-, 30-, and 40-Hz binaural click trains. The ASSR phase locking and evoked power were analyzed with the Morlet wavelet transform.

Results: At 40-Hz stimulation, SZ and AFF had significantly reduced phase locking compared with HC. This deficit was more pronounced over the left hemisphere in SZ. Evoked power at 40 Hz was also reduced in the patients compared with 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 might be common to psychoses in general, whereas others might be specific to particular disorders.

Fig. 1

Time-frequency maps of ASSR phase locking (top) and evoked power (bottom) at electrode Cz for each subject group for 20, 30, and 40 Hz stimulation. Subject groups are healthy controls (HC), schizophrenia patients (SZ), and affective disorder patients (AFF).

Fig. 2

Phase locking time series from the 40 Hz ASSR at left hemisphere (C3), midline (Cz), and right hemisphere (C4) electrodes. The time series were extracted from the frequency bands of the maximal response in Fig. 1 (40.5–44.1 Hz wavelet center frequencies).

Fig. 3

Scatterplots illustrating intersubject variability in phase locking and evoked power, measured at fronto-central electrodes. Horizontal bars indicate the mean values for each group for comparison with the individual values. Outliers are circled for both measures. Note that the y-axis on the 40 Hz ASSR evoked power plot has a different scale than the other plots to accommodate the outlying values.

Fig. 4

Scatterplot illustrating the correlation between the PANSS Positive Symptom Total score and phase locking of the 40 Hz harmonic of the 20 Hz ASSR in SZ. The mean phase locking values for each group are indicated with horizontal bars. Note that only the SZ with the most positive phase locking values were near the mean of the HC group.