Participants
Fifty-four outpatients with SCZ (18 men, 36 women) and 51 outpatients with MDD (13 men, 38 women) were enrolled. The mean ages of the patients with SCZ and MDD were 38.98 ± 16.32 years and 34.72 ± 10.98 years, respectively. Thirty-five HCs (17 men, 18 women) were recruited through the community newspaper. Their mean age was 41.40 ± 11.38 years. The age of all participants ranged from 16 to 82 years (mean age: 38.03 ± 13.56). All participants were native Koreans and were diagnosed and screened using the MINI International Neuropsychiatric Interview of the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition. The Positive and Negative Syndrome Scale (PANSS) [38] was assessed in patients with SCZ by a trained psychiatrist who was not involved in the present study. The Hamilton Depression and Anxiety rating scales (HAMD and HAMA) [39, 40] were evaluated in patients with MDD by a trained psychiatrist. In addition, the Beck Depression Inventory (BDI), which is a self-rating scale, was measured in patients with MDD and HCs[41]. The present study was approved by the Institutional Review Board of Seoul St. Mary’s Hospital College of Medicine, The Catholic University of Korea (approval number: KC09FZZZ0211). Written informed consent was provided, and all experimental procedures followed the relevant institutional guidelines and regulations for all participants.
EEG recordings
Participants were seated in a comfortable chair in a sound-attenuated room. The EEG data were recorded using a NeuroScan SynAmps amplifier (Compumedics USA, El Paso, TX, USA) with a head cap mounted with AgCl electrodes according to the international extended 10–20 system. The following 62 scalp electrodes were employed: FP1, FPz, FP2, AF3, AF4, F7, F5, F3, F1, Fz, F2, F4, F6, F8, FT7, FC5, FC3, FC1, FCz, FC2, FC4, FC6, FT8, T7, C5, C3, C1, Cz, C2, C4, C6, T8, TP7, CP5, CP3, CP1, CPz, CP2, CP4, CP6, TP8, P7, P5, P3, P1, Pz, P2, P4, P6, P8, PO7, PO5, PO3, POz, PO4, PO6, PO8, CB1, O1, Oz, O2, and CB2. Electrooculography (EOG) was placed above and below the left eye to detect vertical movement and at the outer canthus of each eye to measure horizontal movement. Bandpass filtering was applied at 1–100 Hz, with a sampling rate of 1,000 Hz. Reference channels were placed on both mastoids and the ground electrode was placed on the forehead. The impedance was maintained below 5 kΩ.
LDAEP paradigm and analysis
The auditory stimulation protocol comprised 500 stimuli with fixed interstimulus intervals of 2000 ms. Tones of 1000 Hz with a duration of 100-ms (rise and fall time: 10 ms) were delivered at five intensities (60, 70, 80, 90, and 100 dB SPL) through MDR-D777 headphones (Sony, Tokyo, Japan). A total of 500 stimuli comprised of each intensity of 100 stimuli were triggered via the STIM2 system (Compumedics USA, El Paso, TX, USA) to ensure accurate synchronization between the stimuli and EEG recordings. A fixation cross was displayed in the middle of the monitor screen. A trained person with no information about the origin of the data removed gross artifacts through visual inspection. Artifacts related to eye blinks were removed using an established mathematical procedure [42]. Based on vertical EOG, positive and negative components exceeding 300 µV from before and after onset stimulus (-100 ms to 300 ms) were removed. Data were epoched in the range of -100 ms and 700 ms. Pre-stimulus baseline correction and linear detrend were applied to all electrodes. Artifacts exceeding ± 100 µV were rejected at all electrode sites. Off-line bandpass filtering between 1 Hz to 30 Hz was applied. The trials of each intensity were averaged at midline electrodes, respectively (Fz, FCz, Cz, Pz, and Oz). N100-P200 peak detection was performed using MATLAB 2019 software (Mathworks Inc., Natick, MA, USA) and Scan 4.5 software. For each intensity, the most negative peak amplitude of the N100 component was defined between 80 ms and 160 ms after the stimulus onset, while the most positive peak amplitude of the P200 component was defined between 130 ms and 280 ms. Finally, LDAEP was calculated based on the linear regression slope, whereby the slope variations of the sound intensities were ascertained by subtracting N100 from P200. The mean LDAEP was also calculated by averaging the LDAEPs at all five midline electrodes (Fz, FCz, Cz, Pz, and Oz).
Statistical analyses
Demographic data including age, sex, and symptom scores were analyzed using the chi-squared test, multivariate analysis of variance, or t-test as appropriate. To test the interaction effect between LDAEP and group, we analyzed the group differences in LDAEP using repeated measures analysis of covariance. The between-subject factor was group, and the within-subject factors were the LDAEPs at the five electrode sites. Age and sex were controlled as covariates. In addition, for each single electrode, LDAEP was compared among the groups based on the multivariate analysis of covariance (MANCOVA), controlling for age and sex as covariates. LDAEPs were also compared between patients with SCZ and MDD with covariates, age, sex, and drug usage. In MANCOVA analyses, the significance level was set at p < 0.008 (two-tailed), considering the multiple comparison issue based on the Bonferroni correction [43]. Furthermore, partial correlation was performed among age, LDAEP, and symptom severity in each group, controlling for drug and sex. Binary classification of medication was designed for the presence or absence of drug usage that could modulate LDAEP (Table 4). Forty-seven patients with SCZ and twelve patients with MDD administered serotonin-related drugs in the current study. In terms of correlation analyses, p-values were adjusted via the Bonferroni correction with a significance level of p < 0.003. All statistical procedures were performed using IBM SPSS for Windows, version 21.0 (IBM Corp., Armonk, N.Y., USA).