Subjects

We recorded electroencephalograms (EEGs) of 21 individuals with recent-onset schizophrenia (ROSZ), 27 individuals with UHR, and 24 healthy controls (HCs; Table 1). Fourteen of the participants had also participated in our previous MMN study42, and 38 had participated in our previous ASSR study65. Individuals with ROSZ or UHR were recruited from outpatient and inpatient units at the University of Tokyo Hospital. The HC group was recruited through advertisements at several universities in Tokyo. Inclusion criteria for ROSZ individuals were that they were diagnosed using the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV), aged 15–40 years, and had onset of continuous psychotic symptoms within the past 60 months. Among a total of 21 ROSZ patient sample, N = 15 underwent EEG measurement as the biomarker assessments at Time 1 for the Integrative Neuroimaging Studies for Schizophrenia Targeting Early Intervention and Prevention (IN-STEP) project67 [all satisfied the criteria of first-episode schizophrenia (FES): continuous psychotic symptoms within the past 60 months and no history of antipsychotic drug treatment for more than 16 cumulative weeks at entry into the IN-STEP project]; N = 1 did not undergo EEG testing at Time 1, but instead underwent the initial EEG testing during the follow-up period of INSTEP; and N = 5 were newly recruited (N = 3 satisfied FES). Inclusion criteria for individuals with UHR were identified using the Structured Interview for Prodromal Symptoms (SIPS)68, and all were aged 15–30 years. Inclusion criteria of HC were that they were aged 15–40 years and had no personal history of psychiatric disease or a family history of axis I disorders in first-degree relatives. Exclusion criteria for all groups were neurological illness, traumatic brain injury with loss of consciousness for more than 5 min, history of electroconvulsive therapy, low premorbid intelligence quotient (IQ; below 70), previous alcohol/substance abuse or addiction, or a hearing impairment, which was assessed with a hearing test in both ears at 30-dB sound pressure level tone at 1000 Hz and 40-dB at 4000 Hz by audiometer. Written informed consent was obtained from each subject before participation. The Research Ethics Committee of the Faculty of Medicine at the University of Tokyo approved this study (approval No. 629 and 2226).

Table 1 Demographics of participants Full size table

The estimated premorbid IQ was assessed using the Japanese version of the National Adult Reading Test in all participants69,70. The Positive and Negative Syndrome Scale (PANSS)71 and Global Assessment of Functioning (GAF)72,73 were used for assessment of global clinical symptoms and functioning in all participants with ROSZ or UHR. Nineteen patients with ROSZ and 17 individuals with UHR took antipsychotic medication. The antipsychotic dose was converted to an equivalent dose of chlorpromazine74.

Procedure and analyses of the mismatch negativity

A two-tone auditory oddball paradigm with 2000 stimuli was used for MMN. Standard tones (1000 Hz, 50 ms) were 90% of the stimuli and duration-deviant tones (1000 Hz, 100 ms) were 10% of the stimuli. All stimuli were 80 dB and had 1 ms rise/fall time. Stimulus onset asynchrony was 500 ms. While the participants watched a silent cartoon, the tones were presented binaurally through inserted earphones (Multi Trigger System, Medical Try System, Tokyo, Japan).

We used a 64-channel Geodesic EEG System (Electrical Geodesics Inc., Eugene, OR) to obtain EEG data. Electrodes were referenced to the vertex, and impedances were maintained below 50 kΩ. The sampling rate was 500 Hz. The analog filter bandpass was set at 0.1–100 Hz. We analyzed EEG data using EEGLAB75. The continuous EEG data were re-referenced to an average reference, digitally filtered at 0.1–20 Hz, and segmented from −100 to 500 ms relative to the stimulus onset. The mean of the pre-stimulus baseline was subtracted for baseline correction. Independent component analysis was used for eye blink correction. Epochs exceeding ± 100 μV at any electrode were rejected. After averaging across trials, the event-related potential (ERP) waveform in response to standard stimuli was subtracted from the ERP waveform in response to deviant stimuli.

The amplitude of MMN at seven electrodes around the FCz was used for MMN analysis because the largest MMN amplitudes were obtained with them (Supplementary Fig. 1). The MMN amplitude was measured using the mean voltage from 135 to 205 ms post stimuli, in accordance with previous studies36,42,76.

We performed another oddball paradigm with response to frequency deviants, and the oddball paradigms were counterbalanced. However, we did not employ the frequency-deviant MMN in subsequent analyses because the group difference of MMN was specific to the duration-deviant MMN.

Procedure and analyses of the auditory steady-state response

The ASSR paradigm used in this study, which is described in detail elsewhere65, was similar to those used in previous studies from different laboratories56,58. Briefly, subjects were instructed to relax with their eyes open, and they received auditory stimuli presented binaurally through inserted earphones, the same as those used in the MMN session. We performed the ASSR session before the MMN session, within 30 min. We recorded ASSR at first to avoid muscle artifacts because gamma band ASSR is sensitive to muscle artifacts77. We measured both ASSR and MMN in total within 60 min. Click sounds (80 dB, 1 ms) presented in 500 ms trains at 20, 30, and 40 Hz served as the auditory stimuli. Click sound trains were 200 trains at each frequency. The intertrain interval was 500 ms. The sampling rate was 250 Hz. The analog filter bandpass was set at 0.1–100 Hz. We used 40 Hz ASSR data for subsequent analyses because impairment of 40 Hz ASSR in early stages of schizophrenia was confirmed in previous studies63,64,65.

We analyzed EEG data by using EEGLAB75. The continuous EEG data were re-referenced to an average reference, a high-pass filter (1 Hz) and a notch filter (50 Hz) were applied to them in order to remove artifacts, and they were segmented from −250 to 750 ms, relative to the stimulus onset. Independent component analysis was used for eye blink correction, and epochs exceeding ± 100 μV at any electrode were rejected.

We performed time-frequency analyses with a short-term Fourier transformation and then calculated intertrial phase coherence (ITC) and event-related spectral perturbation (ERSP) as indices of ASSR. The ITC indicates phase consistency across trials and ranges between 0 (random phase across trials) and 1 (identical phase across trials). The ERSP indicates event-related changes in power relative to a pre-stimulus baseline. Decreases in ITC and ERSP reflect reduced neural responses to auditory steady-state stimulation. We calculated the mean ITC and ERSP by averaging the data over stimulation time (0–500 ms) and stimulation frequency (40 Hz: 36–45 Hz). Because our previous study65 observed differential alteration of early- and late-time-course components of ITC and ERSP in early stages of psychosis, we additionally calculated the mean ITC and ERSP for each 100 ms-epoch for time-course analyses. We focused on the frontocentral electrode site (FCz) because the most prominent ASSR was found at FCz.

Statistical analyses

We used SPSS (Version 23.0.0.0, IBM Corp., Armonk, NY, USA) for all statistical analyses. We employed χ2 tests, independent t-tests, and one-way ANOVAs for comparison of demographics and clinical characteristics among the groups. We performed one-way ANOVAs for comparison of the number of epochs and found no significant differences among groups [MMN, 161 ± 30 for ROSZ, 156 ± 33 for UHR, and 164 ± 24 for HCs (F 2, 69 = 0.44, p = 0.65); ASSR, 182 ± 21 for ROSZ, 183 ± 16 for UHR, and 187 ± 10 for HCs (F 2, 69 = 0.62, p = 0.54)]. For the main comparison of MMN amplitude, we used repeated measures ANOVA with the three groups as the between-subject factors and with seven electrodes around the FCz as the within-subject factor. For the main comparison of ITC (0–500 ms) and ERSP (0–500 ms), we used one-way ANOVA. As a supplementary analysis, we performed repeated measures ANOVA with the three groups as the between-subjects factor and with time blocks (0–100, 100–200, 200–300, 400–500 ms) as the within-subject factor. Greenhouse-Geisser correction was used for repeated measure ANOVAs when appropriate. In case of a significant group-by-time interaction, we used one-way ANOVA and post-hoc tests with Bonferroni correction for comparison of ITC and ERSP time blocks, and Cohen’s d effect sizes were calculated for group comparisons of MMN, ITC, and ERSP. The significance level was set at p < 0.05 (two-tailed).

The Pearson correlation coefficients (r) of MMN with ITC and ERSP in each group were calculated. p < 0.0014 (0.05/36) was considered statistically significant based on Bonferroni correction (two-tailed). Additionally, we performed correlation analyses between frequency-deviant MMN and ASSR [20, 30, and 40 Hz; ITC (0–500 ms) and ERSP (0–500 ms); p < 0.0028 (0.05/18) was considered statistically significant based on Bonferroni correction (two-tailed)] and between the duration-deviant MMN and ASSR [20 and 30 Hz; ITC (0–500 ms) and ERSP (0–500 ms); p < 0.0042 (0.05/12) was considered statistically significant based on Bonferroni correction (two-tailed)] in ROSZ, UHR and HCs for supplementary information (Supplementary Table 1 and 2, and Supplementary Text).