2.1 Participants
A total of 58 subjects participated in the study, which included 30 MDD patients and 28 healthy controls. However, more than half of MDD patients were excluded from the final analysis because their electrode became detached, more than half of healthy controls were eliminated because their sleep time was less than 6.5 hours on the experimental night. Finally, 11 healthy male adult controls and 11 male MDD patients completed the study. The MDD patients ages’ ranged from 22 to 40 years (mean±SD: 30.64 ± 5.52 years).These patients reported no history of any other psychiatric disorder or prior take of antidepressants. All patients, were from The Peking University Sixth Hospital, and met the criteria for major depression defined in the Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5) (American Psychiatric Association, 2013). Diagnosis was established by experienced psychiatrists using the Structured Clinical Interview for DSM-5: Research Version (SCID) (First et al., 2015). A minimum score of 22 points on the 17 item Hamilton Depression Scale (HRSD - 17) (Snaith 1977) was required to be enrolled in the study. Professional scorers from the Peking University Sixth Hospital conducted the HRSD measurements two times. The first time occurred before the treatment of escitalopram, and the second time occurred on day 57 of escitalopram treatment. The exclusion criteria included: (1) age < 18 or 45 > years, (2) presence of additional psychotic symptoms, (3) cognitive impairment or personality disorders, (4) history of other mental illness, (5) suicidal ideation or behaviors.
Control participants included 11 physically and mentally healthy male volunteers whose ages were between 22 and 38 years (mean ± SD: 27.72 ± 4.79 years). The inclusion criteria included: (1) self-reported good sleep and PSQI < 5, matched age with MDD, (2) absence of psychiatric illnesses diagnosed by the DSM-5 criteria, (3) a maximum score of 7 points on the 17 - item HRSD, (4) a maximum score of 7 points on the 14 - item Hamilton Anxiety Scale (HAMA) (Maier et al. 1988), (5) 18 ≤ BMI < 30.
The exclusion criteria included: (1) any of the exclusion criteria for the MDD group, (2) any past or present history of mental illness that met DSM-5 diagnostic criteria, (3) current or pass chronic physical diseases (e.g., cardiovascular disease, diabetes, rheumatoid arthritis, et al.), (4) shift worker within the preceding year, (5) jet lag travel in the last 2 weeks, (5) total sleep time < 6.5 hours.
All of the participants were Han Chinese. They signed written informed consent forms before participation. The study was approved by the ethics committee of Peking University Sixth Hospital, Beijing, China, in accordance with the Helsinki Declaration.
2.2 Polysomnographic recording
All the depressive patients underwent polysomnographic recording two times. The first time conducted before the treatment of escitalopram, and the second time conducted on day 57 of escitalopram treatment.
Overnight polysomnographic recording included electroencephalography (EEG; including F3, F4, C3, C4, O1, and O2, with reference to the contralateral mastoid; International10 - 20system), electrooculography (EOG), electromyography (EMG), and electrocardiography (ECG). The signals were digitized at a sampling rate of 256 Hz, and an electrode impedance < 5 KΩ. Thirty - second epochs were used for manual analysis, and sleep stages were scored offline according to the criterion of the American Academy of Sleep Medicine (AASM) (Berry, Richard B., et al, 2012), using the standard polysomnographic sleep recordings.
2.3 EEG signal processing
In the processing environment of MATLAB R2016b, using EEGLAB toolkits (University of California San Diego), power frequency interference was eliminated by using a 50 Hz notch, and data was filtered from 0.5 to 30 Hz by using band pass filter (Delorme and Makeig 2004). Each sample had corresponding sleep staging files. However, because the sample duration data was too large, and some data frames had large artifacts, we chose the entire artifact - free frames (30 seconds) from every sleep type (including Wake, rapid-eye-movement (REM), stage-1, stage-2 (including sleep spindles) and stage-3) according to sleep staging files.
The EEG signal processing includes two aspects: linear analysis and nonlinear dynamic analysis.
2.3.1 Linear analysis:Power spectrum
The power spectrum reflects the energy information carried by the brain waves in each frequency band. According to the frequency, the EEG signals are divided into several categories: (0.5 - 2Hz), (2 - 4Hz), θ (4 - 8Hz), α (8 - 13Hz), (13 - 20Hz) and (20 - 30Hz) (Cheng et al. 2019). A previous study revealed that during the night, the frequencies of the most powerful waves are concentrated in the 0.5–2 Hz range and show a continuous tendency to shift towards slower frequencies during sleep. So we divided the delta band into low-delta (0.5–2 Hz)and high-delta (2 - 4Hz)(Lanquart et al. 2018). In the present study, each frequency band power was obtained by using fast Fourier transform (FFT) analysis (Faust et al. 2008; Pardey et al. 1996b; Welch 1988). FFT calculation was performed on 3 second non-overlapping consecutive window (Hamming window). The average values of the different sleep stages were computed form the 30 seconds of data obtained previously. In order to reduce specific individual differences, the relative energy (RE) was computed. The RE corresponds to the ratio between the power value of each frequency band and the sum of the power values in the following calculation:
See formula 1 in the supplementary files.
Correlation dimension, complexity, entropy and Lyapunov exponents are common non-linear features in EEG signal analysis. Correlation dimension and Lyapunov exponents require large data sets and strict dimensional measurements which are not suitable for EEG analysis. Whereas Lempel-Ziv Complexity (LZC) and Co-complexity (C0C) are more suitable, because they require small datasets and have high computation speeds. Therefore, in the present study, LZC and C0C were used to characterize the sleep state of patients with MDD. LZC represents the rate of appearance of a new pattern in a time series from a one dimensional perspective (Li and Wang 2008). A ratio of the area of the disorder component over the area of the original time series is considers as a complexity measurement, which is denoted as C0(Chen et al. 2000). The higher the LZC, the more likely it is that a new model will appear, highlighting complex dynamic behavior. The higher the C0C, the more probability there is that random motion may appear.
In order to obtain 28 seconds sequences, the first and last second of each of them were removed from the 30 seconds previously selected sequences. This shortened sequence was then divided into 7 segments of 4 seconds each for targeted analysis. For each of those time-windows we considered as important EEG features, the mean of the values by itself as well as the characteristic of values according to the sleep stage.
2.4 Statistical analysis
All analyses were performed using the SPSS Statistics version 22.0. We used the paired-samples t-test to investigate the changes in the EEG characteristic parameters between the baseline (before treatment) and the final (after treatment) session, and the independent-sample t-test to compare the results from the patients with MDD and healthy control subjects. We then used the paired-samples t-test to analyze differences between the left and right hemispheres of the cortex. The differences of EEG characteristics in different brain regions (frontal, central, occipital) between baseline and final were analyzed by one-way ANOVA. Differences were considered significant when P< 0.05.