Study design
This study adopted a cross-sectional case-control study design. Data, including demographic data, clinical assessments, neurocognitive assessments, MEG scan data and MRI scan data, were collected from patients with MDD and healthy controls (HCs).
Participants
The MDD patients were recruited from the Affiliated Brain Hospital of Nanjing Medical University between 2012 and 2021. All patients met the diagnostic criteria of major depressive disorder using the Diagnostic and Statistical Manual of Mental Disorders Fourth Edition, Text Revision (DSM-IV-TR). The inclusion criteria for this population were: (1) aged 18-50 years; (2) right-handed; (3) native Han Chinese; (4) education level of junior high school or above; (3) Hamilton Rating Scale for Depression 17-item (HAMD17) >17; (4) Young Mania Rating Scale (YMRS) [54] score < 5; (5) no structured psychotherapy or physical therapy within six months. The exclusion criteria included comorbidity with other mental or physical illnesses, history of substance abuse, contraindications for MEG or MRI and pregnancy or lactation.
One hundred thirty-two individuals, comprising 65 healthy controls and 67 MDD patients, were enrolled in this study. Participants were asked to perform a simple visuomotor task during MEG scanning. This work was approved by the Affiliated Brain Hospital of Nanjing Medical University. It was performed in accordance with the Declaration of Helsinki, and all individuals provided written informed permission before participation.
Clinical assessments
We collected general participant information, such as sex, age, education, ethnicity, dominant hand, disease history, marriage and family history. Depression severity was assessed using HAMD17, and manic symptoms were evaluated by YMRS. Retardation factor score was obtained using 1, 7, 8 and 14 items in HAMD17.
Neurocognitive assessments
Based on previous work, psychomotor alterations could be measured by neuropsychological tests [7]. In this study, psychomotor performance was measured by three different neuropsychological taskes:1) DSST, which assesses psychomotor processing speed by instructing subjects to fill in as many blanks as possible within 90s according to the number symbol correspondence table. Test performance is evaluated by the number of successful completions; 2) TMT-A, a test in which individuals are directed to draw lines through consecutive numbers to determine the time needed to complete the test; and 3) VFT, a task in which participants are asked to name as many animals or vegetables as possible in 60s.
MEG scanning
Subjects were placed in a magnetically shielded room during the entire experiment. Data were collected using a 275-channel CTF system (VSM Med Tech Inc., Port Coquitlam, Canada). The sampling rate is 1,200 Hz. The participants were lying in the MEG machine during a visuomotor task. Three coils, one nasion and two preauricular points were used to check head movements during the recording. Electrocardiography and electrooculography were also recorded. The experiment consists of two blocks, each lasting 5 min. Participants can take a small break between blocks.
Task Paradigm
Participants were asked to perform a visuomotor task during MEG scanning. Participants responded with a right index finger button press to visual stimuli. Fig. 2 shows an illustration of this paradigm. During each trial, grey light was exhibited on a projection screen for 2,500 milliseconds, then a green light lasting 500 milliseconds. Subjects responded with a right index finger button press to visual stimuli. Individuals were asked to concentrate on the projection screen, keep their bodies still, and press the button as quickly as possible. Participants first completed a practice examination to understand the task, followed by a formal exam including 180 trials.
MRI scanning
MRI data were recorded with a Siemens Verio 3T MRI system. The parameters were the same as those used in our earlier articles [55]. Three markers were put in the same location as MEG to facilitate registration between MEG data and structural MRI in the following data analysis.
Data processing
The entire data processing process is illustrated in Fig. 3. The raw data were epoched into a time window (-0.7 to 2.3 s). The 0s represent the emergence of a visual cue or green light. The data was down-sampled to 400 Hz. Then we applied Synthetic third-order gradiometer noise cancellation and removed linear trend and line interference with a 50-Hz band-stop filter. Visual artifact rejection was used to exclude trials and channels with high variation. Furthermore, with a visual examination, an independent component analysis (ICA) was used to eliminate eye artifacts, as well as cardiac and muscular components.
We analysed the MEG data in FieldTrip (version 20210720)[56]. The structural T1 data was imported and segmented using FieldTrip, with 250 anatomical volumes in each direction and “singleshell”.
Beamforming and time course extraction
As PMBR is located in the contralateral M1, we want to extract the beta time series of the M1 in this right finger tap task. Using the linearly constrained minimum variance (LCMV) beamformer [57], we can compute virtual channel time series at the locations of interest. The coordinates of the M1 were defined by the Automated Anatomical Labeling (AAL) template atlas [58]. The spatial filter was computed with the covariance and the forward model for the left M1. Using the spatial filters, virtual channel time series were obtained.
Time-Frequency spectrograms
Using a Hanning taper-based time window of four cycles at each frequency, the source-level time-frequency representation of the time series of the M1 may be obtained (1–100 Hz in steps of 1 Hz). We compared the active energy to the baseline using relative energy percentage. The baseline was defined between the -0.5–0 s window relative to the appearance of green light. The time-frequency spectrograms were averaged across participants in both groups.
PMBR quantification
We averaged mid-beta band (17–25Hz) amplitude from the data baseline adjusted in time windows (1 < t < 2s) acceptable for the PMBR. Eventually, we obtained a value per participant representing the PMBR at the left M1.
Statistical analyses
A two-sample t-test was used to check age and education, whereas chi-square tests were conducted to compare gender between the MDD patients and healthy controls. All statistical analyses were done on SPSS 19.0 software (IBM Corp., Armonk, NY, USA). The differences in neurocognitive test performance (DSST, TMT-A and VFT) between the MDD and HC groups were compared by a two-sample t-test (two-tailed). The difference in PMBR amplitude between groups was analysed using a two-sample t-test. We then computed correlations between PMBR values and retardation factor, neurocognitive tests performance (DSST, TMT-A and VFT), controlling for age, sex and education. To avoid multiple comparison problems, further FDR correction was conducted using a special MATLAB function.