Data acquisition and spatial processing

The experiment was conducted on a 3.0-T scanner (Trio Tim; Siemens, Erlangen, Germany) using a 16-channel birdcage head coil and tightly padded clamps were used to minimize head motion. A routine T1-weighted imaging was first obtained and then the rs-fMRI was obtained using an echo-planar imaging sequence with the following protocols: voxel size: 3.75×3.75×5 mm³, TR: 2000 ms, TE: 30 ms, FOV: 240×240 mm², matrix: 64×64, and slice thickness: 5 mm with no gap. Throughout the scanning, participants were instructed to lie in the scanner supine, relaxed, close their eyes, but remain awake.

The data were preprocessed using SPM8 (The Wellcome Department of Cognitive Neurology, London, UK, http://www.fil.ion.ucl.ac.uk/spm/software/spm8). The first five time points of the resting-state data were discarded because of instability of the initial MRI signal, leaving 175 time points remaining for further processing. Then, the functional images were slice-timing corrected and realigned to the first volume using a six-parameter rigid body transformation. Images whose head translation exceeded 2 mm or rotation exceeded 2° were excluded. The mean image generated was then spatially normalized into a standard stereotactic space using the Montreal Neurological Institute (MNI) echo-planar imaging template. Computed transformation parameters were applied to all functional images, interpolated to isotropic voxels of 2 mm³, and the resulting images were smoothed using a 4-mm full-width half-maximum isotropic Gaussian kernel. Then, using the Data Processing Assistant for resting-state fMRI (DPARSF) package 13, linear drift was removed. A band-pass frequency filter (0.01<f<0.08 Hz) was then applied to reduce physiological high-frequency noise. To further reduce the effects of confounding factors unlikely to be involved in specific regional correlation, we also removed several sources of spurious variance by linear regression, including six head motion parameters, and average signals from cerebrospinal fluid and white matter according to previous fMRI studies 14.