Global resting state fMRI signals reflect state-dependent metabolic differences in brain networks, including external stimulus and vigilance changes between the eyes-closed and eyes-open states.

Test whether patients with Parkinson’s diseases (PD) and REM sleep behavior disorder (RBD) exhibit diminished state-related changes in their resting state fMRI global signal.

24 controls (CON, N=7 scanned twice), 51 PD patients (N=10 scanned twice), and 43 subjects with RBD (confirmed by video polysomnography) without PD (N=9 scanned twice) underwent MRI scanning (3T Philips) including a high resolution T1-weighted anatomical volume and separate eyes open and eyes closed resting state fMRI runs. Using AFNI, the fMRI volumes were motion corrected and aligned to the T1 volume. The mean resting state global signal was extracted from each eyes open and eyes closed run after motion-censoring (0.3 mm threshold) and regression of signal from ventricles and white matter. A single global signal value, defined as the standard deviation of the voxelwise signal across time averaged across all brain voxels, was computed for each eyes closed and eyes open run of each subject. The difference in open global signal was compared within each group of control, PD, and RBD subjects using a 2-tailed paired-test. 

Only the CON group showed a significant difference in the two resting state conditions. Global signal variance was higher during the eyes-closed compared to eyes-open condition (t(30)=2.475, p=0.0192, 0.0469 mean of differences +/- 0.1055 SD). 

No significant differences were found between the eyes closed and eyes open conditions for the PD patients (t(62)=0.2726, p=0.7861, 0.01061 mean of differences +/- 0.3039 SD) or subjects with RBD (t(51)=0.529, p=5991, 0.00963 mean of differences +/- 0.1313 SD).