Parkinson’s disease (PD) is characterized by the asymmetric onset, as exhibited by unilateral motor function deficits.¹ The previous studies showed PD patients with left body side motor symptom (LPD) had worse performance in visuospatial perception while those with the right body side onset (RPD) had more difficulty in language and memory relevant tasks.² Voxel-mirrored homotopic connectivity (VMHC), a novel resting-state fMRI parameter has been developed to calculate the functional connectivity of each voxel in the brain with the corresponding (homotopic) voxel in the contra-lateral brain hemisphere.³ In this preliminary study, we used VMHC to investigate the changes of inter-hemispheric functional connectivity in PD patients and made a voxel-wise comparison among LPD, RPD and normal controls.The study was approved by the local ethical committee and written informed consent was obtained from all the participants. PD patients were recruited according to the criteria of Hoehn & Yahr and UPDRS-III scores, 15 LPD patients (aged 58.3±8.2 years, 8 females) and 16 RPD patients (aged 59.4±7.6 years, 7 females). The UPDRS-III motor scores were 13.5±5.1 for LPD patients and 10.8±2.5 for RPD patients respectively. Age- and gender-matched 19 healthy controls (aged 61.3±5.7 years, 9 females) were recruited for group comparison. All subjects were right-handed. Thirty-five axial slices covering the whole brain were acquired using a 3.0-T HDXT scanner (GE Healthcare, Milwaukee, WI) with an 8-channel phase array head coil (TR/TE 2000/30 ms, flip angle 90°, matrix 64 × 64, FOV 24 cm, thickness/gap 4/0 mm, total 210 volumes). Data preprocessing included slice timing and realignment for temporal and spatial adjustment using SPM8, followed by spatial normalization to warp all the images into the same stereotactic space for group comparison. An in-house software REST was used for VMHC calculation (http://www.restfmri.net). All the time series were de-trended and band-pass filtered (0.01-0.08Hz). VMHC was calculated as the Pearson correlation coefficient between each pair of the voxel and its homotopic counterpart in the contra-lateral brain hemisphere, which was finally standardized by the global mean within the whole brain. The statistical analysis was one-way ANOVA to make comparison among LPD, RPD and control groups. The AlphaSim program implemented in AFNI was used for multiple comparison correction (corrected p<0.05).The UPDRS-III motor scores showed no significant group differences between LPD and RPD. Due to the definition, VMHC maps exhibited a pattern of symmetry. LPD patients showed decreased VMHC in the post-central gyrus, supplementary motor area, cuneus, middle occipital gyrus, lingual gyrus and cerebellum but increased VMHC in the thalamus, as compared to normal controls (Fig. 1). RPD patients showed decreased VMHC in the post-central gyrus and middle frontal gyrus but increased VMHC in the thalamus and superior frontal gyrus when comparing to normal controls (Fig. 2).Both of LPD and RPD patients showed decreased VMHC in post-central gyrus, which was consistent with the clinical symptom of motor function deficits. The decreased VMHC in the cuneus and middle occipital gyrus in LPD patients might affect visual processing function. For RPD patients, the decreased VMHC in the middle frontal gyrus and increased VMHC in the superior frontal gyrus could be relevant to advanced cognitive impairment. Finally, the increased VMHC in the thalamus for both of LPD and RPD might serve as a compensatory mechanism of motor function deficits. Further study is needed to investigate gray matter volume changes and correlation with clinical indexes for PD patients.