Fig. 2.

Forest plot showing the crude effect size of${\mathbf{R}}_{2(\mathbf{I}\mathbf{S}\mathbf{Q}\mathbf{R})}^{\ast }$in the substantia nigra for each Parkinson’s disease (PD) subgroup according to the disease duration: G1 (PD < 5 years), G2 (PD > 5 and < 10 years), G3 (PD > 10 and < 15 years) and G4 (PD > 15 years). G1-G2: effect size between G1 and G2, G1-G3: effect size between G1 and G3, G1-G4: effect size between G1 and G4, G2-G3: effect size between G2 and G3, G2-G4 effect size between G2 and G4, G3-G4: effect size between G3 and G4. SN_D = substantia nigra_dominant side of the diease. SN_ND = substantia nigra_ non dominant side of the disease. The results were expressed using Hedge’s effect size. CI = confidence interval. The box represents the effect size and the whiskers the 95 % CI. A significant difference in the effect size of ${\mathrm{R}}_{2\left(\mathrm{I}\mathrm{S}\mathrm{Q}\mathrm{R}\right)}^{\ast }$ values between Parkinson’s disease subgroups (P < 0.05), as well as when comparing PD patients-G3. vs PD patients-G1 (P_{SN_D} = 0.03, P_{SN_ND} = 0.01) and when comparing PD patients-G4. vs PD patients-G1 (P_{SN_D} < 0.001, P_{SN_ND} = 0.01) was showed.