FIG 3. Thickness of the substantia nigra in the control subjects, the patients with Parkinson's disease, and the patients with secondary parkinsonism. The mean (± SD) thickness of the substantia nigra in the control group (n = 72) was 5.1 ± 0.89 mm, that in the group with Parkinson's disease (n = 47), 4.8 ± 0.75 mm; and that in the group with secondary parkinsonism (n = 10), 3.4 ± 0.53 mm. There was a significant difference between the group with Parkinson's disease and the group with secondary parkinsonism by unpaired Student's t-test (P < .01)