Effect of stimulus orientation on contrast sensitivity in Parkinson's disease.

We studied the effect of stimulus orientation on contrast sensitivity function in 21 patients with Parkinson's disease and in 10 normal subjects. This was done by measuring contrast sensitivity over a range of spatial frequencies for vertical and horizontal sine wave grating stimuli. There was a great test-retest consistency in normal subjects and patients. Fifteen of the 21 patients showed contrast sensitivity deficit in at least one eye. Orientation-specific loss was demonstrated in 17 of the 25 "affected" eyes. The most frequent type of orientation-specific loss was a notch defect, which preferentially affected the middle spatial frequencies. We attribute orientation-specific and spatial frequency-selective loss in Parkinson's disease to a functional disruption of neurons on the visual cortex.