Processing of spatial contrast in peripheral vision in Parkinson's disease

Two experiments were carried out to test the hypothesis, based on anatomical evidence, that contrast gain might be reduced in the retinal periphery in Parkinson's disease. In the first experiment, subjects set contrast thresholds before and after adaptation to a vertical grating of 2 cycles per degree (c/deg), either stationary or oscillating sideways through its spatial period at 8 Hz, presented either in central vision or 7 degrees peripherally. Threshold elevations were similar for central viewing in both patients and controls. However, for peripheral viewing, elevations were greater in the controls, but smaller in the patients, than for central viewing. In the second experiment, a staircase procedure was used to find the contrast of a peripherally viewed grating of either 4 or 1.2 c/deg which apparently matched that of a centrally viewed grating of the same spatial frequency. Patients needed more contrast (about 1.6 times, at both spatial frequencies) than controls for a match. These results suggest that contrast gain may be lowered in the peripheral retina in Parkinson's disease, perhaps because of an abnormality of dopamine amacrine cells, whose density peaks in the peripheral retina.