Deficient perception and cognition in Parkinson's disease (PD) has been attributed to slow information processing, but an alternative explanation may be reduced signal strength. In 18 nondemented individuals with PD and 15 healthy adults, we enhanced the contrast level of rapidly flashed masked letters. The PD group required significantly higher contrast to reach criterion (80% accuracy). Normal motion detection in these participants indicated no gross, general dysfunction of the dorsal visual processing stream. These results suggest that putatively slowed processing in PD may be an artifact of reduced signal strength arising from depletion of dopamine in retina or cortical visual areas.
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