Early cognitive changes and nondementing behavioral abnormalities in Parkinson's disease.

Early cognitive changes in patients with PD are often subtle and influenced by factors that interact with the disease process, including age and age of disease onset, medication, and the specific constellation of motor symptoms. These factors notwithstanding, there is ample evidence that specific cognitive changes occur early in the course of PD. Whereas language processing deficits are infrequent, subtle changes in olfaction and contrast sensitivity have been repeatedly observed. Executive function deficits are often prominent and, as an integral part of many tasks, also influence performance on a wide range of cognitive measures. This is particularly true for memory and visuospatial dysfunction, two areas that rely heavily on executive demands. Finally, depressive symptoms are also frequent in the early stages of the disease. The significance of early behavioral changes and their prognostic implications are largely unknown and need to be assessed prospectively.