Abstract
Many studies have shown that Parkinson's disease (PD) affects not only the ability to generate voluntary saccades but also the ability to suppress reflexive saccades (hyper-reflexivity). To further investigate these apparently contradictory effects of PD on the saccade system we adapted a well-known dual-task paradigm (Deubel, 2008) to measure saccades with and without a peripheral discrimination task. Previously we reported that the concurrent performance of a perceptual discrimination task abnormally reduced the latencies of reflexive saccades in PD. Here we report the effects of the concurrent discrimination task on the generation of voluntary saccades in a PD and a control group. As expected, when saccades were performed without the discrimination task the PD group made voluntary saccades with longer latencies and smaller gain than the control group. The concurrent performance of the perceptual discrimination task facilitated the initiation of voluntary saccades in both groups, but, surprisingly, this facilitatory effect was stronger in the PD group than in the control group. In addition, in the PD group voluntary saccades were abnormally facilitated by the peripheral symbol-changes that occur during saccade planning in this paradigm. The results of this study may help to clarify apparently contradictory oculomotor abnormalities observed in PD.