Adaptive modification of saccade amplitude in Parkinson's disease

The accuracy of saccades (fast eye movements) is maintained over time and is an adaptive ability usually ascribed to the cerebellum. Adaptation might occur elsewhere in certain tasks, such as in the prefrontal cortex for memory-guided saccades. We hypothesized that adaptation of memory-guided saccades would be impaired in Parkinson's disease, as basal ganglia dysfunction can disrupt the operation of the prefrontal cortex, while adaptation of visually guided saccades would be preserved. Adaptation was induced by consistently yet imperceptibly displacing targets as saccades were made toward them, causing artificial saccadic inaccuracy. Twelve Parkinson's disease subjects (OFF medication) and 12 age-matched controls performed 245 visually- and memory-guided horizontal saccades in separate sessions. An infrared eye tracker detected the saccade, during which the target was displaced by 12.5% of the size of the initial jump, either in the same (centrifugal) or the opposite (centripetal) direction. Parkinson's disease subjects made smaller visually guided saccades than did controls [F(1,20) = 9.10, P < 0.01], yet both groups modified saccade size appropriately. Parkinson's disease memory-guided saccades were also smaller than those of controls [F(1,19) = 5.93, P < 0.05]. While controls decreased (by 8.6%) or increased (by 4.1%) the size of these saccades appropriately, Parkinson's disease subjects decreased saccade size in response to both centripetal adaptation (by an excessive 18.3%) and centrifugal adaptation (by 3.5%). Parkinson's disease subjects were less able to modify saccadic size appropriately when the movement size was specified in motor memory: a predilection for excessive hypometria was invoked, regardless of adaptation direction. This indicates that, in certain tasks, saccadic adaptation involves structures other than the cerebellum.