Small saccades restrict visual scanning area in Parkinson's disease.

The purpose of this study was to investigate abnormalities in visual scanning when Parkinson's disease patients view images of varying complexity. Eighteen nondemented Parkinson's disease patients and 18 normal subjects participated in the study. The ocular fixation position during viewing visual images was recorded using an eye-tracking device. The number of saccades, duration of fixation, amplitude of saccades, and scanned area in Parkinson's disease patients were compared with those in normal subjects. We also investigated whether the number of saccades, duration of fixation, or amplitude of saccades influenced the scanned area. While scanning images of varying complexity, Parkinson's disease patients made fewer saccades with smaller amplitude and longer fixation compared with normal subjects. As image complexity increased, the number of saccades and duration of fixation gradually approached those of normal subjects. Nevertheless, the scanned area in Parkinson's disease patients was consistently smaller than that in normal subjects. The scanned area significantly correlated with saccade amplitude in most images. Importantly, although Parkinson's disease patients cannot make frequent saccades when viewing simple figures, they can increase the saccade number and reduce their fixation duration when viewing more complex figures, making use of the abundant visual cues in such figures, suggesting the existence of ocular kinesie paradoxale. Nevertheless, both the saccade amplitude and the scanned area were consistently smaller than those of normal subjects for all levels of visual complexity. This indicates that small saccade amplitude is the main cause of impaired visual scanning in Parkinson's disease patients.