Vestibular, cervical and visual remembered saccades in Parkinson's disease

In order to assess both vestibulo-cervical perception of head rotation and saccadic function in Parkinson's disease, 14 patients with idiopathic Parkinson's disease were subjected to discrete sigmoid-shaped rotational displacements whilst fixating a target aligned with primary gaze in an otherwise dark room. The rotational stimuli were applied to (i) the whole body (vestibular stimulus); (ii) the trunk whilst the head remained stationary in space (cervical stimulus); (iii) to the head alone whilst the trunk remained stationary (combined vestibular and cervical stimulus). The fixation target was then extinguished and the subjects had to estimate the angle travelled by the head or trunk with an ocular-pointing task using information from the preceding rotational stimulus (vestibular and cervical 'remembered' saccades). It was found that, although these saccades in Parkinson's disease patients were multiple-step and hypometric, the final position of the eyes matched the rotational stimulus as accurately as in normal subjects. A complementary experiment in six patients showed that visual remembered saccads were hypometric, but significantly less so than vestibular remembered saccades. It is concluded that (i) vestibular and cervical perception of head/neck rotation is normal in Parkinson's disease; (ii) abnormalities of 'remembered' saccades, previously reported in Parkinson's disease, are not confined to the visual modality but involve other sensory modalities as well; (iii) across different modalities of memory-guided saccades, visual input improves saccadic performance. This result demonstrates that the known increased visual dependence found in Parkinson's disease extends to memory-driven tasks.