Parkinson's disease (PD) is classically characterized by a decline in motor function, marked by the hallmark symptoms of akinesia, bradykinesia, rigidity and tremor as well as impaired posture and balance. However, non-motor symptoms are also recently becoming recognized as a major part of the disease. Non-motor symptoms may include sleep disorders, mood disturbances, hallucinations, cognitive impairment, and various sensory and perceptual deficits. In contrast to the motor symptoms, non-motor symptoms are less observable by nature, and can therefore go unnoticed if not tested directly.

Already, early studies revealed broad visual dysfunction in PD. This includes delays in visual evoked responses and abnormalities in contrast, spatiotemporal and color sensitivity. PD patients also have altered perception of visual orientation as well as complex visual impairments. Yet, despite their visual deficits, PD patients seem to be functionally more dependent on vision, versus controls. This seems to contradict established principles of optimal sensory integration, according to which, impaired cues should be less relied upon. However, this can only be gauged within a principled framework that measures, quantifies and compares the precision of relevant perceptual cues. Namely, it is the relative reliabilities of sensory cues that should, according to schemes of optimal (Bayesian) integration, set the extent to which the cues are relied upon (related to further below).

Research has demonstrated impairments in sensory systems, other than vision, such as proprioceptive and vestibular function. Interestingly, many sensory deficits in PD may be closely associated with "classic" motor symptoms. For example: : i) dysfunctional vestibular signals may lead to impaired balance control in PD, (ii) proprioceptive deficits impair voluntary and reflexive motor commands, (iii) impairments in spatial perception may contribute to freezing of gait (FOG), and (iv) PD patients overestimate the volume of their own speech, likely reflecting perceptual deficits either by impaired sensorimotor integration or by impaired self-awareness of motor deficits. Also, higher perceptual functions, such as perception of emotion from facial expression, is impaired in parkinsonian patients. Perception of self-motion arises primarily from inertial motion (vestibular) and optic flow (visual) cues. When presented with radial expanding optic flow patterns, PD patients demonstrate altered navigational veering and altered perception of the egocentric midline as well as reduced activation in visual brain areas versus controls. However, thresholds of self-motion perception from optic flow have not yet been investigated, and will thus be measured in this study.

Vestibular abnormalities might also affect perception of self-motion in PD. Recently, Bertolini et al. (2015) found impaired tilt perception in PD, but here too, vestibular thresholds of linear self-motion perception have not been researched directly. Hence, the first aim of this study is to determine the thresholds of unisensory (visual and vestibular) perception of self-motion in PD, using a rigorous and well used paradigm of heading discrimination.

However, in addition to deficits in visual and vestibular perception of self-motion, PD patients may suffer from sub-optimal integration of these cues. Hence, the second major aim is to specifically investigate the integration of visual and vestibular cues for self-motion perception. This will be done in the Bayesian framework of multisensory integration.

The integration of information from different modalities ("multisensory integration") is vital for intact perception of the world. Theoretical studies, based on Bayesian statistics, have provided a framework to study multisensory-integration with predictions for an 'optimal' strategy. Assuming Gaussian distribution and a flat prior, optimal integration of multiple cues reduces to straight forward linear equation, according to which the multisensory percept is a weighted combination of the underlying cues. Many human and animal studies have indeed demonstrated near optimal cue-integration. Yet, while multisensory integration is an active topic of research in normal brain function, with well-established tools, it has not been studied in PD.The investigators hypothesize, based on the apparent over-dependence in PD on visual cues. PD patients might demonstrate non-optimal multisensory integration (namely overweighting of visual cues). This can have profound effects on basic function.

Adding sensory noise to a stimulus reduces its reliability. In the optic flow stimuli of self-motion through a 3D cloud of dots, reliability can be controlled by manipulating the coherence of the moving dots. For 100% coherence (no added noise), all the dots move coherently according to the direction of simulated self-motion. When noise is added, e.g. to 75% coherence, 75% of the dots move coherently according to the direction of self-motion, whereas the remaining 25% move in a random direction As coherence is decreased the visual stimulus reliability reduces. Recently the investigators showed that different clinical groups (e.g. autism) can respond differently to the addition of visual noise. Hence, as part of these experiments, the investigators will also compare visual perception in the absence and presence of added visual noise. The pathophysiology of PD is often understood to reflect increased neuronal noise (e.g. beta oscillations) hence the investigators hypothesize that external sensory noise might have a stronger effect on PD patients vs. controls (perhaps by the stimulus aggravating, rather than reducing, neuronal fluctuations in PD).

Hence, in this study the investigators have 3 main aims: i) to observe the basic (unisensory) visual and vestibular perception of self-motion in PD, ii) to observe the multisensory integration in PD patients, within the framework of Bayesian inference, and iii) to observe the effects of reducing visual reliability (the addition of visual stimulus noise) on performance in PD. All three aims will be addressed with the same experiment. All participants will come for two visits. PD patients will perform the same procedure once "on" medication and once "off" medication (the order of which will be counterbalanced between patients; determined randomly in advance). For the "off" medication condition, patients will stop taking their PD medication 12 hours before the experiments (until after the experiment). The control group will also perform the experiment twice in order to control the possible artifact of learning effects.