Optic Flow and Vestibular Sensory Integration in Self-motion Perception in Parkinson's Disease

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.