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Abstract

Perception of the relative orientation of the self, and objects in the environment, requires integration of visual and vestibular sensory information, and an internal representation of the body's orientation. Here I present and discuss findings on the relative contribution of these cues for orientation perception in Parkinson’s disease (PD; Barnett-Cowan et al., Neuroscience, In Press). The orientation of a line relative to vertical (i.e., subjective visual vertical) and the orientation in which a letter character is perceived as the ‘right way up’ (i.e., the perceptual upright) were measured in PD patients (on- and off-medication) and age-matched controls. Visual, vestibular and body cues were manipulated using a polarized visual room presented in various orientations while observers were upright or lying right-side-down. Patients were more influenced by vision than controls but only when setting a line to vertical. Patients relied less on the internal representation of the body when identifying characters as indicated by a greater influence of vestibular information. Consistent with reports of impaired proprioception among medicated PD patients, our effects were significant only when patients were medicated. I will discuss these results within a framework suggesting that this ‘impaired’ internal representation of the body may reflect physiological impairments in PD.