Content area
Abstract
Our perceived orientation in relation to the world and its contents is derived from multisensory information and prior knowledge of our actions. The senses each provide an estimate of orientation which may or may not agree with that provided by the other senses. Often these sources of information are coherent and unambiguous perception of orientation is attained. Occasionally it is discordant and orientation is ambiguous. How the brain can resolve some of these ambiguities was investigated. In a series of experiments participants were exposed to various combinations of physical and visual tilts in roll with respect to the direction of gravity's force. Several probe measures designed to directly and indirectly assess perceived self orientation were employed. The perceived orientation of gravity, of the self and the perceptual upright (PU: the orientation which objects appear most upright) provided distinct referents for making spatial perception judgments. Psychophysical techniques were used to measure perceptual judgments from different probe measures. We manipulated the position of the body relative to gravity as well as visual orientation cues to investigate (1) the connection between probes and their ability to investigate perceived allocentric and egocentric space; (2) measuring relative cue weight using a weighted vector sum model; (3) testing assumptions of this model; (4) measure and model sensory dependence in intact and (5) impaired populations. Results revealed a common internal representation of gravity using saccadic and tactile measures. The internal representation of gravity, and of, the body were both found to be significantly affected by visual tilt. The relative contribution of these sensory and internal cues was well defined using a weighted vector sum model, despite the model's assumption of mirror symmetry being found violated. Independent assertions in the literature of females and patients with Parkinson's disease (PD) being 'visually dependent' when estimating the subjective visual vertical (SVV) were reassessed using the PU and SVV measures. Females were more influenced by vision than males for the SVV but not the PU. PD patients when medicated were more influenced by vision for the SVV but less influenced by the internal representation of their body than age matched controls. Neurological correlates which support this observation lead to my suggestion that this 'impaired' internal representation of the body likely reflects physiological impairments in PD. In addition I conclude that because these deficits are only found when patients are medicated it suggests that areas outside the basal ganglia are associated with these effects. These findings contribute to the fields of multisensory integration, perception and neurology by clarifying how sensory information is combined differently to define distinct and simultaneous representations of space tailored for different perceptual purposes. Finally, these results provide clinicians with a new theoretical framework with which to assess the progression of Parkinson's disease.
