Visual mechanisms of spatial disorientation in Alzheimer's disease

Impaired optic flow perception may contribute to the visuospatial disorientation of Alzheimer's disease (AD). We find that 36% of AD patients have elevated perceptual thresholds for left/right outward radial optic flow discrimination. This impairment is related to independent visual motion processing deficits affecting the perception of left/right motion-defined boundaries and in/out radial motion. Elevated optic flow thresholds in AD are correlated with greater difficulty in the Road Map test of visuospatial function (r = -0.5) and in on-the-road driving tests (r = -0.83). When local motion cues are removed from optic flow, subjects must rely on the global pattern of motion. This reveals global pattern perceptual deficits that affect most AD patients (85%) and some normal elderly subjects (21%). This deficit might combine with impaired local motion processing to undermine the alternative perceptual strategies for visuospatial orientation. The greater prevalence of global pattern deficits suggests that it might precede local motion processing impairments, possibly relating to the sequence of early hippocampal and later posterior cortical damage that is typical of AD.