Temporal integration of 3D coherent motion cues defining visual objects of unknown orientation is impaired in amnestic mild cognitive impairment and Alzheimer's disease

The nature of visual impairments in Alzheimer's disease (AD) and their relation with other cognitive deficits remains highly debated. We asked whether independent visual deficits are present in AD and amnestic forms of mild cognitive impairment (MCI) in the absence of other comorbidities by performing a hierarchical analysis of low-level and high-level visual function in MCI and AD. Since parietal structures are a frequent pathophysiological target in AD and subserve 3D vision driven by motion cues, we hypothesized that the parietal visual dorsal stream function is predominantly affected in these conditions. We used a novel 3D task combining three critical variables to challenge parietal function: 3D motion coherence of objects of unknown orientation, with constrained temporal integration of these cues. Groups of amnestic MCI (n = 20), AD (n = 19), and matched controls (n = 20) were studied. Low-level visual function was assessed using psychophysical contrast sensitivity tests probing the magnocellular, parvocellular, and koniocellular pathways. We probed visual ventral stream function using the Benton Face Recognition task. We have found hierarchical visual impairment in AD, independently of neuropsychological deficits, in particular in the novel parietal 3D task, which was selectively affected in MCI. Integration of local motion cues into 3D objects was specifically and most strongly impaired in AD and MCI, especially when 3D motion was unpredictable, with variable orientation and short-lived in space and time. In sum, specific early dorsal stream visual impairment occurs independently of ventral stream, low-level visual and neuropsychological deficits, in amnestic types of MCI and AD.