Perceiving collision impacts in Alzheimer’s disease: The effect of retinal eccentricity on optic flow deficits

eKim, Nam-Gyoon · 2015 · DOAJ

DOI: 10.3389/fnagi.2015.00218

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Summary

This study investigates whether the optic flow deficits observed in Alzheimer’s disease (AD) extend beyond radial flow patterns to those specifying collision impacts, specifically examining the influence of retinal eccentricity. Previous research established that AD patients struggle to process radial optic flow, which aids in determining heading direction. However, it remained unclear if this deficit was specific to radial flow or generalized to other optical patterns, such as those governed by tau-dot ($\dot{\tau}$), a variable used to estimate time-to-contact and avoid collisions. Additionally, the role of retinal location—central versus peripheral vision—in these deficits was not fully understood, particularly given the "magnocellular deficit hypothesis," which suggests AD-related degeneration of parasol ganglion cells primarily affects the peripheral retina. To address these questions, the researcher employed a psychophysical task involving 23 AD patients and 23 age- and education-matched healthy elderly controls. Participants viewed simulated displays of observer movement toward obstacles, where optical expansion was modulated by varying $\dot{\tau}$. The visual field was masked either centrally (isolating peripheral vision) or peripherally (isolating central vision) using masks ranging from 10° to 30° in diameter. Participants were asked to judge whether their approach would result in a "collision" or "no collision." This design allowed for the assessment of sensitivity to $\dot{\tau}$ under different retinal conditions, testing whether AD patients’ impairments were confined to specific visual field regions or flow types. The results demonstrated that AD patients’ sensitivity to $\dot{\tau}$ was severely compromised in both central and peripheral vision conditions compared to controls. This finding indicates that the optic flow deficit in AD is not limited to radial flow patterns but also encompasses the optical patterns engendered by $\dot{\tau}$. Furthermore, the impairment persisted regardless of whether the central or peripheral retina was stimulated, suggesting that the deficit is not solely attributable to peripheral magnocellular pathway dysfunction. The data reveal a generalized inability in AD patients to extract critical collision-avoidance information from optic flow, irrespective of retinal eccentricity. These findings have significant implications for understanding visuospatial disorientation in AD. The inability to process both heading information from radial flow and collision-impact information from $\dot{\tau}$ exacerbates difficulties in navigation and spatial orientation. The study challenges the notion that AD-related visual deficits are strictly peripheral or limited to specific flow types, pointing instead to a broader impairment in processing optic flow cues essential for safe locomotion. This generalized deficit likely stems from widespread neural degeneration affecting both central and peripheral visual processing pathways, contributing to the severe navigational challenges faced by individuals with Alzheimer’s disease.

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• perception action locomotion