O2‐07‐01: REDUCED MODULATION OF SCANPATHS IN RESPONSE TO TASK DEMANDS IN POSTERIOR CORTICAL ATROPHY
Timothy J. Shakespeare, Yoni Pertzov, Keir X. X. Yong, Jennifer M. Nicholas, Sebastian James Crutch · Alzheimer s & Dementia · 2014
Visual dysfunction in Posterior Cortical Atrophy (PCA; most often caused by Alzheimer's disease) has been studied using a variety of artificial neuropsychological and experimental tests. However we know very little about how PCA patients perceive the real world. Here we showed patients more ecologically valid scenes under different task conditions, investigating how their impairments are reflected in eye movement patterns. Eye movements when viewing scenes reflect high-level (top-down; e.g. scene identity) and low-level (bottom-up; e.g. colour, contrast) influences. In contrast to the majority of studies which have focused upon the influence of low-level image features, the current study tested the interaction between high- and low-level influences upon scene perception. Participants were 7 PCA patients, eight typical Alzheimer's disease (AD) patients and 19 healthy age-matched controls. Each participant viewed 10 scenes under four task conditions (encoding recognition search description) whilst eye movements were recorded. We examined how the response to different task instructions differed between groups. We also compared participants' scan-paths to computer generated low-level salience maps. The results reveal significant differences between the three groups in the impact of test instructions upon scanpaths. Across tasks without a search component, PCA patients were significantly less consistent than typical AD patients and controls in where they were looking. By contrast, when comparing search and non-search tasks, it was controls who exhibited lowest between-task similarity, suggesting they were much better able than PCA or typical AD patients to respond appropriately to high-level needs by looking at task-relevant regions of a scene. PCA patients had a significant tendency to fixate upon more low-level salient parts of the scenes than controls irrespective of the viewing task. This study provides a detailed characterisation of scene perception abilities in PCA. The results improve our understanding of patients' impairments, suggesting that a tendency to fixate regions with low-level salience and an inability to modify scanpaths to match task demands are important contributory factors. This improved understanding has implications for the design of practical therapeutic interventions to increase independence in everyday tasks. The study also provides insight into the mechanisms by which high-level cognitive schemes interact with low-level perception. Scanpaths and salience maps. Example scanpaths from two healthy controls (A and B) and two PCA patients (C and D) demonstrating greater modulation of the scanpath for the search task (green) compared with a non-search task (encode; red) in controls than patients. The between-task similarity is abnormally high for patients (C=83%; D=84%) relative to controls who view different parts of the scene in a more task-appropriate manner (A=-17.7%; B=-60.5%). The highlighted area shows the search target are (all vegetation). Panels E and F show example computer generated low-level salience maps, from which normalised salience at fixation is calculated for scanpaths.