Posterior Cortical Atrophy: The role of simultanagnosia in deficits of face perception
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When viewing a face, healthy individuals tend to fixate on upper regions, particularly the eyes, which provide important configural information about the spatial layout of the face. In contrast, individuals with face blindness (prosopagnosia) rely more on local features – particularly the mouth. Presented here is an examination of face perception deficits in individuals with Posterior Cortical Atrophy (PCA). PCA is a rare progressive neurodegenerative disorder that is characterized by atrophy in occipito-parietal and occipito-temporal areas. PCA primarily affects higher visual processing, while memory, reasoning, and insight remain relatively intact. Common among individuals with PCA is simultanagnosia, an inability to perceive more than one object or detail simultaneously. One might consider simultanagnosia the most extreme form of a feature-based approach. In a series of investigations, individuals with PCA and their healthy control participants completed a same/different discrimination task in which images of faces were presented as cue-target pairs. Eye-tracking equipment (Experiment 1) and the newly developed Viewing window paradigm (Experiment 2) were used to investigate scanning patterns when faces were presented in full view, and through a restricted viewing aperture, respectively. In contrast to previous prosopagnosia research, individuals with PCA each produced unique scan paths that focused on one aspect of the face. Individuals with PCA tended to focus on areas of high-contrast but many of these areas were not diagnostically useful, suggesting that they were having difficulty processing the face even at a featural level. These results suggest a role of simultanagnosia in the scan patterns of PCA patients that is not reflective of ‘typical’ prosopagnosia, and instead points to simultanagnosia, sometimes matched with basic perceptual impairments, as a significant contributor to the face perception deficits seen in PCA.