What is the cortical basis of agnosia and abnormal foveal-crowding phenomena?
In this case-control study, 5 patients with posterior cortical atrophy with simultanagnosia and foveal crowding underwent a behavioral assessment and population receptive field (pRF) size evaluation along the visual hierarchy via functional magnetic resonance imaging. Findings demonstrate a striking perceptual abnormal fovea-to-periphery gradient that is associated with an abnormal pRF size mapping along the eccentricity axis.
Smaller peripheral and larger foveal pRFs could explain simultanagnosia and foveal crowding, respectively; these findings explain high-order visuocognitive functions with basic cortical characteristics and may suggest new approaches to rehabilitation.
The neuronal mechanism of visual agnosia and foveal crowding that underlies the behavioral symptoms of several classic neurodegenerative diseases, including impaired holistic perception, navigation, and reading, is still unclear. A better understanding of this mechanism is expected to lead to better treatment and rehabilitation.
To use state-of-the-art neuroimaging protocols to assess a hypothesis that abnormal population receptive fields (pRF) in the visual cortex underlie high-order visual impairments.
Design, Setting, and Participants
Between April 26 and November 21, 2016, patients and controls were recruited from the Hadassah-Hebrew University medical center in a cross-sectional manner. Six patients with posterior cortical atrophy (PCA) were approached and 1 was excluded because of an inability to perform the task. Participants underwent functional magnetic resonance imaging–based cortical visual field mapping and pRF evaluation and performed a masked repetition priming task to evaluate visuospatial perception along the eccentricity axis. The association between pRF sizes and behavioral impairments was assessed to evaluate the role of abnormal pRF sizes in impaired visual perception. Posterior cortical atrophy is a visual variant of Alzheimer disease that is characterized by progressive visual agnosia despite almost 20/20 visual acuity. Patients with PCA are rare but invaluable for studying visual processing abnormalities following neurodegeneration, as atrophy begins in visual cortices but initially spares other brain regions involved in memory and verbal communication.
Participants underwent a magnetic resonance imaging scan.
Main Outcomes and Measures
Population receptive field sizes and their association with visual processing along the fovea-to-periphery gradient.
Five patients with PCA (4 men [80%]; mean [SEM] age, 62.9 [3.5] years) were compared with 8 age-matched controls (1 man [25%]; mean [SEM] age, 63.7 [3.7] years) and demonstrated an atypical pRF mapping that varied along the eccentricity axis, which presented as abnormally small peripheral and large foveal pRFs sizes. Abnormality was seen in V1 (peripheral, 4.4° and 5.5°; foveal, 5.5° and 4.5° in patients and controls, respectively; P < .05) as well as in higher visual regions, but not in intermediate ones. Behaviorally, an atypical fovea-to-periphery gradient in visual processing was found that correlated with their pRF properties (r = 0.8; P < .01 for the correlation between pRF and behavioral fovea-to-periphery slopes).
Conclusions and Relevance
High-order visuocognitive functions may depend on abnormalities in basic cortical characteristics. These results may fundamentally change approaches to rehabilitation in such conditions, emphasizing the potential of low-level visual interventions.
de Best PB, Raz N, Guy N, et al. Role of Population Receptive Field Size in Complex Visual Dysfunctions: A Posterior Cortical Atrophy Model. JAMA Neurol. Published online August 12, 2019. doi:10.1001/jamaneurol.2019.2447
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