Multiple models to explain visual agnosia, an impaired visual recognition despite near-normal acuity, were previously suggested. However, contradicting behavioural patterns within this entity hampers a unified explanation. The current study uses behavioural experiments and functional MRI-based population receptive field (pRF) models to study this complexity. Two cardinal, presumably-contradicting symptoms manifest in PCA patients; simultanagnosia, the inability to perceive several objects at once; and foveal crowding, in which recognition is impaired by the interference of nearby stimuli. While the first is attributed to restricted spatial perception, the latter may suggest the opposite.

pRF properties in 5 PCA patients and 8 age-matched controls were reconstructed, using a 3T Skyra MRI. Major outcomes included center and surround pRF size, representing spatial extent of the pRF. These were assessed along the eccentricity axis in the primary visual cortex, as well as in higher visual areas. To directly assess the association between patients’ pRF center-periphery gradient and visual processing, we performed a masked repetition priming experiment. In this task, the reaction time for identifying a foveal target was assessed as a function of primes' eccentricity.

Findings demonstrate a striking perceptual abnormal foveal-to-periphery gradient in visual perception which strongly correlated to an abnormal gradient in cortical pRF size mapping along the eccentricity axis. Smaller peripheral and larger foveal pRFs were shown in PCA patients, and may explain their manifested simultanagnosia and foveal crowding respectively. Cortical changes were already evident in the primary visual cortex and are suggested to arise from inactivation of feedback connections from atrophied higher order visual regions.