Convergence Along the Visual Hierarchy Is Altered in Posterior Cortical Atrophy

Abstract

Purpose: Posterior cortical atrophy (PCA) is a rare neurodegenerative syndrome manifesting with visuospatial processing impairment. We recently suggested that abnormal population receptive field properties are associated with the symptoms of PCA patients. Specifically, simultanagnosia, the inability to perceive multiple items simultaneously, can be explained by smaller peripheral population receptive fields, and foveal crowding, in which nearby distractors interfere with object perception, may result from larger foveal population receptive fields. These effects occurred predominantly in V1, even though atrophy mainly involves high-order areas. In this study, we used connective field modeling to better understand these inter-area interactions.

Methods: We used functional magnetic resonance imaging to scan six PCA patients and eight controls while they viewed drifting bar stimuli. Resting-state data were also collected. Connective field modeling was applied for both conditions: once when the source was V1 and the targets were extrastriate areas and once for the opposite direction. The difference between the two was defined as convergence magnitude.

Results: With stimulus, the convergence magnitude of the controls increased along the visual pathway, suggesting that spatial integration from V1 becomes larger up the visual hierarchy. No such slope was found in the PCA patients. The difference between the groups originated mainly from the dorsal pathway. Without stimulus, the convergence magnitude was negative, slightly more so for the PCA patients, with no slope, suggesting constant divergence along the visual hierarchy.

Conclusions: Atrophy in one part of the visual system can affect other areas within the network through complex intervisual area interactions, resulting in modulation of population receptive field properties and an ensemble of visuocognitive function impairments.

Figure 1.

Convergence magnitude. V1 to region x sampling extent is the circular area in V1 that was sampled by a single voxel in (extrastriate visual) region x, and vice versa for region x to V1 sampling extent. Convergence magnitude is defined as the difference between the two.

Figure 2.

Convergence magnitude in the visual system. Stimulus-based and resting-state-based convergence magnitudes are shown for PCA patients (empty circles) and controls (filled). The different visual areas were positioned along the x-axis using their hierarchical levels.^, In the main graphs, a significant difference between groups in a specific region is denoted by an asterisk (P < 0.05, corrected for multiple comparisons). Linear trend lines were fitted to the data for each individual and were used for statistical comparisons between groups (inlay graphs). Asterisks inside the inlay graphs denote significant differences in slopes between groups (P < 0.05, corrected for multiple comparisons). For visualization purposes, linear trend lines were also calculated using the individual values from each group and are presented in the main graphs. Error bars represent standard error of the median.