Numerosity Impairment in Corticobasal Syndrome

Objective: We assessed the representation of numerosity in corticobasal syndrome (CBS), a neurodegenerative condition affecting the parietal lobe. Method: Patients judged whether a target numerosity (e.g., " 3" ) falls between two bounding numerosities (e.g., " 1" and " 5" ). We manipulated the format for representing numerosity (Arabic numerals or dot arrays), the size of the gap between the two bounding numerosities, the absolute magnitude of the numerosities, and the order for presenting the bounding numerosities. In a subset of patients with available imaging, we related performance to cortical atrophy using voxel-based morphometry. Results: CBS patients were significantly impaired overall (65.7% ± 16.2 correct) compared to healthy seniors (96.6% ± 2.4 correct), and required three times longer than controls to judge correct stimuli. This deficit was equally evident for Arabic numeral and dot array formats. Controls were significantly slower with smaller gaps than larger gaps, consistent with the greater challenge distinguishing between numerosities that are more similar to each other than very different numerosities. However, CBS patients were equally slow and inaccurate for all gap sizes. Controls also were significantly slower with larger numerosities than smaller numerosities, but CBS patients were equally slow and inaccurate with all numerosity magnitudes. Voxel-based morphometry revealed significant cortical atrophy in parietal and frontal regions in CBS compared to controls, including the intraparietal sulcus. Conclusions: These observations are consistent with the claim that the representation of numerosity is degraded in CBS.