Dissociation of numbers and objects in corticobasal degeneration and semantic dementia

Semantic memory is thought to consist of category-specific representations of knowledge that may be selectively compromised in patients with neurodegenerative diseases, but this has been difficult to demonstrate reliably across object categories. The authors evaluated performance on several simple measures requiring number representations (including addition and magnitude judgments of single digits), and on a task that requires object representations (an object naming task) in patients with corticobasal degeneration (CBD; n = 13) and semantic dementia (SD; n = 15). They also examined regional cortical atrophy using voxel-based morphometric analyses of high resolution structural MRI in subgroups of five CBD patients and three SD patients. CBD patients were consistently more impaired on simple addition and magnitude judgment tasks requiring number representations compared to object representations. Impaired performance with numbers in CBD was associated with cortical atrophy in right parietal cortex. By comparison, SD patients demonstrated a greater impairment on a naming task requiring object representations relative to their performance on measures involving number representations. This was associated with left anterior temporal cortical atrophy. The cognitive and neuroanatomic dissociations between CBD and SD are consistent with the hypothesis that number and object representations constitute distinct domains in semantic memory, and these domains appear to be associated with distinct neural substrates.