Understanding quantity in semantic dementia

Patients with semantic dementia (SD) show relative preservation of number skills, contrasting with their severe multimodal semantic impairment. Underpinning this preservation, it is argued, is spared understanding of numerical quantity, a competence likened to a spatial map and subserved by the parietal lobes. The study investigated quantity knowledge in 14 SD patients to determine whether it is consistently preserved irrespective of disease severity or whether there are constraints on this preserved knowledge domain. Performance was well preserved on Piagetian conservation tasks, estimating object numerosity, and understanding of basic numerical magnitude. However, patients showed impairment on real-world estimation tasks, increasing with semantic severity. More surprisingly, on an analogue scale task, they produced implausible responses, suggesting degraded knowledge of precise numerical relationships. The findings challenge the view that knowledge of quantity is totally preserved in SD and suggest that the temporal lobes have a contributory role in the conceptual understanding of quantity.