Meeting an “Impossible Challenge” in Semantic Dementia: Outstanding Performance in Numerical Sudoku and Quantitative Number Knowledge

Objective: This study describes a follow-up investigation of numerical abilities and visuospatial memory in a patient suffering from semantic dementia whose progressive decline of semantic memory variably affected different types of knowledge. Crucially, we investigated in detail her outstanding performance with Sudoku that has been only anecdotally reported in the previous literature. Method: We tested spatial cognition and memory, body representation, number processing, calculation, and Sudoku tasks, and we compared the patient's performance with that of matched controls. Results: In agreement with the neuroanatomical data, showing substantial sparing of the parietal lobes in the face of severe atrophy of the temporal (and frontal) regions, we report full preservation of skills known to be supported by intact parietal-basal ganglia networks, and impaired knowledge related to long-term stored declarative information mediated by temporal regions. Performance in tasks sensitive to parietal dysfunction (such as right-left orientation, finger gnosis, writing, and visuospatial memory) was normal; within the numerical domain, preserved quantity-based number knowledge dissociated from increasing difficulties with nonquantitative number knowledge (such as knowledge of encyclopedic and personal number facts) and arithmetic facts knowledge. Conclusions: This case confirms the relation between numbers and space, and, although indirectly, their anatomical correlates, underlining which abilities are preserved in the case of severe semantic loss. In addition, although Sudoku is not inherently numerical, the patient was able to solve even the most difficult pattern, provided that it required digits and not letters, showing that digits have, in any case, a specific status.