Number processing in posterior cortical atrophy--a neuropsycholgical case study

Posterior cortical atrophy (PCA) is an uncommon syndrome of dementia with early onset, characterised by disorders of higher visual function, variable symptoms of Balint's syndrome, visual agnosia, alexia, agraphia, finger agnosia, right-left disorientation and dyscalculia [Benson D. F., Davis R. J., & Snyder B. D. (1988). Posterior cortical atrophy. Archives of Neurology, 45, 789-793]. In a single case study specific numerical deficits were observed which may be predicted by parietal neurodegeneration (more pronounced on the right side; verified by SPECT). Besides impairments in all tasks involving visuo-spatial abilities (e.g., dot counting, analog number scale task), deficits appeared in tasks requiring access to an internal representation of numbers such as mental number bisection, approximation, estimation and semantic facts. In number comparison an increased distance effect was found. In simple arithmetic, a striking dissociation between operations was found-multiplication and addition facts being preserved at a superficial level, subtraction and division being severely impaired. The study confirms the close relation between spatial and numerical processing and highlights the modular organisation of the semantic system (number semantics impaired). Moreover, the study adds evidence about the clinical manifestation of the particular degenerative syndrome.