Why semantic dementia drives you to the dogs (but not to the horses): A theoretical account.

This paper describes a patient (IH) with semantic dementia and severe impairment in all semantic categories except for numerical knowledge, which was preserved. IH showed a severe deficit in reading and writing non-number words (e.g., candle, juice) and nonwords, and preservation of reading and writing number words (e.g., one, forty) and numerals (e.g., 1, 40). IH's pattern of performance can be explained by the combination of a selective sparing of one semantic category--i.e., numbers--with a total deficit of nonsemantic processes for mapping letters and sounds. As number was the only spared semantic category in the presence of these other nonsemantic deficits, it follows that the semantic route is sufficient for accurate reading and spelling. Our data clarify the nature of reading and writing processes and support the functional and neuroanatomical independence of the number domain.