Separate mechanisms for number reading and word reading: Evidence from selective impairments.

Do number reading and word reading use the same cognitive mechanisms? We examined this question through the looking glass of dissociations between impairments in number and word reading. We report two women with selective deficits in number reading, who read words normally. An examination of their impairment pattern indicated that the specific locus of their number reading deficits is in processes that handle the number's structure: both were impaired in parsing a digit string into triplets, and one of them was also impaired in generating the number's verbal structure. In contrast to their structural deficits in number reading, their word reading was completely intact, including the structural processes in word reading (morphological analysis and assembly). We present this dissociation in the framework of a broader effort to examine dissociations between specific components in number and word reading. We went beyond general word-number dissociations: we used detailed cognitive models for word reading and for number reading, and analyzed them in order to identify which components of the number reading process are homologous to which components of words reading. We then show that even these homologous processes are dissociable: an examination of previously-reported dissociations, completed by the case studies presented here, indicated that each of the specific homologous sub-processes of word/number reading can be selectively impaired. We conclude that although the word and number reading pathways show much similarity, they are almost entirely separate.