Written language is an evolutionarily recent human invention whose neural substrates cannot, therefore, be determined by the genetic code. How, then, does the brain incorporate skills of this type? One possibility is that written language is parasitic on evolutionarily older skills such as spoken language, while another is that dedicated substrates develop with expertise. If written language is parasitic on spoken language, then acquired deficits of spoken and written language should necessarily co-occur. Alternatively, if there are at least some dedicated written language substrates, these deficits may (doubly) dissociate. We report on five individuals with aphasia, documenting a double dissociation in which the production of affixes (e.g., jumping) is disrupted in writing but not speaking, or vice versa. The findings reveal considerable independence of the written and spoken language systems in terms of morpho-orthographic operations. Understanding these properties of the adult orthographic system has implications for the education and rehabilitation of written language.
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