What semantic errors restricted to either speaking or writing in aphasia tell us about lemmas

Theories about word production differ in whether they assume that lexical selection involves modality-specific representations or modality-neutral ones, called lemmas. The observation that semantic errors in aphasia may be restricted to either speaking or writing has long been taken to challenge lemma models (Hillis et al., 1999; Kemmerer, 2015, 2019). Whereas patients RGB, HW, and RR make semantic errors in speaking but not in writing, patient RCM exhibits the opposite dissociation. Here, using WEAVER++ simulations and a simple mathematical analysis, a model with lemmas is shown to account for the double dissociation observed in patients. In particular, the model captures the patterns of performance of the patients on vocal picture naming, written picture naming, auditory word comprehension, and writing to dictation, explaining 98% of the variance. The challenge is now for modality-specific models to account for the findings that the lemma model was designed to explain in the first place.