Origins of nonword phonological errors in aphasic picture naming.

A recent theory of lexical access in picture naming maintains that all nonword errors are generated during the retrieval of phonemic segments from the lexicon (Dell, Schwartz, Martin, Saffran, & Gagnon, 1997b). This theory is challenged by "dual origin" theories that postulate a second, post-lexical mechanism, whose disruption gives rise to "phonemic paraphasias" bearing close resemblance to the target. We tested the dual origin theory in a corpus of 457 nonword errors drawn from 18 subjects with fluent aphasia. The corpus was divided into two parts, based on degree of phonological overlap between error and target, and these parts were separately examined for proposed diagnostic characteristics of the postlexical error mechanism: serial order effects across the word, sensitivity to target length, and insensitivity to target frequency. Results did not support the dual origin theory but were consistent with a single, lexical origin account in which segment retrieval operates from left to right, rather than in parallel. Findings from this study also shed new light on how individual differences in the severity of the retrieval deficit modulate the expression of phonological errors in relation to target characteristics.