Origin of phoneme substitution and phoneme movement errors in aphasia

Despite a general consensus about the lexical-phonological origin of phonological errors, a debate persists concerning a single or multiple origins of such errors. In particular, a similar post-lexical origin has been attributed to milder phonological paraphasias, such as phoneme substitution errors, and to normal slips of the tongue. However, most slips of the tongue have a contextual origin, while most phonological paraphasias are not contextual errors. Here we explore the possibility that even at post-lexical encoding levels different errors are generated by distinct processes. We take advantage of the production of an unusual proportion of within-word phoneme movement errors in a patient with conduction aphasia (SJ) and tackle the question of their origin in comparison to phoneme substitution errors. Error properties relative to phoneme substitution and phoneme movement errors produced by SJ were analysed and compared with those of a second patient (GF) who produced a similar proportion of substitution errors but no movement errors. Very similar profiles between the two patients emerged on substitution errors, while phoneme movement errors displayed different properties. The observation that substitution and movement errors are not affected by the same factors favours a different origin of these errors. The sub-lexical frequency and similarity effects and the lexical bias in substitution errors indicate an interaction between multiple encoding levels during the production of these errors; by contrast, movement errors seem related to a different and independent process bearing no interaction with other levels of representation.