Phonological processes and similarity constraints in consonant and vowel substitution errors: Insights from individuals with conduction aphasia.

Individuals with conduction aphasia (CA), associated with post-lexical impairments, frequently produce phonological substitution errors. This study examined whether such substitutions reflect phonological simplification principles (e.g., preference for unmarked phonemes, sonority-based tendencies, assimilation) or are mainly constrained by phonological similarity. Under a similarity account, substitutions should preserve key phonological features of the target, even when the result is more complex. We analysed 339 consonant and 98 vowel substitution errors produced by 14 individuals with CA across naming, reading, and repetition tasks, and compared them with errors from one individual with apraxia of speech (AOS) without aphasia. CA errors were strongly guided by phonological similarity, with most substitutions differing from the target by only one or two features. Although these errors did not violate phonological principles, they often maintained salient target features even when producing more complex sounds. In contrast, the AOS participant showed a tendency toward simpler, unmarked phonemes. These findings indicate distinct underlying mechanisms in CA and AOS and highlight feature-level preservation in CA phonological encoding.