Impairments of speech production and speech perception in aphasia

The basis of speech production and speech perception deficits in aphasia relates to implementation and access rather than to the underlying representation or knowledge base of the sound structure of language. Speech production deficits occur on the phonological level in which the incorrect phonological form of the word is selected but is implemented correctly, and the phonetic level in which the correct sound segments are selected but articulatory implementation is impaired. Phonological deficits emerge regardless of lesion site, whereas phonetic deficits have a specific localized neuroanatomical substrate. Phonetic deficits are not linguistic but affect particular articulatory movements. Speech perception impairments emerge in nearly all aphasic patients, suggesting that the neural basis for speech perception is broadly distributed in the language hemisphere. The impairment reflects the misperception of phonetic features rather than a deficit in the auditory processing of speech and emerges particularly as the sound properties of speech contact the lexicon.