Conduction aphasia

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Conduction aphasia, also called associative aphasia, is an acquired language disorder, most commonly caused by left-hemisphere cerebrovascular injury. It is characterized by fluent, grammatically correct speech with frequent phonemic paraphasias and a disproportionately severe impairment of verbatim repetition (difficulty repeating words, nonwords, and sentences) despite relatively preserved auditory comprehension. Affected people are fully capable of understanding what they are hearing, but fail to encode phonological information for production. This deficit is load-sensitive as the person shows significant difficulty repeating phrases, particularly as the phrases increase in length and complexity and as they stumble over words they are attempting to pronounce. People have frequent errors during spontaneous speech, such as substituting or transposing sounds. They are also aware of their errors and will show significant difficulty correcting them. Conduction aphasia is associated with disruption of the dorsal auditory-motor/phonological system in the left temporo-parietal region (often implicating the Sylvian-parietal-temporal area, posterior superior temporal and supramarginal gyri, and in some cases underlying white matter such as the arcuate fasciculus). Modern lesion-mapping and functional imaging work emphasize cortical temporo-parietal contributions to the syndrome in addition to classical white-matter disconnection hypotheses. In 1970, Tim Shallice and Elizabeth Warrington were able to differentiate two variants of this constellation: the reproduction and the repetition type. These authors suggested an exclusive deficit of auditory-verbal short-term memory in repetition conduction aphasia whereas the other variant was assumed to reflect disrupted phonological encoding mechanism, affecting confrontation tasks such as repetition, reading and naming in a similar manner.