The double dissociation of short-term memory for lists and sentences. Evidence from aphasia

We document the sentence and list repetition skills of 3 aphasic patients. Cases 1 and 2 were classified as span-impaired conduction aphasics and Case 3 was classified as a span-preserved transcortical sensory aphasic. We found that repetition by the span-impaired cases was facilitated by increasing the 'meaningfulness' of lists, whereas this had no effect on the performance of the span-preserved case. The patients' ability to repeat sentences was contrasted with their ability to repeat three words contained in the sentences, and a double dissociation was obtained: the span-impaired cases were better at repeating sentences than in repeating three word lists whereas the span-preserved case showed the opposite pattern of difficulty. The repetition of sentences containing a known and unknown vocabulary was tested in the span-preserved case. His repetition of sentences containing words which he still knew was significantly better than his ability to repeat sentences containing a vocabulary of words which he had 'forgotten' as a consequence of his aphasia. The patients' ability to repeat complete, incomplete, and nonsense sentences was contrasted: overall the span-impaired patients were somewhat better at repeating complete sentences than incomplete sentences. Finally we examined the patients' ability both to comprehend and repeat word strings in an abbreviated 'naming from description' task: the span-impaired patients were able to comprehend three word strings, but unable to repeat them, the span-preserved case was able to repeat but not comprehend. On the basis of these double dissociations we conclude that the evidence indicates two dissociable short-term memory systems, one a relatively passive phonological store subserving list repetition, the other a dynamic, anticipatory, and integrative memory system which underpins sentence repetition.