Recovery in deep dysphasia: evidence for a relation between auditory - verbal STM capacity and lexical errors in repetition.

This study investigates the changes in auditory-verbal short-term memory (AVSTM) and error patterns in repetition observed in a Wernicke's aphasic, NC, over a period of about 2 years following the onset of a left middle cerebral artery aneurysm. When first tested, NC demonstrated deep dysphasia, a disorder characterized by the production of semantic errors in repetition and a severe disability in repeating nonwords. At this stage, his AVSTM span, assessed in a pointing task, was less than one item. As NC recovered somewhat, his performance on AVSTM tasks improved (span increased to two items), and his pattern of error in word repetition changed (fewer semantic errors, more formal paraphasias and neologisms). Other features of his span performance after some recovery resembled patterns associated with STM-based repetition impairments (reduced recency effects and reduced word length effects). In a series of computer simulation and empirical studies, we show that NC's repetition performance can be accounted for by varying two parameters of an interactive activation model of repetition adapted from Dell and O'Seaghdha's (1991) model of production: decay rate and temporal interval. These results provide support for the view that AVSTM performance depends on storage capacities intrinsic to the language processing system. Such a model allows deep dysphasia and STM-based repetition disorders to be seen as quantitative variants of the same underlying disturbance.