The importance of short-term memory for order in dissociating short-term memory and language deficits

Selective verbal short-term memory (STM) deficits are rare, and when they appear, they are often associated with a history of aphasia, raising doubts about the selectivity of these deficits. We explore here the distinction between STM for item information and STM for order information to separate STM and language impairments. Recent models of STM consider that STM for item information depends upon activation of the language system, and hence item STM deficits should be associated with language impairment. By contrast, STM for order information is considered to recruit a specific system, distinct from the language system. In this view, order STM should be impaired in patients with STM deficits that cannot be accounted for by language impairment. We applied this rationale to the exploration of STM profiles of patients MB and CG. Patient MB showed mild phonological impairment and associated STM deficits. As predicted, these were characterized by poor item STM but preserved order STM. Patient CG showed verbal STM deficits with no associated language deficits. His STM deficit was characterized by poor order STM but relatively preserved item STM. This study presents the first double dissociation between item and order STM deficits, demonstrating the necessity of this distinction for understanding selective STM impairment.