Dissociating short-term memory and language impairment :the importance of item and serial order information.

BACKGROUND: 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. 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: this system could be impaired in patients with language-independent STM deficits.
AIM: We demonstrate here the power of the item-order distinction to separate STM and language impairments in two brain damaged cases with STM impairment and a history of aphasia.
METHODS & PROCEDURES: Recognition and recall STM tasks, maximizing STM for either item or order information were administered to patients MB and CG.
OUTCOMES & RESULTS: Patient MB showed mild phonological impairment. As predicted, associated STM deficits were characterized by poor item STM but preserved order STM. On the other hand, patient CG showed no residual language deficits. His STM deficit was characterized by poor order STM but perfectly preserved item STM.
CONCLUSIONS: This study presents the first double dissociation between item and order STM deficits, and demonstrates the necessity of this distinction for understanding and assessing STM impairment in patients with and without aphasia.