On the Structure of Verbal Short-term Memory and its Functional Role in Sentence Comprehension: Evidence from Neuropsychology

Abstract

We present a case of a patient with a disorder of short-term memory. BO has a reduced span (2 to 3 items), no recency effect in free recall, and rapid forgetting in Brown-Peterson tasks, establishing her as a patient with impaired short-term verbal memory functions. She shows no effect of either phonological similarity or word length in recall of auditory or written word lists, but some recency effect under recall-from-end conditions and better performance on Brown-Peterson tasks in an unfilled than in a filled condition. This pattern of performance is interpreted as being consistent with a primary disturbance of the articulatory rehearsal processes of S.T.M. and possibly some impairment of the phonological store (using Baddeley's 1986 terminology). Her aphasic disturbance—apraxia of speech—is also consistent with a disturbance of articulatory rehearsal. BO shows a retained ability to extract phonology from print, including an ability to apply sublexical grapheme-phoneme correspondences, thus indicating that the rehearsal and transcoding functions associated with articulatory mechanisms are dissociable. Her improved performance on unfilled delays in Brown-Peterson testing, as well as her overt attempts to rehearse in this condition, also establishes a dissociation between rehearsal in span and in delayed recall tasks. BO shows excellent comprehension of a wide variety of syntactic structures with auditory, written, and speeded written presentations, indicating that articulatory rehearsal is not needed for the assignment of syntactic structure and its utilisation to establish aspects of prepositional semantics (thematic roles and co-indexation of noun phrases). BO makes errors referable to the maintenance of particular items in propositional memory systems, consistent with the view that the role of articulatory rehearsal mechanisms in sentence comprehension involves maintenance of items in an interpreted (semantic) structure.