Visual short-term store can compensate for a defective phonological short-term store in patients with apraxia of speech

OBJECTIVE: The aim of the study was to investigate whether, within the phonological short-term memory (STM) system, speech articulation disorders primarily due to cortical damage are associated with deficits in the phonological recoding of visual-verbal material, and whether the visual short-term store (STS) can compensate for inefficient access to the phonological STS, as suggested in previous reports. METHOD: Two patients (AE and TM) with apraxia of speech due to atrophic cerebral damage were administered a battery of tasks devised to explore the organization of the phonological STM. RESULTS: AE's span was normal and TM's span was markedly reduced. Phonological similarity and word-length effects: Both patients showed the effects in visual presentation; the effect was less evident in verbal presentation. This suggests a phonological STS disorder and preserved rehearsal/phonological recoding, consistently with involvement of the left parietal regions, which was documented by the MRI in both patients. Unexpectedly, TM had a longer span for similar than for dissimilar words in visually presented stimuli. Silent phonology (on written material): At variance with controls, stress assignment improved during unattended speech both in AE and TM, while in both patients and controls a detrimental effect of unattended speech was documented in the initial sound task. CONCLUSIONS: To account for this unusual pattern of results, we hypothesized that whenever possible, AE and TM adopt the strategy that takes advantage of the visual STM store to compensate for a defective phonological STS.