The serial order system in word production and working memory: A case series approach

Serial order is important in verbal behavior, such as sequencing words in working memory (WM) or arranging phonemes during speech. In both WM and word production, distinct processes are found for item identity and their serial order. In the current study, we investigated whether a shared system supports the serial order of verbal items (phonemes or words) across cognitive functions (WM and production) and tasks (repetition and naming). We recruited 30 participants with chronic stroke-induced aphasia. We examined WM abilities to recall item and serial order information using immediate serial recall tasks of words. We also assessed the ability to accurately sequence phonemes in word repetition and naming tasks, with its impairment indexed by the proportion of misordered phonemes among all incorrect phonemes compared to chance in phonologically related word and nonword responses. We examined how variability of this index of serial order impairment in repetition and naming relates to item and serial order WM capacities. Our findings reveal that serial order WM capacity, but not item WM capacity, was associated with the severity of serial order impairment in repetition, indicating a shared serial order system for WM and repetition. We also found that item WM, but not serial order WM, was associated with serial order impairment in naming, implying an item WM buffer for phonemic sequencing in naming. These results suggest distinct sequencing processes for repetition and naming, each linked to different WM mechanisms. Implications for word production models and the relationship between WM and word production are discussed.