Independent contributions of semantic and phonological working memory to spontaneous speech in acute stroke

Patients with left hemisphere stroke often have language deficits which impair their ability to produce phrases and sentences. One possible source of these speech impairments is the disruption of verbal working memory (WM). Single-case studies of chronic stroke have suggested the existence of a WM capacity specific to maintaining semantic information that is critical for preparing multiple words in phrases prior to speech onset (Freedman, Martin, & Biegler, 2004; Martin & Freedman, 2001; Martin & He, 2004; Martin, Miller, & Vu, 2004). The current study tested this hypothesis by examining spontaneous narrative language production and working memory capacities in a large sample of individuals at the acute stage of stroke (N = 36), prior to the reorganization of function or strategy development. Here we show using a multiple regression approach that patients' semantic but not phonological WM capacity had an independent contribution in predicting phrasal elaboration and increasing utterance length whereas patients' phonological but not semantic WM capacity had an independent contribution in predicting a more rapid speech rate. Importantly, neither WM capacity independently predicted grammatical abilities in speech, implying that the other relations did not result from overall severity. These results indicate that separable semantic and phonological WM components exist that support different aspects of narrative speech. To our knowledge, this is the first study to examine spontaneous speech in a large group of acute stroke patients demonstrating a critical relationship between working memory and the ability to produce more words in phrases and longer utterances.