Frontiers in Psychology (May 2015)
Structural prediction in aphasia
Abstract
There is considerable evidence that young healthy comprehenders predict the structure of upcoming material, and that their processing is facilitated when they encounter material matching those predictions (e.g., Staub & Clifton, 2006; Yoshida, Dickey & Sturt, 2013). However, less is known about structural prediction in aphasia. There is evidence that lexical prediction may be spared in aphasia (Dickey et al., 2014; Love & Webb, 1977; cf. Mack et al, 2013). However, predictive mechanisms supporting facilitated lexical access may not necessarily support structural facilitation. Given that many people with aphasia (PWA) exhibit syntactic deficits (e.g. Goodglass, 1993), PWA with such impairments may not engage in structural prediction. However, recent evidence suggests that some PWA may indeed predict upcoming structure (Hanne, Burchert, De Bleser, & Vashishth, 2015). Hanne et al. tracked the eyes of PWA (n=8) with sentence-comprehension deficits while they listened to reversible subject-verb-object (SVO) and object-verb-subject (OVS) sentences in German, in a sentence-picture matching task. Hanne et al. manipulated case and number marking to disambiguate the sentences’ structure. Gazes to an OVS or SVO picture during the unfolding of a sentence were assumed to indicate prediction of the structure congruent with that picture. According to this measure, the PWA’s structural prediction was impaired compared to controls, but they did successfully predict upcoming structure when morphosyntactic cues were strong and unambiguous. Hanne et al.’s visual-world evidence is suggestive, but their forced-choice sentence-picture matching task places tight constraints on possible structural predictions. Clearer evidence of structural prediction would come from paradigms where the content of upcoming material is not as constrained. The current study used self-paced reading study to examine structural prediction among PWA in less constrained contexts. PWA (n=17) who had varying levels of sentence-comprehension impairment read sentences where an upcoming disjunction either could (1b) or could not (1a) be predicted, based on the presence of either (Staub & Clifton, 2006; see Figure 1 for example). If either spurs PWA to predict an upcoming disjunction and this prediction is facilitative, then reading times on the or and second disjunct (or a beautiful portrait) should be faster in the Either condition than in the No Either condition. Results confirmed this prediction (see Figure 1; β=352, t=2.36). The magnitude of this facilitation was related to overall language-impairment severity on the Comprehensive Aphasia Test (CAT: Swinburn, et al., 2004): PWA with milder language impairments showed more facilitation for either than PWA with more severe language impairments (r=.594, p.05). This finding represents strong and novel evidence that PWA can use a lexical cue to predict the structural form of upcoming material during comprehension. However, the lack of relation between these PWA’s degree of structural facilitation and their sentence comprehension ability may indicate that structural predictions could speed reading without improving comprehension.
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