When task demands induce asyntactic comprehension - a study of sentence interpretation in aphasia

We investigated sentence processing in two aphasic patients who appeared to have asyntactic comprehension when tested using sentence-picture matching. It was found that neither patient could handle the nonlinguistic cognitive demands of the original task: Specifically, processing two semantically incongruous inputs (sentence plus reverse-role picture) overloaded Working memory. Their ability to deal with semantic conflict in the absence of multiple inputs was examined in an interleaved meaning-classification/actor-identification task. The patients rarely accepted misordered sentences like The cheese ate the mouse as plausible, but performed poorly when asked to identify the ''actors'' in such sentences, often selecting the more likely alternative (the mouse). We concluded from this dissociation between tasks that semantic conflict only overtaxed their limited processing capacity when the conflicting options were explicitly available and directly relevant to the decision process. There was, therefore, no adverse effect in meaning classification, where the alternative (lexically-based) sentence reading (1) had to be computed by patients, and (2) bore no direct relevance to the plausibility judgement. By contrast, in actor identification, the patients had to choose between two explicitly available candidates for the actor role, one syntactically based and the other more plausible. Since our patients made errors in identifying actors immediately after correctly classifying sentences with regard to plausibility, we argued that their inaccurate performance under conditions of high processing load was more likely to reflect an inability to perform the necessary decision processes than an impaired capacity to analyse linguistic structure in the face of increased (nonlinguistic) task demands.