WHEN TASK DEMANDS INDUCE ASYNTACTIC COMPREHENSION - A STUDY OF SENTENCE INTERPRETATION IN APHASIA

We investigated sentence processing in two aphasic patients who appear ed to have asyntactic comprehension when tested using sentence-picture matching. It was found that neither patient could handle the nonlingu istic cognitive demands of the original task: Specifically, processing two semantically incongruous inputs (sentence plus reverse-role pictu re) overloaded Working memory. Their ability to deal with semantic con flict 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 plausi ble, but performed poorly when asked to identify the ''actors'' in suc h sentences, often selecting the more likely alternative (the mouse). We concluded from this dissociation between tasks that semantic confli ct only overtaxed their limited processing capacity when the conflicti ng options were explicitly available and directly relevant to the deci sion process. There was, therefore, no adverse effect in meaning class ification, 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 afte r correctly classifying sentences with regard to plausibility, we argu ed that their inaccurate performance under conditions of high processi ng load was more likely to reflect an inability to perform the necessa ry decision processes than an impaired capacity to analyse linguistic structure in the face of increased (nonlinguistic) task demands.