Selective impairment of grammatical morphology due to induced stress in normal listeners: implications for aphasia.

The traditional clinical picture for English nonfluent aphasics has generally presented the deficit as one of total loss of control over grammatical morphology, with some sparing of word order. This is at odds with recent research involving nonfluent aphasic speakers of highly inflected languages, which has shown that agrammatic performance is characterized by morphological substitution rather than omission errors. If the deficit associated with focal brain damage cannot be adequately accounted for in syndrome-specific ways, we may need to look for language-specific processing explanations. One such explanation has to do with language-specific response to global processing difficulty. The current experiment is designed to study the effects of a stress-related limitation on morphological processing. Normal speakers of a language with a relatively rich morphological system (German) are compared with those of a comparatively impoverished system (English) on different forms of a sentence comprehension task. In one form, "clean" stimuli permit full reliance on all available cues to meaning in each language. In another test, a low-level noise mask partially obscured the stimulus sentences. English speakers, who rely almost exclusively on word order cues, were not affected by the noise manipulation. German speakers relied heavily on morphological and semantic information rather than on word order under "clean" conditions. However, under noise Germans made significantly less use of grammatical morphology, with a trend toward compensatory reliance on word order. The results indicate that a global reduction in processing capacity can affect some aspects of language more than others and suggest that such factors must be taken into account in trying to understand specific impairment of morphology in aphasia.