Syntactically and semantically based computations: processing of prepositions in agrammatism.

Six Broca's and six Wernicke's aphasics were tested for their processing of prepositions in different linguistic tasks which placed increasing emphasis on the use of syntactic knowledge. Subjects performed a variety of tasks involving the production and perception of sentences containing prepositions. Agrammatics' production performance was very poor. Although their comprehension performance was superior to production when asked to complete a sentence selecting the target item out of four prepositions their comprehension performance was no longer superior to production in tasks where correct performance strongly depends upon the knowledge of sentence form. These results support the view of a general incapacity of agrammatics to use knowledge of sentence structure in production and in comprehension.