Individuals with agrammatism show selective impairments in functional category production. The Tree Pruning Hypothesis attributes this deficit to an inability to project higher nodes in the syntactic tree. This study examined complementizer production (situated in the highest node, the Complentizer Phrase (CP)), and verb tense and agreement (located in lower nodes, the Tense Phrase (TP) and Agreement Phrase (AgrP), respectively) in four agrammatic speakers. Results of two sentence production experiments showed unimpaired CP level structures, however deficits in both TP and AgrP were noted. In addition, TP was not more impaired than AgrP and substitution errors were found for both. These findings suggest that functional category deficits are not syntactic, rather they are due to faulty implementation of morphological rules.
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