Background: Individuals with agrammatism show selective deficits in functional categories. The tree-pruning hypothesis (TPH; Friedmann & Grodzinsky, 1997) suggests that this results from inability to project certain nodes in the syntactic tree. On this account, higher nodes in the tree are more vulnerable than lower ones. Other theories, however, suggest that functional category impairments can be explained in the context of a morphological deficit (e.g., Arabatzi & Edwards, 2002; Penke, 2003; Thompson, Fix, & Gitelman, 2002).

Aims: This study examined production of complementisers, tense, and agreement morphology in four English-speaking agrammatic participants to test the hierarchical nature of functional category deficits. The consistency of verb inflection errors was also tested under conditions examining a minimal set versus a full array of English inflected forms.

Materials & Procedures: In Experiment 1 participants were asked to produce sentences by using a complementiser (i.e., whether, that, and if), a tense (-ed), or agreement marker (-s), in structured sentence elicitation tasks. In Experiment 2 the participants' production of both finite and non-finite verb inflection forms was examined.

Outcome & Results: All participants produced complex sentences successfully using a complementiser, indicating intact projection to the Complementiser Phrase (CP). As for tense and agreement—structures within the Inflection Phrase (IP)—the agrammatic speakers were impaired in both categories and they showed higher scores in non-finite vs finite verb conditions. Further, their errors were dominated by substitutions, rather than omissions, with various non-target morphemes.

Conclusions: Our agrammatic participants' deficits are morphological, rather than syntactic. The participants were able to project to the uppermost structure, CP. They showed the ability to project verb inflection and to implement inflectional rules in their grammar. However, instantiation of grammatical markers sometimes failed to operate, resulting in incorrect inflectional forms. These findings suggest that within the domain of functional categories, IP- and CP-level deficits may result from disruption of differing underlying mechanisms and, therefore, they may require separate treatment strategies.