Sentence Comprehension in Aphasia

Aphasia is a disorder of language that sometimes occurs in the setting of brain damage. Aphasic disturbances of syntactic comprehension have been described only in recent decades, and have been characterized as the loss of the ability to understand non-canonical sentences. Several interesting hypotheses have been advanced to explain this phenomenon. Some of these hypotheses rest on the assumption that heuristics play a role in producing the aphasic comprehension pattern. Evidence from the past decade suggests that heuristic processes do play a role in normal sentence comprehension, and act in concert with slower processes that more closely resemble the syntax of linguistic theory. The slower syntactic mechanisms appear to be more readily disturbed by brain damage, leading to greater difficulty with sentences that cannot be interpreted heuristically. However, the dichotomy between canonical and non-canonical sentences accounts for little of the variance in the performance of aphasic individuals. Instead, the comprehension of each sentence structure is disrupted according to the complexity of the structure and the severity of the individual's aphasia. This observation argues against hypotheses that posit a deficit of a specific linguistic operation and favors reduction of a hypothetical cognitive resource that is necessary for normal comprehension. In this case, the chief contribution from linguistic theory is the syntactic tree itself, which affords a concise characterization of the difficulty of parsing and interpreting a given sentence. The field now faces several challenges. The neural basis of heuristic, parsing, and interpretive mechanisms must be elucidated. Achievement of this objective entails a neural characterization of the cognitive resource that is disrupted in sentence comprehension disorders and of the semantic representations onto which morphemes and sentences are mapped.