Syntactic predictions and asyntactic comprehension in aphasia: Evidence from scope relations

• We investigated comprehension of predictable and unpredictable dependencies.
• Participants performed a truth-value judgment task and a picture-selection task.
• The non-agrammatic patients performed well in all conditions.
• The agrammatic patients performed relatively well in unpredictable dependencies.
• Syntactic prediction increases complexity resulting in parsing failures.

People with aphasia (PWA) often fail to understand syntactically complex sentences. This phenomenon has been described as asyntactic comprehension and has been explored in various studies cross-linguistically in the past decades. However, until now there has been no consensus among researchers as to the nature of sentence comprehension failures in aphasia. Impaired representations accounts ascribe comprehension deficits to loss of syntactic knowledge, whereas processing/resource reduction accounts assume that PWA are unable to use syntactic knowledge in comprehension due to resource limitation resulting from the brain damage. The aim of this paper is to use independently motivated psycholinguistic models of sentence processing to test a variant of the processing/resource reduction accounts that we dub the Complexity Threshold Hypothesis. According to this hypothesis, PWA are capable of building well-formed syntactic representations, but, because their resources for language processing are limited, their syntactic parser fails when processing complexity exceeds a certain threshold. The source of complexity investigated in the experiments reported in this paper is syntactic prediction. We conducted two experiments involving comprehension of sentences with different types of syntactic dependencies, namely dependencies that do not require syntactic prediction (i.e. unpredictable dependencies in sentences that require Quantifier Raising) and dependencies whose resolution requires syntactic predictions at an early stage of processing based on syntactic cues (i.e. predictable dependencies in movement-derived sentences). In line with the predictions of the Complexity Threshold Hypothesis, the results show that the agrammatic patients that participated in this study had no difficulties comprehending sentences with the former type of dependencies, whereas their comprehension of sentences with the latter type of dependencies was impaired.