Two Sources of Age-Related Decline in Comprehension of Complex Relative Clause Sentences

Working memory capacity is thought to underlie many higher cognitive processes including sentence comprehension. Sentences that contain object-extracted relative clauses are more complex and impose higher demands on working memory than comparable sentences that contain subject-extracted relative clauses. We report two experiments examining the role of the central executive component of working memory in adult age-related declines in comprehension of these sentences. Two different conceptualizations of executive capacity were employed. One approach quantifies load in terms of simultaneous storage and computational demands of the task and assesses this capacity using complex span tasks such as Reading Span. The other (newer) approach quantifies load in terms of the complexity of the relational information in the task and assesses capacity using relational processing tasks such as n-term premise integration. Participants aged 20 years to 88 years read objectand subject-cleft sentences (Study 1) and subjectand object-relative clause sentences (Study 2) at their own pace then responded to comprehension questions. As expected, object-extracted sentences were more difficult to understand than subject-extracted sentences. Multiple regression analyses showed that after controlling for subject-clefts/relatives, N-term premise integration task accounted for age-related and age-independent variance in objectclefts/relatives whereas Reading Span accounted for age-related variance only. The findings suggest that both N-term and Reading Span tap processes that are required for comprehension of object-extracted sentences. Moreover, relational processing (N-term) and simultaneous storage and computation (Reading Span) are partly independent. Both are vulnerable to agerelated decline, but relational processing declines earlier than capacity for simultaneous storage and computation.