Functional Compensation in Response to Increasing Task Difficulty: Comparing Semantic and Episodic Memory Tasks in Young and Older Adults

Previous fMRI studies have demonstrated that older adults who perform as well as young adults on certain cognitive tasks recruit additional brain regions relative to younger adults while performing these tasks. This phenomenon has been interpreted as a compensatory response and may reflect an effort to maintain performance in the face of increasing changes in cognitive difficulty or age-related brain changes in structure and/or function. Whether the compensatory response is specific to older adults or represents a more general response of any individual to increasing task difficulty is unclear. The present fMRI experiment explored age differences in brain activity associated with increases in task difficulty in two tasks, an episodic-retrieval task that is expected to be difficult for older compared to young adults, and a lexical-semantic task that is expected to be more difficult for young compared to older adults. In the lexical-semantic task, participants judged whether pairs of words were synonyms or antonyms. In the episodic task that followed, participants made yes/no memory judgments for the word pairs previously presented. Difficulty was manipulated using word frequency -low frequency words are more difficult in the lexical-semantic judgment task and easier in the episodic task. Young (ages 18-24) and older healthy adults (ages 60-83) were scanned on a 3T GE magnet using a single-shot spiral pulse sequence. Behavioral results showed a double dissociation – older adults were adversely affected by word frequency in the episodic but not the semantic task, while young adults were adversely affected by word frequency in the semantic but not the episodic task. Both groups showed activation in similar task-related and task-general regions regardless of difficulty level). Age-related differences were observed for task-specific and linear increases due to difficulty. Linear increases in fMRI activation were associated with younger adults showing increased task difficulty in bilateral task-related regions during the lexical semantic task, whereas in the episodic retrieval task only activating bilateral posterior cortices. As difficulty increased, older adults showed unilateral brain activations: left regions for the lexical-semantic task and medial and right hemisphere regions for the episodic retrieval task. Most importantly, difficulty load increases paralleled the groups' behavioral results: younger adults showed greater increases in activity in the lexical-semantic task compared to older adults, but not in the episodic retrieval task, whereas older adults showed the opposite pattern, with greater increases in activation only in the episodic task when compared to younger adults. Thus, younger and older adults recruit regions differently in response to increases in difficulty. Our results suggest that increases in fMRI activation as difficulty increases occur as an interaction to deal with task difficulty and the inherent abilities of the individuals, rather than occurring only in older adults, or in older adults across all tasks, regardless of their abilities in that domain.Release after 21-Jun-201