On why the elderly have normal semantic retrieval but deficient episodic encoding: a study of left inferior frontal ERP activity.

Age-related left inferior prefrontal cortex (LIPFC) blood flow reductions during semantic retrieval are associated with reduced subsequent episodic recognition memory performance but are inconsistent with age-invariant semantic retrieval performance. Therefore, we compared brain activity in young and elderly persons during low- and high-selection versions of a semantic task using ERPs recorded at 62 scalp locations. In an early interval (400-800 ms), both age groups showed more negativity over left inferior frontal scalp while performing the high- compared to the low-selection task. This early semantic selection-related negativity was associated with age-equivalent accuracy and reaction time on the semantic tasks. Further, in the early time interval, the ERPs of the young were more negative than those of the elderly. In addition, only the young showed a late selection-related negativity (1,200--1,400 ms). These age-related differences in left frontal ERP activity were associated with significant decrements in subsequent recognition for the elderly. Moreover, additional analyses revealed that larger amounts of negativity over left inferior frontal scalp locations during semantic retrieval were correlated with enhanced subsequent episodic recognition in both groups. In sum, the data suggest that (1) semantic retrieval and episodic encoding processes overlap temporally in similar brain regions, most likely including the LIPFC, and (2) the failure to recruit these regions in the service of episodic encoding underlies the age-related deficit in episodic memory.