Cortical thickness in the right inferior frontal gyrus mediates age-related performance differences on an item-method directed forgetting task.

Evidence suggests that older adults have difficulty relative to younger adults in forgetting irrelevant information. Here we sought to understand the physical basis of this deficit by investigating the relationship between cortical thickness and intentional forgetting, using an item-method directed forgetting task. We tested younger (n = 44) and older (n = 54) adults' memories for words that they were instructed to either remember or to forget, and then extracted cortical thickness values from brain regions previously shown, using functional neuroimaging, to be associated with memory suppression, including the right inferior frontal gyrus, the right postcentral gyrus and the left superior/middle frontal gyrus. Results from a parallel mediation model indicated that variations in cortical thickness in the right inferior frontal gyrus, but not the right postcentral gyrus or left superior/middle frontal gyrus, partially explained age-related differences in directed forgetting: older adults with thinner cortices in this area showed worse forgetting ability. This is the first study to explore how neuromorphological differences affect the ability to intentionally suppress items in memory. The results suggest that age-related differences in directed forgetting may be partly driven by cortical thickness in a brain structure known to be functionally involved in directed forgetting, and inhibitory control more broadly, supporting a contribution of deficient inhibition to this phenomenon.