Better statistical regularity with aging? Age-related difference in the neural processing of idioms

To process information efficiently, people extract regular/consistent patterns from the outside world to better predict upcoming events. Indeed, infant studies suggest that prior experience is necessary for such statistical regularity to be used. Yet, little work has been done to investigate if older adults—with their longer life experiences—would lead to greater influence of statistical regularity compared to their younger counterparts. We examined older and younger adults’ processing of Chinese idioms—four-word phrases with semantic regularity acquired through experience. Thirty younger and 27 older neurologically-intact right-handed adults underwent an idiom recognition fMRI experiment. Participants viewed sequentially presented quartets of Chinese characters and, upon onset of the fourth characters, judged whether the quartets constituted idioms or not. Quartet sequences comprised frequent idioms, infrequent idioms, scrambled characters, and non-characters. Compared to younger adults, older adults had higher recognition hits for frequent idioms and higher false alarms for scrambled characters, with no difference in d’, but smaller beta (more likely to make idioms judgment). fMRI results showed that older adults engaged higher neural responses to idioms relative to scrambled characters in left frontal, left parietal, and bilateral temporal areas, and to scrambled words relative to non-characters in left superior frontal gyrus. Critically, greater distinctive fronto-temporal engagement between idioms (either frequent or infrequent) and scrambled characters correlated with higher d’ only in older but not younger adults. Our findings indicate that older adults were more prone to judging four-word phrases as idioms, with those who were more selective (greater d’) for idiom judgment engaging more selective neural processing, possibly reflecting interaction of aging and experience. These findings suggest that different neural mechanisms are recruited for making the same idiom judgment in younger and older adults, reflecting the differential influence of statistical regularity extracted from the environment.