IQ moderation of cognitive decline supports cognitive reserve and not brain maintenance

Abstract Background While cognitive decline has been frequently reported in aging research, moderating factors for cognitive changes in healthy aging have been inconclusive. This study evaluated 5-year changes in four cognitive abilities and the potential moderation of age and cognitive reserve (CR) factors on cognitive changes. Methods Participants included 254 healthy adults initially aged 20 – 80 years. Six tasks estimated each of the four abilities: fluid reasoning, processing speed, memory and vocabulary. The proxies for CR included years of education and IQ. Cognitive changes and moderating factors were examine using multiple indicator latent change score model. Change point analysis pinpointed inflection points after which cognitive changes accelerated. Results There was significant decline over five years in fluid reasoning, processing speed and memory, with age moderation such that older age was associated with steeper decline. Accelerated decline was observed earlier for reasoning and speed, at ages 58 and 59 years respectively, than for memory, at age 70 years. Vocabulary continued to improve until reaching peak performance at 67 years. For moderation of cognitive changes by CR proxies, while education did not show significant moderation, higher IQ was associated with reduced 5-year decline in reasoning and memory but not processing speed. CR moderation effect was found to be independent of mean cortical thickness. Conclusions Using a robust statistical model to estimate the latent change in four cognitive abilities over 5 years, the results showed that cognitive reserve rather than brain maintenance is the potential mechanism underlying IQ’s protective effect on cognitive decline. Highlights Reasoning and processing speed show accelerated decline after ages 59 and 58 years, respectively. Memory shows accelerated decline after age 70 years. Vocabulary performance peaks at age 67 years. Higher IQ is protective of cognitive decline in reasoning and memory but not for processing speed. IQ’s protective effect on cognition is independent of brain maintenance.