More than 1,100 older community residents without dementia rated frequency of participation in cognitively stimulating activities. After a mean of about 6 years, they underwent clinical evaluation for MCI and AD and then were followed with brief cognitive performance tests for another 5 to 6 years. More frequent cognitive activity was related to slower cognitive decline in those without cognitive impairment and more rapid cognitive decline in AD, with no effect in MCI. The results suggest that late-life cognitive activity compresses the cognitive morbidity of AD by delaying its onset and by hastening cognitive decline after dementia onset.
In prior research, higher level of cognitive activity has been associated with reduced cognitive decline in persons without dementia.2,9–11
However, its association with cognitive decline within no cognitive impairment or MCI subgroups has not been examined. The present results suggest that the association of frequent cognitive activity with reduced cognitive decline is primarily due to its association with the initial development of cognitive impairment rather than with slowing the progression of cognitive impairment thereafter. Further, few studies have examined the relation of premorbid cognitive activity to cognitive decline in AD and results have been mixed. One study found, like the present one, that higher premorbid activity predicted more rapid cognitive decline, but the cohort was selected and premorbid activity was assessed by retrospective informant report.12
The other study, like the present one, used a population-based sample and self-report of cognitive activity prior to dementia onset, but findings were mainly negative. Higher level of cognitive activity was not related to more rapid cognitive decline following dementia onset though there was some evidence of the association when cognitive function assessments prior to dementia onset were used in analyses.13
The present results show the correlation of premorbid cognitive activity with cognitive decline in AD to be statistically robust and comparable in magnitude, but opposite in direction, to cognitive activity's well-established association with change in cognitive function prior to dementia onset.
These findings suggest that cognitive activity somehow enhances the brain's ability to maintain relatively normal function despite the accumulation of a mild to moderate neuropathologic burden, perhaps due to activity-dependent changes in the function and structure of neural systems underlying cognitive functioning.33,34
Any protection provided by cognitive activity must be limited, however, because cognitively active people do develop dementia. If cognitive activity does somehow allow the brain to tolerate more pathologic changes, those with high premorbid cognitive activity are likely to have a higher pathologic burden than those with low premorbid activity at the time of dementia onset and therefore to experience a more rapidly progressive dementia course. In effect, these results suggest that the benefit of delaying the initial appearance of cognitive impairment comes at the cost of more rapid dementia progression.
Because AD gradually evolves over many years, some factors that predict dementia, such as impaired olfaction,35
are really early signs of the disease. Although a reverse causality hypothesis can account for cognitive activity's association with risk of dementia, it cannot easily explain the correlation between premorbid cognitive activity and cognitive decline in AD observed here or the apparent lack of association between cognitive activity and the neuropathologic lesions underlying AD.11
These observational data suggest that interventions designed to enhance cognitive plasticity36
may prove beneficial in compressing the cognitive morbidity of AD. In this regard, narrow interventions targeting executive control processes37,38
and multimodal interventions that engage older persons in challenging pursuits such as taking acting classes39
or working in an elementary school40
seem particularly promising. The present results suggest that cognitive enrichment interventions may need to be initiated before the development of cognitive impairment, possibly because many persons with MCI already have substantial levels of AD pathology.21–23
This study has several strengths. Participants were sampled from a defined population and represent a broad spectrum of cognitive function from no impairment to frank dementia, suggesting the results are generalizable. Clinical classification was based on a uniform evaluation and widely used criteria, and cognitive function and cognitive activity were assessed with previously established psychometrically sound measures, enhancing our ability to model cognitive activity's association with cognitive decline in each diagnostic subgroup.
Study limitations should also be noted. Differences between diagnostic subgroups could have affected results. The composite measures of cognitive function and cognitive activity do not allow determination of whether results vary across domains of cognitive function, as suggested by some prior research,2,10,11
or whether some activities are more important than others. In addition, with a mean of 2 to 3 observations per individual, we were not well-positioned to capture nonlinear change in cognitive function within diagnostic groups.