Abstract :
[en] Summary
Background: Aging would be associated with brain changes that have an impact on certain cognitive functions. However, there is a huge interindividual heterogeneity in age related cognitive decline. This variability may be explained by differences in cognitive reserve (CR), a very recent and complex concept which suggests that the central nervous system can maintain its optimal performance over a longer period of time in individuals with certain lifestyle characteristics, evolving in "enriched environments". Thus, the aim of the present thesis was to deepen our understanding of the relationship between activity and brain; in particular, we examined the role of a variable that decisively influences subjects’ level of activity: retirement.
Studies: First, we have shown that people aged 60 and over may maintain their cognitive skills through the continuation of their professional and non-professional activities (European Share survey). A cross-country analysis also revealed that cognitive scores of older adults were better in countries in which the eligibility age for retirement benefits was higher (e.g., 65 years old in Sweden) as compared to those where it was lower (e.g., 60 years old in France), suggesting that people benefit from staying longer in employment. Then, we extended our work to the effect of retirement on pathological aging and, in particular, to Alzheimer's disease (AD). We used data collected from the Ictus cohort, a European study of AD patients. We showed that previous research suffered from a methodological bias that led to overestimate the effect of retirement age. After taking into account this bias, we showed that although there is an association between retirement age and onset of the disease, the strength of the link is reduced. These initial results highlight the need for cautiousness when studying such complex relationships and for prospective studies. Thus, we used longitudinal data from the 3 Cities, a French population-based study of older adults. This study revealed that even though deferred retirement is associated with a lower risk of dementia, a longer working life is not associated with such reduced risk. This indicates that the retirement age does not directly contribute to CR but rather that earlier retirement should be considered as a factor of cognitive and psycho-social vulnerability. In the fourth study, we identified psychosocial factors associated with retirement that may influence cognitive functioning using data from the AMI study, which is conducted in rural areas in southwestern France. We found that holding positive consideration towards work and the beneficial effect of activity after retirement favored better cognitive functioning. Finally, we have developed and compared two indices of CR measuring all life experiences (a standard and a detailed indices). This study revealed that while both indices were associated with a minimization of the effects of age on cognition, the detailed index seems to best reflects the level of one’s CR.
Conclusion: Although later retirement age is associated with cognition, caution is needed, as the current evidence should not merely serve economic interests aiming to justify an increase in the retirement age. Indeed, one should not suggest, based on these initial results, that increasing the age of retirement would be beneficial for everyone’s health. On the contrary, research on retirement and cognition mostly underlines the importance of activity in a general sense, rather than being limited to the professional occupation. The retirement transition is a complex life event that should be viewed in light of the various individual and social factors that may influence cognitive functioning. A better understanding of retirement-related factors that contribute to cognitive vitality is necessary in order to optimize health policies and to ensure that they are well adjusted to the diverse professional and psychological issues affecting the older population.
