During the last decade, the field of neuroimaging has been marked by considerable interest in the effects of cognitive training on both cognition and the brain. Several studies involving cognitive training paradigms have shown positive training effects in both young and older adults. However, a number of questions remain: Are training effects intervention specific? Why do some individuals benefit more from cognitive training relative to others? How does plastic potential change with age? Are structural brain alterations a neurobiological substrate for behavioural improvements? Is continuous cognitive training a premise for the persistence of training-related brain changes?
The objective of this thesis was to address these questions through a longitudinal memory-training study including 217 young and older participants. In summary, the results indicate that 1) the memory training had a unique effect on memory performance relative to active and passive control conditions. 2) Individual differences in white matter (WM) microstructural brain characteristics, measured by diffusion tensor imaging, influence the potential to benefit from memory training in older age. Thus, conditions that promote preservation of brain integrity throughout the lifespan should represent an important focus for future research. 3) Cognitive plasticity can be found in both young and older adults. However, the present study identified WM microstructural changes in relation to cognitive improvement in the older adults only. While these results provide promising evidence of preserved plasticity in older age, further research is needed to determine the conditions that may evoke microstructural brain changes in different age groups. 4) Cognitive training has the potential to moderate the magnitude of age-related decline in WM microstructure, but continuous training may be a premise for the persistence of such effects. Further investigations are required to determine the nature of training effects in the longer term, and whether these results also apply to other brain characteristics. In conclusion, the results contribute to the understanding of possible premises for WM microstructural plasticity in young and older adults. As the brain is highly vulnerable to the impact of age, knowledge about the potential to influence cognitive and brain function throughout the lifespan may have important implications at both the individual and community level, by promoting preventive interventions, longer lasting function in older age and the reduction of health care cost.