| Cognitive neuroscience of aging research examines neurobiology in relation to cognitive stability and decline in adults of different ages. Because most research has focused on a set of cognitive functions, namely explicit forms of learning and memory (e.g., episodic and working memory), relationships between aging, the brain and implicit forms of learning have remained relatively ignored. To broaden our current understanding, this dissertation examined the neural and cognitive bases of implicit associative learning and how these differ in young versus old adults. The first study revealed that healthy older adults are poorer than younger at such unintentional learning of probabilistic, associative regularities, and that age deficits appear late, but not early, in training. The second study examined the neural mechanisms of implicit associative learning, by assessing relationships between variations of the dopamine transporter gene (DAT1), which influence dopamine transporter expression in the striatum, and implicit associative learning in healthy young adults. Results showed that DAT1 genotype predicted how well individuals learned late, but not early in training for implicit sequential associative events. This finding suggests a role for striatal processes (e.g., striatal dopamine) as training progresses in implicit associative learning of sequential events. The final study examined age differences in functional brain activity during implicit associative learning in young and old adults, finding age group differences in the balance of neural learning systems during early and late training. Both age groups recruited the hippocampus early in training, perhaps because this structure is relatively preserved with age. But, with training, the young recruited the caudate whereas the old continued to rely on the hippocampus, since the caudate shows significant age-related morphological and neurochemical declines. This pattern of brain activity enabled old to maintain near-young levels of performance only early in training, but not later. Taken together, these three studies demonstrate that implicit associative learning of probabilistic regularities is characterized by age deficits that become more pronounced with practice, presumably due to age-related striatal losses and related changes in the relative balance of supporting neural systems with age. |
