It is well established that older adults show a deficit in episodic memory. The associative deficit hypothesis (ADH) (Naveh-Benjamin, 2000) suggests that an age-related reduced ability to create links between units of information is a major contributor to the episodic deficit. It has been a robust finding that older adults show a disproportionate decline in associative memory relative to item memory when compared to young adults. Previous researchers have investigated the role of the frontal lobes (FL) by studying the effect of reduced attentional resources in the associative deficit. However, they have not found that divided attention in young adults produces the disproportionate associative decline seen in aging and it is thought that some cognitive process other than the allocation of attentional resources may contribute to the associative deficit. The present study intended to use a divided attention (DA) task that also engages medial temporal brain regions (MTL) in order to tax additional parts of the network involved in creating associations and provide indirect support for the role of the MTL in the associative deficit. However, the associative memory deficit in older adults was not replicated due to unique poor associative memory performance of some young adults in the study. Analyses excluding these participants show support for the role of the MTL in the associative deficit. However, the young poor performers may provide support for the role of FL function in the associative deficit and show that poor associative memory may not be limited to the older adult cohort.

One of the few areas in the brain that still exhibits experience-dependent neuroplasticity in adulthood is found in the medial temporal lobe (MTL) system. Within the MTL, this plasticity has been observed in the hippocampus in both humans and animal models. Rodent model studies focusing on the effect of aerobic exercise have shown a positive increase of neuroplasticity in the dentate gyrus subregion of the hippocampus. Another area in the MTL, the entorhinal cortex (EC), serves as a primary input to the hippocampus, and studies on environmental enrichment have reported greater EC volume in rodents supplied with toys and running wheels. Previous work in our lab working with healthy young adults showed a positive correlation between right EC volume, and aerobic fitness (VO2 max). In this thesis, I examined two aims, first whether aerobic fitness predicts changes in thickness or volume of the MTL as well as performance in an MTL dependent task in healthy young adults. Additionally, whether the brain morphology measures of the MTL can predict performance on the memory task. The second aim looks at the longitudinal effect a 12-week exercise intervention has on thickness or volume in the MTL and performance on an MTL dependent task in the same population. Results indicate that there is a positive baseline correlation between aerobic fitness and thickness of the EC on the left hemisphere but there are no longitudinal changes in morphology after the exercise intervention. These data extend previous work on the effects aerobic exercise has on MTL structure and offer interesting venues to combat neurodegenerative diseases that affect the MTL memory system like Alzheimer’s disease.