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This experiment explored whether a long-term aerobic exercise program may induce significant structural and functional changes in the hippocampus, an area of the brain that is important for spatial navigation and memory formation. Based on existing rodent studies, we hypothesize that exercise will cause a shift to allocentric processing, away from a less robust egocentric learning strategy. It is possible that exercise-induced relief of chronic stress, which contributes to improved hippocampal function, will increase reliance on allocentric spatial navigation. Neurogenesis, which occurs in the dentate gyrus region of the hippocampus, is another indicator of hippocampal function that may influence this shift to allocentric learning. The current study examines whether six weeks of aerobic exercise enhances allocentric processing in healthy young adults. Forty-nine young adults (35 female; age range 18-29 years) were randomly assigned to one of three groups: 1) High intensity interval training group, 2) Moderate intensity training group, or 3) Non-exercising control group. Hippocampus-dependent memory was assessed before and after the intervention on a Virtual Reality Water Maze task, and a high interference memory task, the Mnemonic Similarity Task (MST) which may be dependent on hippocampal neurogenesis. Levels of chronic stress and depression were measured using the Beck Depression Inventory II. It was expected that exercise would improve spatial memory performance on the water maze task, and that performance would improve in proportion to enhanced fitness levels. This improvement in spatial memory performance was expected to correlate with the two indicators of hippocampal function that were assessed in the current study—chronic stress and performance on the high interference memory task. Six weeks of regular aerobic exercise resulted in a 21.5% improvement in spatial memory performance on the water maze task, indicating improved hippocampus-mediated spatial memory function. Improvements displayed by high intensity exercisers were greater than those observed in the moderate intensity exercisers, suggesting that higher intensity exercise may be more effective in enhancing hippocampal function. Importantly, low responders to exercise exhibited a 30% improvement in water maze performance, suggesting that even minor fitness improvements can lead to significant cognitive gains. Chronic stress and depression, and performance on the MST were not significantly associated with changes in spatial memory performance; however trends observed may offer some explanation to the aforementioned changes in spatial memory. Findings from the current study have important implications for treatment options in populations that are currently, or at risk of suffering from impaired hippocampal function. / Thesis / Master of Science (MSc)

In this essay, changes in the human hippocampal volume and function induced by stress, depression and physical exercise are examined. Hippocampus is crucially involved in the acquisition and retrieval of episodic and spatial memory, and hippocampal volume correlates with episodic and spatial memory performance. Hippocampus has substantial plasticity and changes with age, but also in response to experiential factors across life. Stress and, under at least some circumstances, also depression have negative effects on hippocampal volume and memory function. The negative effects are believed to accelerate age-related decline in volume and function, mediated by exaggerated cortisol levels and dysfunction in the HPA-axis. Physical exercise is examined from two perspectives; aerobic and strength exercise. Aerobic exercise increases hippocampal volume across various ages and decelerates age-related hippocampal degeneration, whereas support for strength exercise-induced effects are mixed and need to be studied further. The positive effects are believed to be mediated by increased BDNF levels and regional cerebral blood volume. Although hippocampal volume normally correlates with hippocampus-dependent memory, studies on exercise-induced changes in human hippocampus-dependent memory have reported inconsistent results. Animal studies have observed both the negative and positive effects on hippocampal volume to relate to changes in neurogenesis, cell proliferation, and dendritic complexity. The negative and positive effects on hippocampal volume have been observed to be non-permanent, suggesting that physical exercise may prevent, attenuate and possibly reverse hippocampal degeneration induced by stress and depression. Further, more studies on sex and age differences, exercise intervention designs and functional values of physical exercise would be of value.