Negative Neuroplasticity in Moderate-to-severe Traumatic Brain Injury and Healthy Aging, and the Development of an Intervention to Attenuate Hippocampal-related Decline

The present dissertation examined neurocognitive processes associated with negative neuroplasticity related to declines in enriching activities (i.e., new learning), neurally in patients with chronic moderate-to-severe traumatic brain injury (TBI), and behaviourally in healthy older adults. Additionally, it explored a potential avenue for offsetting negative neuroplasticity in older adults. In TBI, recent longitudinal studies show that neurodegeneration continues chronically, and is associated with negative environmental factors (e.g., low levels of enriching activities) that can lead to a cycle of negative neuroplasticity. Similarly, healthy aging involves degeneration and related cognitive declines, which are exacerbated by previous incidences of trauma (e.g., TBIs), while counterproductive environmental factors promote negative neuroplasticity. A region that is vulnerable to damage in both populations is the hippocampus (HPC), which also shows heightened experience-dependent neuroplasticity. Furthermore, the HPC is implicated in memory and navigation, domains that are negatively impacted by both TBI and aging. Although hippocampal degeneration has been previously described in TBI, Study 1 examined patterns of decline across all gray matter structures in patients with chronic moderate-to-severe TBI, finding that other structures also show decline and may be affected by negative neuroplasticity. Study 2 investigated injury- and age-related deficits in two hippocampal-dependent domains – spatial cognition and memory, finding broad age-related effects across all tasks, while TBI-related deficits were more specific to memory. The behavioural relationship between these domains was examined in younger and older adults. Although spatial cognition significantly predicted memory processes in younger adults, these two domains seem to become more independent with age. Finally, Study 3 assessed the feasibility and preliminary efficacy of a novel, remotely delivered intervention in healthy older adults, which behaviourally targets the HPC through allocentric navigation. Although retention rates suggest this population may need more initial support, compliance was encouragingly high. There was evidence of near- and some medium-transfer effects, but improvements did not generalize to memory. Together, this dissertation deepens our understanding of how negative neuroplastic processes may be manifested in affected populations (i.e., TBI and healthy older adults), how specific its effects may be in each population, and examines environmental enrichment as a way of counteracting this negative cycle.Ph.D