Objective: Following a sport related concussion (SRC) visible symptoms generally dissipate in 7-10 days post-injury. However, little is known about the cumulative effects of SRCs both in terms of structural damage to the white matter of the brain and neurocognitive performance. To address this issue, the relationship between the number of SRCs (frequency), axonal white matter (WM) damage and neurocognitive performance was examined. There were three predictions. First, increases in SRC frequency will be associated with decreases in performance on neurocognitive tests. Second, the frequency of SRC will be associated with axonal injury measured three WM tracts: the corpus callosum, the fronto-occipital fasciculus and the inferior longitudinal fasciculus. Third, less accurate and slower performance on a response inhibition task (STOP-IT) will be associated with greater axonal injury. Methods: A cross-sectional correlational design was utilised. Participants were rugby players with a history of SRC, rugby players with no history of SRC and control athletes (N=40) who completed a neurocognitive test battery and had a DTI brain scan. The neurocognitive battery consisted of the following standardised tests: Speed and Capacity of Language Processing Test, CogState Electronic Battery, Stroop Colour and Word Test, Controlled Oral Word Association Test, the Trail Making Test and the experimental test STOP-IT Electronic Test. White matter axonal injury was measured by DTI using fractional anisotropy (FA) and mean diffusivity (MD) metrics. The DTI data was processed using FSL to extract FA and MD DTI metrics in three a-priori regions of interest. Results: Spearman’s correlation analyses did not find significant associations between SRC frequency and neurocognitive performance on the FAS (rs=0.053, 95% CI [-0.27, 0.36]), TMT-A (rs=0.058, 95% CI [-0.26, 0.37]), TMT-B (rs= -0.046, 95% CI [-0.27, 0.36]) and the Stroop Interference (rs= -0.25, 95% CI [-0.07, 0.52]). Similarly, no significant Spearman’s correlations were found between SRC frequency and the computerised neurocognitive tests STOP-IT-SSRT (rs= -0.04, 95% CI [-0.28, 0.35])), STOP-IT–Accuracy (rs= -0.05, 95% CI [-0.27, 0.36]), CogState Detection subtest (rs= -0.15, 95% CI [-0.17, 0.44]), CogState Identification subtest (rs= -0.065, 95% CI [-0.26, 0.37]), CogState One card learning subtest (rs= 0.24, 95% CI [-0.08, 0.52]) or the CogState One back task subtest (rs= 0.06, 95% CI [-0.26, 0.37]). In terms of the DTI data there were no significant associations between SRC frequency and axonal injury measured by FA values in the CC (rs= 0.005, 95% CI [-0.31, 0.32]), ILF (rs= 0.028, 95% CI [-0.29, 0.34]) or FOF (rs= -0.022, 95% CI [-0.30, 0.33]). The same was pattern was found for MD values in the CC (rs= 0.081, 95% CI [-0.24, 0.39]), ILF (rs= -0.16, 95% CI [-0.16, 0.45]) or FOF (rs= -0.15, 95% CI [-0.17, 0.44]) Finally, there were no significant Spearman’s correlations between axonal injury FA values and the STOP-IT SSRT in any of the ROIs: CC (rs= 0.005, 95% CI [-0.31, 0.32]), ILF (rs= 0.028, 95% CI [-0.29, 0.34]) or FOF (rs= -0.022, 95% CI [-0.30, 0.33]). Equally, there were no significant correlations between MD values STOP-IT SSRT in the CC (rs= -0.028, 95% CI [-0.29, 0.34]), ILF (rs= -0.16, 95% CI [-0.16, 0.45]) or FOF (rs= -0.15, 95% CI [-0.17, 0.44]). Likewise, there were no significant Spearman’s correlations between accuracy on the STOP-IT and FA values and in any of the ROIs: CC (rs= 0.19, 95% CI [-0.13, 0.48]), ILF (rs= -0.045, 95% CI [-0.27, 0.35]) and FOF (rs= -0.032, 95% CI [-0.29, 0.34]), or MD values in the CC (rs= -0.11, 95% CI [-0.21, 0.41]), ILF (rs= 0.017, 95% CI [-0.30, 0.33]) or FOF (rs= 0.082, 95% CI [-0.24, 0.39]). This study did not find support for the hypothesis that cumulative SRCs are associated with poorer performance on neurocognitive tests or with axonal injury as measured by FA and MD DTI metrics. Conclusion: The null findings suggest that there are no cumulative effects of SRCs. The current findings are inconsistent with previous cross-sectional research that indicates that there are long-term changes to diffusivity measures present after single SRCs as well as cumulative effects in contact sport athletes. Likewise they are at odds with evidence suggesting that after three SRCs neurocognitive performance can be affected. The study needs to be extended to include a larger sample to ensure the results are not due to low statistical power.

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[Truncated abstract] Controlled and automatic processing are broad categories, and how best to measure these constructs and their impact on functioning after mild traumatic brain injury (TBI) remains uncertain. The purpose of this thesis was to examine automatic and controlled processing aspects of attention after mild TBI using the Paced Auditory Serial Addition Task (PASAT) and event-related potentials (ERPs). The PASAT is one of the most frequently used tests to evaluate attentional functioning. It has been demonstrated to be a measure sensitive to both acute and longer-term effects of mild TBI, presumably due to demands for rapid processing and executive attentional control. ERPs provide a noninvasive neurophysiological index of sensory processing and cognitive functions and have demonstrated sensitivity to even minor cognitive dysfunction. The parameters provided by this functional technique may be those most likely to distinguish individuals with mild TBI from controls. Initially, it was hypothesized that successful novice PASAT performance requires the engagement of executive attention to establish novel controlled information processing strategies. Ten individuals who had suffered a mild TBI an average of 15.20 months previously were therefore expected to demonstrate processing abnormalities on the PASAT, relative to 10 healthy matched controls. Although the mild TBI group reported significant intensification of subjective symptoms since their injury, compared to controls, the mild TBI group provided a similar amount of correct PASAT responses. ... In the first experiment a visual search task consisting of an automatic detection and a controlled search condition was developed. In the second experiment the search task was performed concurrently with the PASAT task in a dual-task paradigm. In the mild TBI group, prior failure to establish more efficient forms of information processing with practice was found to significantly interfere with simultaneous performance of the PASAT task and the attention demanding condition of the search task. The pattern of impaired performance was considered to reflect a reduction in processing resources rather than a deficit in resource allocation. Dual-task performance in the control group was not associated with a large interference effect. In general, the results of this thesis suggest that individuals with mild TBI are impaired in their ability to progress from the stage of effortful controlled information processing to a stage of more efficient, automatic processing, and thus suffer a subtle attentional deficit. Following mild TBI, performance levels equivalent to controls may only be achieved with an abnormal expenditure of cognitive effort. As a result of the neuropathologic consequences of injury, individuals who have sustained a mild TBI are less able to benefit from practice, experience difficulty coping with simultaneous performance of secondary task, and are susceptible to distressing subjective symptomatology.