Intraindividual variability in cognitive performance is associated with white matter organisation in chronic mild traumatic brain injury

Mild traumatic brain injury (mTBI) or concussion, can result in persistent cognitive deficits (particularly in attention, processing speed, and working memory), even years after the injury. Majority of studies have focussed on averaged cognitive performance scores, such as averaged accuracy, reaction time or latency scores. However, less is understood about how mTBI affects intraindividual variability (IIV) in cognitive performance over time. In this study, we investigate IIV in cognitive performance in chronic mTBI patients (n = 11) relative to healthy controls (n = 22). Participants underwent a single testing session (incorporating the Rivermead Post-Concussion Symptom Questionnaire and a processing speed task) and a multi-shell diffusion MRI scan. This was followed by a 30-day ecological momentary assessment (EMA) assessment using a mobile app, which measured symptoms and cognitive performance on a daily basis. Our results revealed that mTBI patients exhibited higher IIV than controls in both trial-by-trial and daily EMA measures. Higher daily IIV in cognitive performance coincided with higher daily fluctuations in post-concussive symptoms. Additionally, mTBI patients showed reduced white matter organization, as indexed by fixel-wise fibre density and fibre density cross-section, in the left superior longitudinal fasciculus-II compared to controls. Finally, trial-by-trial IIV was positively associated with white matter alterations in the SLF-II in mTBI. Our findings suggest that mTBI results in dynamic performance deficits that persist into the chronic phase of injury. In addition, the white matter organization of a major fronto-parietal tract seems to play an important role in supporting the consistency of cognitive performance over time, highlighting its potential as a biomarker for understanding cognitive dynamics in healthy adults and clinical populations.