# Association Between Intraindividual Variability in Cognitive Performance and White Matter Organisation in Chronic Mild Traumatic Brain Injury

## TL;DR

This study shows that chronic mild traumatic brain injury is linked to unstable cognitive performance and changes in brain white matter structure.

## Contribution

The study is the first to combine smartphone-based EMA and fixel-based MRI analysis to link cognitive instability with white matter changes in chronic mTBI.

## Key findings

- mTBI patients showed higher intraindividual variability in cognitive performance compared to controls.
- Daily cognitive instability in mTBI correlated with daily symptom fluctuations and reduced white matter organization in the left superior longitudinal fasciculus-II.
- Trial-by-trial cognitive variability was associated with white matter alterations in mTBI patients.

## Abstract

Mild traumatic brain injury (mTBI) can result in persistent cognitive deficits (particularly in attention, processing speed, and working memory), even years after the injury. The majority of behavioural studies have focussed on averaged cognitive performance scores, such as average reaction time or accuracy scores after mTBI. However, less is understood about how mTBI affects intraindividual variability (IIV) in cognitive performance across repeated sessions or measurement occasions 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 behavioural testing session (incorporating the Rivermead Post‐Concussion Symptom Questionnaire and a computerised processing speed task) and a multi‐shell diffusion MRI scan. This was followed by a 30‐day ecological momentary assessment (EMA) protocol using a smartphone app which measured symptoms and cognitive performance on a daily basis. Our results revealed that mTBI patients exhibited higher IIV than controls in both single‐session 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.

This study combined smartphone‐based ecological momentary assessment (EMA) and fixel‐based analysis (FBA) of diffusion MRI data to examine intraindividual variability (IIV) in cognitive performance and its neural correlates in chronic mild TBI. Our findings revealed persistent cognitive instability linked to daily symptom fluctuations and structural changes in the left superior longitudinal fasciculus‐II in mTBI patients.