# Time course of functional and structural brain network changes after mild traumatic brain injury

## TL;DR

This study shows that mild traumatic brain injury causes early functional brain network disruptions that recover over time, while structural changes remain stable and influence cognitive performance later.

## Contribution

The study provides longitudinal evidence of functional recovery and later structure-cognitive coupling after mild traumatic brain injury.

## Key findings

- Early functional disconnection between the default mode network and attention networks was observed in mild traumatic brain injury patients.
- Functional connectivity normalized by follow-up, but structural connectivity remained stable with diffusion metrics linking to cognitive outcomes later.
- Attentional performance in mild traumatic brain injury patients correlated with fractional anisotropy during the follow-up phase.

## Abstract

Traumatic brain injury is a progressive and potentially persistent pathophysiological condition affecting multiple cognitive domains. Large-scale brain networks, particularly those supporting attention, are closely linked to these cognitive impairments. Additionally, functional network connectivity, which captures statistical dependencies among network time courses, has revealed disrupted coupling between attentional networks. However, longitudinal evidence on how functional network connectivity changes over time and whether such changes are related to structural connectivity or cognitive outcomes remains limited. To address these gaps, this study investigated functional and structural connectivity among the default mode network, dorsal attention network, and ventral attention network in 41 patients with mild traumatic brain injury (mean age: 48.7 ± 15.8 years) and 35 matched controls (mean age: 44.6 ± 12.8 years). Both groups underwent brain imaging, clinical and neuropsychological assessments, and two cognitive tasks, including the Wisconsin Card Sorting Test and Digit Span Test, during the baseline (<1 month) and follow-up (>3 months) phases. Functional networks were defined using the Schaefer atlas and structural connectivity was constructed using these networks as nodes. Diffusion metrics, including fractional anisotropy, axial diffusivity, radial diffusivity, and mean diffusivity, were assessed. Clinically, symptom scores of depression, symptom severity, and quality of life improved over the three-month period (P < 0.001), whereas outcomes assessed by the Korean version of the Montreal Cognitive Assessment and the Frontal Assessment Battery showed limited change (P > 0.05). Cognitive performance was generally comparable between groups, except for a significant main effect of group in the backward Digit Span Test, with the mild traumatic brain injury group showing poorer performance than controls. In the baseline phase, functional network connectivity between the default mode network and dorsal and ventral attention network was significantly reduced in the mild traumatic brain injury group, correlating with impaired attentional control but not with working memory capacity. These disruptions were no longer observed at the follow-up assessment. Structural connectivity remained largely stable throughout the period. Diffusion metrics in controls were associated with attentional performance but not with working memory performance. Positive associations were observed between attentional performance and fractional anisotropy only during the follow-up phase in the mild traumatic brain injury group, possibly reflecting shifts in the association over time. In contrast, working memory performance was consistently linked to diffusion metrics in the mild traumatic brain injury group. Taken together, these findings highlight a temporal dissociation between early functional disconnection and later structure-cognitive coupling following mild traumatic brain injury and highlight the value of multimodal longitudinal imaging in understanding post-injury recovery.

Kim et al. report early (<1 month) functional disconnection between the default mode network and attention-related networks in mild traumatic brain injury, despite relatively stable structural connectivity. Functional network connectivity became comparable to controls at follow-up (>3 months), whereas fractional anisotropy became related to attentional performance.

Graphical AbstractFor image description, please refer to the figure legend and surrounding text.