Objective We investigated the signature of the C9orf72 repeat expansion on resting-state cortical activity in asymptomatic individuals and explored its relationship with structural, gene expression, and cognitive measures.

Methods High-density resting-state EEG was compared between 90 asymptomatic family members of patients with familial amyotrophic lateral sclerosis (ALS), dichotomised into carriers of the pathological C9orf72 repeat expansion (N = 37) and non-carrier controls (N = 53). Periodic (oscillatory) and aperiodic (1/f) components of the power spectrum were analysed at the sensor- and source-level. Regional EEG power changes were correlated with C9orf72 expression from the Allen Human Brain Atlas, cortical MRI thickness and performance on the Complementary Cognitive ALS Screen (C-CAS).

Results Asymptomatic carriers exhibited significantly lower periodic power in the α and β frequency bands (10-30 Hz) across posterior cortical regions, including the parietal, occipital, and temporal lobes. The magnitude of this power reduction was associated with increased C9orf72 expression and reduced cortical thickness in the same regions. In carriers, reduced β-band power was associated with poorer performance on Visuoconstructive Immediate and Body Representation tasks. The aperiodic component of the EEG power did not differ between groups.

Interpretation In asymptomatic C9orf72 repeat expansion carriers, resting-state EEG reveals differences in oscillatory power in the posterior brain regions. The correlational findings suggest that C9orf72 repeat expansion may be involved in functional and structural changes in the posterior cerebral cortex, which may contribute to deficits in tasks requiring visuospatial processing.

The authors have declared no competing interest.

This study was financially supported by Stichting ALS Nederland, as part of the "GoALS" programme (AV2022-0004, AV2022-0005, AV2023-0006) and the ALS Society of Canada (Stevie Fever Foundation-ALS Canada Acceleration Grant).

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Ethics committee of University Medical Centre Utrecht gave ethical approval for this work.

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