Divergent aperiodic slope and alpha dynamics expose cortical excitability gradients in fragile X syndrome.

Background: Fragile X syndrome (FXS) is characterized by cortical hyperexcitability, a core neurophysiological feature that contributes to sensory hypersensitivity, cognitive dysfunction, and other disabling symptoms. This disruption in excitatory-inhibitory balance is a key pharmacological target, yet reliable biomarkers to quantify it noninvasively remain limited. Spectral slope, derived from the aperiodic component of the EEG power spectrum, has emerged as a potential index of cortical excitability. Here, we evaluated spectral slope and theta-alpha peak frequency in individuals with FXS to assess their utility as candidate neurophysiological biomarkers.

Methods: Five minutes of resting state EEG data were collected from 70 subjects with FXS (mean age 20.5 ± 10 years; 32 females) and 71 age-matched controls (mean age 22.2 ± 10.7 years; 30 females). The Spectral Parameterization toolbox (SpecParam) was used to separate periodic and aperiodic components of the source localized power spectra and characterize aperiodic slope and theta-alpha peak frequency.

Results: Statistical modeling of aperiodic slope revealed a significant two-way interaction between sex and diagnostic group, but no interaction with brain lobe. Among males, the aperiodic slope was significantly decreased in FXS, indicating greater cortical excitability, compared to typically developing controls (TDC), whereas no difference was observed between FXS and TDC females. For peak alpha frequency, statistical modeling identified significant two-way interactions between sex and diagnostic group, and between brain lobe and diagnostic group, but no significant three-way interaction.

Limitations: This study is limited by the absence of non-invasive measures of cortical fragile X mental retardation protein (FMRP). Additionally, participants were not stratified by mosaic status and FMRP levels were not quantified, which could affect variability and interpretation.

Conclusion: Compared to traditional band-limited power measures, aperiodic slope provides a more direct and validated index of excitation-inhibition balance. Our findings of reduced aperiodic slope in male subjects with FXS align with preclinical circuit-level evidence of increased excitability in FXS and are consistent with previous findings of reduced individual alpha peak frequency, supporting with thalamocortical dysrhythmia models of FXS. Importantly, aperiodic slope measurements can be applied directly to various modalities of local field potential data, enabling more robust cross-species and translational comparisons.