Abstract
Disruptions in functional connectivity and dysfunctional brain networks are considered to be a neurological hallmark of neurodevelopmental disorders. Despite the vast literature on functional brain connectivity in typical brain development, surprisingly few attempts have been made to characterize brain network integrity in neurodevelopmental disorders. Here we used resting-state EEG to characterize functional brain connectivity and brain network organization in eight males with fragile X syndrome (FXS) and 12 healthy male controls. Functional connectivity was calculated based on the phase lag index (PLI), a non-linear synchronization index that is less sensitive to the effects of volume conduction. Brain network organization was assessed with graph theoretical analysis. A decrease in global functional connectivity was observed in FXS males for upper alpha and beta frequency bands. For theta oscillations, we found increased connectivity in long-range (fronto-posterior) and short-range (frontal-frontal and posterior-posterior) clusters. Graph theoretical analysis yielded evidence of increased path length in the theta band, suggesting that information transfer between brain regions is particularly impaired for theta oscillations in FXS. These findings are discussed in terms of aberrant maturation of neuronal oscillatory dynamics, resulting in an imbalance in excitatory and inhibitory neuronal circuit activity.
Highlights
Fragile X syndrome (FXS) is the most common inherited neurodevelopmental disorder caused by a single gene defect, and provides a unique opportunity to study the neurobiological mechanisms of brain development and cognitive function
Functional connectivity was lower in FXS individuals for alpha oscillations, but these differences were only significant for the upper alpha spectral band (10–13 Hz), t(18) = 22.56, p = .020, gp2 =
We only observed a negative correlation between phase lag index (PLI) and spectral power of Beta oscillations (13–30 Hz), r = 2.725, p = .04, which could possibly have underestimated the functional connectivity in the Beta band in FXS individuals
Summary
Fragile X syndrome (FXS) is the most common inherited neurodevelopmental disorder caused by a single gene defect, and provides a unique opportunity to study the neurobiological mechanisms of brain development and cognitive function. The neurobiological hallmark of FXS is the silencing of a single gene (FMR1) located on the X-chromosome [1,2], resulting in reduced or absent levels of its gene product – the fragile X mental retardation protein (FMRP) [3]. Both humans and rodents with the FXS full mutation consistently display an excess of long and thin dendritic spines, resembling immature cortical networks [4,5,6,7]. Disturbed glutamatergic and GABAergic activity is argued to disrupt these neurobiological processes, resulting in cortical hyperexcitability FXS [10,15]
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