Auditory steady-state response deficits in Fragile X Syndrome implicate deficits in stimulus representation maintenance and GABAergic modulation

BackgroundFragile X Syndrome (FXS) is a rare, neurodevelopmental disorder caused by a mutation to the Fragile X messenger ribonucleoprotein 1 (Fmr1) gene and characterized by sensory processing abnormalities and sensitivities, including neural auditory oscillatory disruptions and reduced neural entrainment to chirp stimuli. The present study aims to evaluate the 40 Hz auditory steady state response (ASSR) in FXS to evaluate stimulus representation maintenance in FXS. MethodsAdolescents and adults (N = 67; 34 FXS and 33 age, sex-matched typically developed controls (TDC)) completed a 40 Hz auditory steady state task during electroencephalography (EEG). Time-frequency analyses using Morlet wavelets were completed to evaluate intertrial phase coherence (ITC) and event-related spectral perturbation (ERSP), including characterization of the transient and sustained components of the 40 Hz ASSR. ResultsBoth ITC (p = .003) and ERSP (p = .004) at 40 Hz were reduced for FXS compared to TDC. Interestingly, TDC exhibited a significantly elevated early, transient component (100 - 400 ms) which reduced in both ITC and ERSP during transition to the sustained component (650 - 3000 ms) whereas FXS were consistently reduced across the ASSR suggesting a reduced ability for FXS to mount a transient response. ConclusionsIndividuals with FXS exhibit robust reductions in magnitude and temporal precision of neural entrainment to the steady state stimulus. The reduced ability to mount a transient response may represent reduced GABAergic modulation where the overall reduction in ITC and ERSP may reflect reduced excitatory/inhibitory balance between NMDA and GABAergic input.