Auditory processing atypicalities for pure tones and complex speech sounds in Rett Syndrome – towards neuromarkers of disease progression

Abstract

Due to severe motor impairments and the lack of expressive language abilities seen in most patients with Rett Syndrome (RTT), it has proven extremely difficult to obtain accurate measures of auditory processing capabilities in this population. Here, we examined early auditory cortical processing of pure tones and more complex phonemes females with confirmed mutation of the MECP2 gene. We recorded high-density auditory evoked potentials (AEP), which allow for objective evaluation of the timing and severity of processing deficits along the auditory processing hierarchy. We compared AEPs of 12 females with RTT to those of 21 typically developing (TD) peers aged 4-21 years, interrogating the first four major components of the AEP (P1: 60-90ms; N1: 100-130ms; P2: 135-165ms; N2: 245-275ms). Atypicalities were evident in RTT at the initial stage of processing. Whereas the initial P1 showed increased amplitude to phonemic inputs relative to tones in TD participants, this modulation by stimulus complexity was absent in RTT. Interestingly, the subsequent N1 did not differ between groups, whereas the following P2 was hugely diminished in RTT, regardless of stimulus complexity. The N2 was similarly smaller in RTT, and did not differ as a function of stimulus type. The P2 effect was remarkably robust in differentiating between groups with near perfect separation between the two groups despite the wide age range of our samples. Given this robustness, along with the observation that P2 amplitude was significantly associated with RTT symptom severity, the P2 has the potential to serve as a biomarker of treatment efficacy. Significance statement: Our study points to dramatic reduction of the P2 component of the auditory evoked potential (AEP) as a potentially reliable biomarker of Rett Syndrome severity, with prospective applicability as an objective readout (neuromarker) of change in functional brain activity following therapeutic interventions administered in the context of clinical trials. 4 Compellingly, the reduction of P2 amplitude in patients with RTT mimics findings in animal models of RTT, providing a translational bridge between pre-clinical and human research. While there is some AEP research using frequency-specific tone-pip stimuli in RTT 1 2 syndrome, AEPs in response to more complex sounds such as speech, have not yet been 1 3 closely examined. Given that a key question in the RTT population is the extent to which non-or 1 4 minimally-verbal individuals can process and understand the spoken word, there is an 1 5 imperative to map cortical auditory processing abilities in these individuals. Recent reports by 1 6Key and colleagues suggest that there may be quite profound processing atypicalities for 1 7 speech tokens. They reported abnormalities in the AEP to speech stimuli in the latency range 1 8 from 200-500 ms post-stimulus when the responses to real words were compared to those to 1 9 non-words (Key et al., 2019) and from 250-450 ms and 450-750 ms when AEPs to a 2 0 participant's own name were compared to those ...