Cross-modal impact of auditory experience on visual cortical entrainment in children who are hard-of-hearing.

Cross-modal neuroplasticity is a well-studied phenomenon whereby in the absence of sensory input (e.g., in individuals who are deaf or blind), the sensory cortices of the affected modality are "taken over" by the unaffected sensory systems. This neuroplasticity is often coupled with changes in the dynamics of the unaffected primary sensory cortices themselves. Nonetheless, whether these cross-modal changes extend to those with mild-to-severe sensory loss where sensory input is degraded but not absent, as is the case for children who are hard-of-hearing (CHH), remains poorly understood. Visual entrainment, sometimes referred to as the visual steady-state response, is the process by which the primary visual cortices entrain to the frequency of a rhythmic exogenous visual stimulus. Importantly, visual entrainment dynamics are thought to serve as an important proxy for how visual stimuli are processed in the brain. The current study sought to identify the impact of mild-to-severe hearing loss and individual differences in auditory experience on visual entrainment dynamics in youth. To this end, CHH and an age- and sex-matched group of children with normal hearing (CNH) were presented with a visual entrainment stimulus that flickered at 15 Hz during magnetoencephalography. Neural responses to the fundamental (15 Hz) and first harmonic (30 Hz) were imaged using beamforming, and the power envelope of the peak responses was extracted as a function of time and submitted to linear mixed effects modeling. We found a significant group-by-time interaction, whereby CHH exhibited a stronger increase in power during entrainment than CNH, suggesting altered entrainment dynamics. We also ran whole-brain correlations between 15 and 30 Hz neural responses and hearing aid use in CHH, controlling for degree of hearing loss. We found a significant negative relationship between hearing aid use and activity in the left primary auditory cortex and left lateral parietal cortex, such that CHH who consistently wore their hearing aids showed the smallest amount of cross-modal neuroplasticity in these regions. Crucially, reductions in cross-modal neuroplasticity in the left lateral parietal cortex were related to better verbal outcomes. These data provide important new information regarding how consistent auditory experience may serve to normalize the neural dynamics serving sensory processing, and how these changes may cascade into improvements in verbal ability in CHH.