Viji Easwar, PhD – Slide of the Week

Viji Easwar Slide of the Week

Title: Cortical processing of interaural time differences in children with normal hearing and bilateral cochlear implants

Legend: A. One of the cues that we use to locate sounds in space is the difference in arrival time of sound between our ears called interaural time differences (ITDs). ITD detection is affected by prelingual deafness and could be facilitated during development by early provision of  bilateral cochlear implants (CIs) with short inter-implant delays. We compared cortical processing of ITDs in 16 children with bilateral CIs provided in the same surgery (chronological age = 6.13±0.79 years; age of implantation = 2.10±0.97 years; time-in-sound = 4.29±0.97 years) and 16 hearing-age matched peers (chronological age/time-in-sound = 5.24±0.95 years) to assess if simultaneous provision of bilateral CIs promotes development of cortical ITD processing. B. Children with normal hearing and children with bilateral CIs show similar right hemispheric dominance during ITD processing.  C. ITD processing in children with normal hearing is characterized by reduced activity to stimuli leading from the same side of each auditory cortex (ipsilateral, I). D. In contrast to children with normal hearing, sensitivity to ITDs is significantly reduced in children with bilateral CIs. In summary, simultaneous provision of bilateral CIs promotes the expected right hemispheric dominance during ITD processing, however, it does not appear to support the development of typical cortical ITD detection.

Citation: Easwar V, Yamazaki H, Deighton M, Papsin B, Gordon K. (2017). Cortical representation of interaural time difference is impaired by deafness in development: evidence from children with early long-term access to sound through bilateral cochlear implants provided simultaneously. Journal of Neuroscience, 37(9), 2349-2361.

Abstract: Accurate use of interaural time differences (ITDs) for spatial hearing may require access to bilateral auditory input during sensitive periods in human development. Providing bilateral cochlear implants (CIs) simultaneously promotes symmetrical development of bilateral auditory pathways but does not support normal ITD sensitivity. Thus, although binaural interactions are established by bilateral CIs in the auditory brainstem, potential deficits in cortical processing of ITDs remain. Cortical ITD processing in children with simultaneous bilateral CIs and normal hearing with similar time-in-sound was explored in the present study. Cortical activity evoked by bilateral stimuli with varying ITDs (0, ±0.4, ±1 ms) was recorded using multichannel electroencephalography. Source analyses indicated dominant activity in the right auditory cortex in both groups but limited ITD processing in children with bilateral CIs. In normal-hearing children, adult-like processing patterns were found underlying the immature P1 (∼100 ms) response peak with reduced activity in the auditory cortex ipsilateral to the leading ITD. Further, the left cortex showed a stronger preference than the right cortex for stimuli leading from the contralateral hemifield. By contrast, children with CIs demonstrated reduced ITD-related changes in both auditory cortices. Decreased parieto-occipital activity, possibly involved in spatial processing, was also revealed in children with CIs. Thus, simultaneous bilateral implantation in young children maintains right cortical dominance during binaural processing but does not fully overcome effects of deafness using present CI devices. Protection of bilateral pathways through simultaneous implantation might be capitalized for ITD processing with signal processing advances, which more consistently represent binaural timing cues. SIGNIFICANCE STATEMENT: Multichannel electroencephalography demonstrated impairment of binaural processing in children who are deaf despite early access to bilateral auditory input by first finding that foundations for binaural hearing are normally established during early stages of cortical development. Although 4- to 7-year-old children with normal hearing had immature cortical responses, adult patterns in cortical coding of binaural timing cues were measured. Second, children receiving two cochlear implants in the same surgery maintained normal-like input from both ears, but this did not support significant effects of binaural timing cues in either auditory cortex. Deficits in parieto-occiptal areas further suggested impairment in spatial processing. Results indicate that cochlear implants working independently in each ear do not fully overcome deafness-related binaural processing deficits, even after long-term experience.

About the Investigator: Viji’s research focuses on the development of hearing in children with or without hearing loss. In particular, she is interested in investigating the effects of age, auditory deprivation, and hearing prostheses like hearing aids or cochlear implants on how sounds, particularly speech stimuli, are encoded by the auditory system using electroencephalography (EEG) techniques. In addition, she is interested in evaluating how these factors affect the ability of the auditory system to decipher where sounds are located in space. The goal of her research is to develop evidence that can help inform intervention strategies in children with hearing loss and extend the use of EEG techniques to clinical practice to help clinicians (audiologists) evaluate intervention outcomes in children.

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