Cross-Modal Functional Reorganization of Visual and Auditory Cortex in Adult Cochlear Implant Users Identified with fNIRS.

Martin G. Bleichner,Pascale Sandmann,Ling-Chia Chen,Stefan Debener,Jeremy D. Thorne

doi:10.1155/2016/4382656

Abstract

Cochlear implant (CI) users show higher auditory-evoked activations in visual cortex and higher visual-evoked activation in auditory cortex compared to normal hearing (NH) controls, reflecting functional reorganization of both visual and auditory modalities. Visual-evoked activation in auditory cortex is a maladaptive functional reorganization whereas auditory-evoked activation in visual cortex is beneficial for speech recognition in CI users. We investigated their joint influence on CI users' speech recognition, by testing 20 postlingually deafened CI users and 20 NH controls with functional near-infrared spectroscopy (fNIRS). Optodes were placed over occipital and temporal areas to measure visual and auditory responses when presenting visual checkerboard and auditory word stimuli. Higher cross-modal activations were confirmed in both auditory and visual cortex for CI users compared to NH controls, demonstrating that functional reorganization of both auditory and visual cortex can be identified with fNIRS. Additionally, the combined reorganization of auditory and visual cortex was found to be associated with speech recognition performance. Speech performance was good as long as the beneficial auditory-evoked activation in visual cortex was higher than the visual-evoked activation in the auditory cortex. These results indicate the importance of considering cross-modal activations in both visual and auditory cortex for potential clinical outcome estimation.

Highlights

Modern cochlear implants (CI) allow deafened adults to partially regain their hearing ability [1]
Two-way follow-up analysis of variance (ANOVA) were performed separately for the CI users and the normal hearing (NH) controls to follow up the group main effect
The present functional near-infrared spectroscopy (f NIRS) study observed residual cross-modal reorganization of the auditory cortex of CI users, which was possibly induced during the period of deafness prior to implantation

Summary

Introduction

Modern cochlear implants (CI) allow deafened adults to partially regain their hearing ability [1]. Patterns of cortical plasticity caused by deafness on one hand and partially restored input on the other may help to explain the large degree of variability. The lack of auditory input has been shown to induce reorganization of the auditory cortex for visual processing in deaf-born individuals [2] and in postlingually deafened individuals [3]. On the other hand it has been shown that successful speech perception depends on the adaptive plasticity to the new electrical input from the CI [4]. It is of clinical relevance to understand whether postimplantation adaptation in postlingually deafened adults completely reverses the preimplantation reorganization of auditory cortex and, if not, whether residual preimplantation reorganization of auditory cortex is beneficial or detrimental for speech perception with a CI

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Discussion

Conclusion