Cortical Activation Patterns Correlate with Speech Understanding After Cochlear Implantation

Abstract

Objectives Cochlear implants are a standard therapy for deafness, yet the ability of implanted patients to understand speech varies widely. To better understand this variability in outcomes, we used functional near-infrared spectroscopy (fNIRS) to image activity within regions of the auditory cortex and compare the results to behavioral measures of speech perception. Design We studied 32 deaf adults hearing through cochlear implants and 35 normal-hearing controls. We used fNIRS to measure responses within the lateral temporal lobe and the superior temporal gyrus to speech stimuli of varying intelligibility. The speech stimuli included normal speech, channelized speech (vocoded into 20 frequency bands), and scrambled speech (the 20 frequency bands were shuffled in random order). We also used environmental sounds as a control stimulus. Behavioral measures consisted of the Speech Reception Threshold, CNC words, and AzBio Sentence tests measured in quiet. Results Both control and implanted participants with good speech perception exhibited greater cortical activations to natural speech than to unintelligible speech. In contrast, implanted participants with poor speech perception had large, indistinguishable cortical activations to all stimuli. The ratio of cortical activation to normal speech to that of scrambled speech directly correlated with the CNC Words and AzBio Sentences scores. This pattern of cortical activation was not correlated with auditory threshold, age, side of implantation, or time after implantation. Turning off the implant reduced cortical activations in all implanted participants. Conclusions Together, these data indicate that the responses we measured within the lateral temporal lobe and the superior temporal gyrus correlate with behavioral measures of speech perception, demonstrating a neural basis for the variability in speech understanding outcomes after cochlear implantation.

Multisensory Integration in Cochlear Implant Recipients

“…These findings seem to align with studies in other imaging modalities that correlate broader cortical activation (including across both hemispheres) to both auditory (Olds et al 2016) and visual stimuli (Doucet et al 2006) with lower CI outcomes. Instead, more focal activity (either intramodal or crossmodal) may indicate more efficient, localized processing without the need for more expansive cortical recruitment.…”

Measuring Cortical Activity During Auditory Processing With Functional Near-Infrared Spectroscopy

“…Hence, a major step was to utilize multi-channel fNIRS systems which allow the possibility of measuring cortical hemodynamics from several cortical locations and construct topographic activity maps (29–32). Recently, researchers have developed a 140-channel fNIRS system to enhance local sensitivity – measured with several source–detector distances over overlapping regions to enable three-dimensional image reconstructions (33,34). The method resembles the topographic mapping techniques familiar with fMRI measurements.…”

Functional near‐infrared spectroscopy as a tool for assessing speech and spoken language processing in pediatric and adult cochlear implant users

“…The spatial resolution, typically estimated at 1–2 cm (Ferrari & Quaresima, ; Scholkmann et al, ), enables localization of cortical responses with reasonable precision, and this can be further manipulated through variations in the arrangement and density of sources and detectors. For example, increasing the density of channels over a target area achieves finer sampling of the cortex (e.g., Olds et al, ; Pollonini et al, ). Moreover, it is possible to generate three‐dimensional images of the optical properties of the brain given a sufficient number of sources and detectors (Eggebrecht et al, ).…”

“…Because the Pollonini et al () study did not involve cochlear implant users, our next move was to test adult implant users with a similar paradigm (Olds et al, ), using comparable stimulus conditions and fNIRS instrumentation. Our goal in designing this study was to better understand the variability in speech perception across cochlear implant users with different speech perception abilities.…”