Auditory cortex activation to natural speech and simulated cochlear implant speech measured with functional near-infrared spectroscopy

Abstract

The primary goal of most cochlear implant procedures is to improve a patient’s ability to discriminate speech. To accomplish this, cochlear implants are programmed so as to maximize speech understanding. However, programming a cochlear implant can be an iterative, labor-intensive process that takes place over months. In this study, we sought to determine whether functional near-infrared spectroscopy (fNIRS), a non-invasive neuroimaging method which is safe to use repeatedly and for extended periods of time, can provide an objective measure of whether a subject is hearing normal speech or distorted speech. We used a 140 channel fNIRS system to measure activation within the auditory cortex in 19 normal hearing subjects while they listed to speech with different levels of intelligibility. Custom software was developed to analyze the data and compute topographic maps from the measured changes in oxyhemoglobin and deoxyhemoglobin concentration. Normal speech reliably evoked the strongest responses within the auditory cortex. Distorted speech produced less region-specific cortical activation. Environmental sounds were used as a control, and they produced the least cortical activation. These data collected using fNIRS are consistent with the fMRI literature and thus demonstrate the feasibility of using this technique to objectively detect differences in cortical responses to speech of different intelligibility.

Cortical Activation Patterns Correlate with Speech Understanding After Cochlear Implantation

“…The two-dimensional interpolated T-statistic map was projected to the cortical surface and the cortex surface was colored based on the projected T-statistic values. This strategy is reasonable given our previously published work demonstrating that the responses measured with fNIRS derive from this region of the brain (Sevy et al 2010; Pollonini et al 2014). However, it is important to note that this plotting scheme has no impact on the individual or group statistical analyses performed in this manuscript.…”

“…Previously, our group demonstrated that fNIRS can detect speech-evoked activation of the LTL/STG, a core region of the auditory cortex in normal hearing and implanted participants. In addition, we found that adults with normal hearing exhibit greater cortical activation to speech than non-speech sounds (Sevy et al 2010; Pollonini et al 2014). Here, we expand this approach in participants with CIs to explore the use of fNIRS as an objective measure of speech perception.…”

“…This prediction was confirmed after fNIRS and fMRI demonstrated similar response patterns within the LTL/STG to auditory stimuli. We then validated the expanded fNIRS headset we use here by comparing measurements made using fNIRS and fMRI, and finding a similar pattern of responses within the LTL/STG to the various stimuli (Pollonini et al 2014). …”

“…The light from each source was received by neighboring detectors in the array, giving a potential of 96 source-detector pairings (or channels) on each side of the head. In order to optimally probe the cortex, we only analyzed the 31 channels in which the source-detector distance was 3.0-3.3 cm (Sevy et al 2010; Beauchamp et al 2011; Pollonini et al 2014). …”

“…The analysis of the fNIRS data was performed in a similar fashion to our previous publication (Pollonini et al 2014). In essence, this involved removing channels from the analysis that did make good contact with the scalp, removing motion artifacts, bandpass filtering to the time course of the stimuli, and the calculation of changes in oxygenated and deoxygenated hemoglobin using the modified Beer-Lambert Law.…”