Functional Near-Infrared Spectroscopy as a Measure of Listening Effort in Older Adults Who Use Hearing Aids

Abstract

Listening effort may be reduced when hearing aids improve access to the acoustic signal. However, this possibility is difficult to evaluate because many neuroimaging methods used to measure listening effort are incompatible with hearing aid use. Functional near-infrared spectroscopy (fNIRS), which can be used to measure the concentration of oxygen in the prefrontal cortex (PFC), appears to be well-suited to this application. The first aim of this study was to establish whether fNIRS could measure cognitive effort during listening in older adults who use hearing aids. The second aim was to use fNIRS to determine if listening effort, a form of cognitive effort, differed depending on whether or not hearing aids were used when listening to sound presented at 35 dB SL (flat gain). Sixteen older adults who were experienced hearing aid users completed an auditory n-back task and a visual n-back task; both tasks were completed with and without hearing aids. We found that PFC oxygenation increased with n-back working memory demand in both modalities, supporting the use of fNIRS to measure cognitive effort during listening in this population. PFC oxygenation was weakly and nonsignificantly correlated with self-reported listening effort and reaction time, respectively, suggesting that PFC oxygenation assesses a dimension of listening effort that differs from these other measures. Furthermore, the extent to which hearing aids reduced PFC oxygenation in the left lateral PFC was positively correlated with age and pure-tone average thresholds. The implications of these findings as well as future directions are discussed.

Keywords: aging; hearing loss; n-back task; neuroimaging; working memory.

Figure 1.

The average audiogram of 16 participants. Error bars show standard errors of the mean.

Figure 2.

The fNIRS sensor pad (bottom) that we used in this study was secured to the forehead like a headband (top). The system contains four light sources and 10 light detectors. Each light source emits photons of two wavelengths into the scalp. Some of the photons reflect at the scalp-air interface, while others are absorbed by extracranial or intracranial tissues, and the remainder are scattered through the layers of cortex beneath the skull. Some of these scattered photons will follow a banana-shaped path to an adjacent light detector. This source-detector pair constitutes a channel. The proportions of photons of each wavelength that reach this adjacent light detector are used to calculate the concentration of oxygenated and deoxygenated hemoglobin in that channel. The top of the figure shows one source-detector pair (channel) as an example of this concept, although the same concept applies to all 16 channels, shown at the bottom of the figure (numbered in white).

Figure 3.

Overall PFC oxygenation as a function of n-back condition (working memory demand) and modality, averaged over the aided and unaided conditions. Error bars show standard errors of the mean. Oxygenation is reported in relative units: the device used was unable to determine the absolute concentration of oxygen in the PFC in each condition, but rather only the concentration of oxygen in each condition relative to other conditions (with oxygenation of zero representing the baseline). For both the visual and auditory n-back tasks, overall PFC oxygenation increased with task demand until the 2B, at which point it leveled.

Figure 4.

Overall PFC oxygenation during the auditory n-back task as a function of n-back condition and hearing aid use. Error bars show standard errors of the mean. The lack of an interaction between modality and hearing aid use on overall PFC oxygenation suggested that hearing aids did not reduce overall PFC oxygenation during the auditory n-back task.

Figure 5.

Reaction time during the auditory n-back task as a function of n-back condition and hearing aid use. Error bars show standard errors of the mean. The lack of an interaction between modality and hearing aid (visual data not shown) on reaction time suggested that hearing aids did not reduce reaction time during the auditory n-back task.

Figure 6.

Self-reported effort during the auditory n-back task as a function of n-back condition and hearing aid use. Error bars show standard errors of the mean. The lack of an interaction between modality and hearing aid use (visual data not shown) on self-reported effort suggested that hearing aids did not reduce self-reported effort during the auditory n-back task.

Figure 7.

Percent-correct during the auditory n-back task as a function of n-back condition and hearing aid use. Error bars show standard errors of the mean. The lack of an interaction between modality and hearing aid use (visual data not shown) on percent-correct suggested that hearing aids did not increase percent-correct during the auditory n-back task.

Figure 8.

Oxygenation during the auditory n-back task as a function of n-back condition and hearing aid use, in the L-LPFC (top-left), R-LPFC (top-right), L-MPFC (bottom-left), and R-MPFC (bottom-right). Error bars show standard errors of the mean. The lack of an interaction between modality and hearing aid use (visual data not shown) on oxygenation in all PFC subregions suggested that hearing aids did not reduce oxygenation in any PFC subregion during the auditory n-back task. R-MPFC = right medial prefrontal cortex; L-MPFC = left medial prefrontal cortex; R-LPFC = right lateral prefrontal cortex; L-LPFC = left lateral prefrontal cortex.

Figure 9.

During the auditory n-back, hearing aid benefit for oxygenation in the L-LPFC increased with participant PTA (left panel) and participant age (right panel). Shaded regions indicate the 95% confidence interval of the regression line. L-LPFC = left lateral prefrontal cortex; HA benefit = hearing aid benefit for PFC oxygenation (unaided oxygenation -- aided oxygenation).