Objective Measures in Adults with Unilateral Hearing Loss and Normal Hearing: A Speech in Noise Study

Background: Individuals with unilateral severe to profound hearing loss (UHL) have known deficits, particularly when background noise degrades the incoming signal. Comprehension of connected speech depends on left lateralized cortical language areas that support semantic processing. Very little is known about the cognitive processes used by UHLs when attempting to recognize speech in challenging environments.

Objective: To determine whether individuals with UHL engage cortical areas differently compared to normal hearing (NH) individuals when listening to degraded speech.

Methods: UHL (right ear deaf) and NH individuals participated in a behavioral session to assess accuracy for degraded speech. Participants listened to sentences that varied in 1) predictability of the last word based on semantics (low or high predictability), and 2) intelligibility due to spectral degradation (4, 8 or 16 band noise-vocoded speech). A functional magnetic resonance imaging (fMRI) protocol used an event-related design with stimuli similar to the behavioral task.

Results: Accuracy was lower in UHL versus NH for the least intelligible (4-band) and moderately intelligible (8-band) sentences but similar for 16-band (highly intelligible) stimuli. For both groups, predictability affected accuracy; low predictability sentences were less accurate than high predictability sentences. Imaging data showed group differences in semantic control regions located in inferior frontal cortex and inferior parietal cortex based on the semantic context of degraded speech. In contrast, there were no significant group differences in nearly all auditory region responses despite uniformly smaller responses in UHL for less intelligible sentences. Both groups appeared to process speech in early auditory regions similarly irrespective of spectral degradation. UHL, however, showed larger amplitude responses in several areas of dorsolateral prefrontal cortex (DLPFC). No predictability effects (low or high) were evident in the imaging data.

Conclusions: In UHL, reduced activity in key semantic processing areas in inferior frontal and inferior parietal cortex potentially parallels the behavioral findings of lower accuracies for less intelligible sentences. The group differences in DLPFC could reflect cognitive efforts in UHL to extract semantic information when speech is less intelligible. Increased knowledge of the effects of UHL on cognitive processes may inform studies of treatment options in those with unilateral input.