One of the most common complaints of adults visiting an audiology clinic is difficulty in understanding speech in the presence of background noise. Performance in noise is often attributed to sensorineural hearing loss. However, there are many individuals with normal hearing who report difficulty hearing in background noise, which demonstrates how inadequate a typical audiometric evaluation is in evaluating the demands of speech comprehension in complex listening situations. It is also unclear which anatomical structures are responsible for our performance in background noise. Discovering the roles of inner hair cells (IHCs), outer hair cells (OHCs), and auditory neurons in speech understanding in quiet and in the presence of background noise is currently a hot topic in the scientific community.
Previous studies have suggested that the eighth cranial nerve, or Auditory Nerve (AN), may play a role in speech understanding in the presence of background noise. In Temporary Threshold Shift (TTS) studies in animals (Kujawa. J Neurosci 2009;29:14077-85 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2812055/), noise exposure has been shown to cause temporary damage to OHCs (as measured by Distortion Product Otoacoustic Emissions [DP-OAEs]) and permanent damage to the AN (measured by a decrease in the wave I amplitude of the Auditory Brainstem Response [ABR]). The authors of these studies speculate that a decrease in AN activity would affect speech in noise performance (Furman. J Neurophysiol 2013;110:577-86 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3742994/). However, measuring speech in noise among humans is a difficult task.
Our group recently presented preliminary data https://www.researchgate.net/publication/294732622_Speech_Detection_in_the_Presence_Of_Background_Noise_Is_Effected_by_both_Spiral_Ganglion_and_Outer_Hair_Cell_Function in this realm at the 2016 annual meetings of the Association for Research in Otolaryngology, the American Auditory Society, and the American Academy of Audiology. The aim of our study was to investigate the roles that OHCs and the AN play in audition in quiet and performance in noise. Participants included 40 English speaking adults (mean 45 years old) recruited from our audiology clinic. All subjects underwent an audiological evaluation that included tympanometry, air and bone conduction thresholds, speech reception thresholds (SRT), and word recognition in quiet (WR-Q) using recorded NU6 word lists. These measures were performed for a single ear and stimuli were presented at 0, 10, 20, and 40 dB SL to control for hearing loss. OHC function was evaluated by measuring DP-OAE amplitudes and thresholds. AN function was measured by the amplitude, latency, and threshold of wave I of the ABR.
Our preliminary findings revealed that those who exhibited better word recognition in noise were younger people with better hearing, more robust OHC function (both DPOAE amplitude and DPAOE threshold), and longer spiral ganglion absolute latencies and higher thresholds. Unlike animal studies, wave I amplitude failed to be a reliable metric of AN function in humans (ABR amplitudes are known to be notoriously variable in awake human subjects). Rather, AN latency and threshold produced repeatable measurements of AN function. However, our preliminary results of AN function were not what we had expected. We found that people who performed poorer in background noise exhibited a shorter wave I latency and a lower wave I threshold, which on the surface was the opposite of what we expected based on animal models. However, this discrepancy is likely due to the dysfunction of the tuning properties of the OHCs and AN. In the aforementioned animal models, the experimental model exhibited OHC function but a decrease in wave I amplitude. Therefore, it appears that normal OHC function is required for the expected lower wave I amplitude that would also exhibit a longer wave I latency and higher wave I threshold. In our current study, both OHCs and AN were abnormal in persons exhibiting poorer performance in background noise. Based on other animal studies, OHC damage should result in a decrease in wave I latency and a lowering of wave I threshold, which is what we see in our human data. These preliminary results suggest that both OHCs and the AN contribute to speech recognition in the presence of background noise. However, OHC damage may play a primary role in hearing difficulties in this complex listening situation.
Both of these cell types likely played a role in word recognition in quiet at or near threshold. Older people who exhibited poorer audiometric thresholds, diminished OHC responses, and abnormal (shorter, see above) AN latencies also exhibited better word recognition at 0 and 10 dB SL than younger persons with better PTAs. This suggests that people with sensorineural hearing loss are more sensitive to speech at or near thresholds than their normal hearing counterparts. This difference may also be due to OHC damage, which results in decreased tuning of the acoustic signal and in loudness recruitment (hyperacusis) that appears to occur at the level of the eighth cranial nerve.
Considering the prevalence of people who report difficulty understanding speech in background noise, a better understanding of the roles that OHCs and ANs play in hearing and hearing loss has implications for the evaluation, education, and treatment of these individuals. An improved understanding can help hearing care providers in making more objective evaluations of hearing in noise and in offering better counseling and treatment. For example, the selection of more advanced hearing aid technologies, such as active compression, interaural synchronization or adaptive directional microphones, could improve the outcomes for these patients. Finally, future biotechnological therapies aimed at regenerating IHCs, OHCs, and AN could be employed if better assessments of these cell types existed.
Speech Detection in the Presence Of Background Noise Is Affected by both Spiral Ganglion and Outer Hair Cell Function
Hoben R, Easow G, Nyatepee-Coo E, et al.
Preliminary data presented at the annual meetings of the Association for Research in Otolaryngology, the American Auditory Society, and the American Academy of Audiology