Aging Effects on Suppression and Gain Reduction Estimates and Their Relationship to Speech Perception in Noise

Erica L Hegland, Purdue University

Abstract

Speech perception in noise is difficult, and it becomes more challenging as people get older. Many times, older adults will begin avoiding noisy situations, causing them to curtail their participation in social activities, which can lead to social isolation and a reduced quality of life. Difficulty understanding speech in noise even happens when older adults have audiological thresholds similar to those of younger adults, and it can happen as early as middle age. By better understanding the age-related changes underlying poor speech perception in noise, it is possible that quality of life could be improved for countless older adults. It is likely that a combination of causes results in poor speech understanding in noise. And it is possible that there are declines in hearing that are not captured by the traditional audiological examination. One of these such mechanisms is two-tone suppression, which occurs as a result of cochlear nonlinearity due to normal outer hair cell function. Two-tone suppression is a decrease in the amplitude of one tone in the presence of a second tone of a higher amplitude. Tones above and below another tone can cause suppression. It causes a nearly instantaneous decrease in cochlear gain in the region being suppressed. It is thought to improve speech perception - formants, or regions of higher energy, are thought to suppress regions of lower energy, resulting in an emphasis of the formants and improved speech perception. This may also cause the formants to stand out more in noise. If there is a decrease in outer hair cell function, this could decrease suppression. In fact, sensorineural hearing loss results in a decrease or loss of suppression. Suppression may also decrease with age, although there is disagreement about this in the literature. Another mechanism not evaluated in the clinic is the medial olivocochlear reflex (MOCR). This is a sound-elicited feedback loop that decreases cochlear gain after approximately a 25-ms delay. The MOCR is thought to improve speech-in-noise perception by perceptually decreasing the level of the background noise, thus improving the signal-to-noise ratio. This hypothesis has been challenged by several studies that have found no correlation between the MOCR and speech perception or have even found a negative correlation, where those with better speech perception in noise have a weaker MOC response. The MOCR may decrease in strength with age, although there is controversy in the literature about this as well. If suppression occurs while the MOCR is elicited, there is evidence from two physiological papers that suppression would decrease. If gain is reduced due to MOCR elicitation, there may be less gain to reduce further by suppression. The ability of suppression to decrease with MOCR elicitation may mean that healthy auditory systems can actively adapt to the changing acoustic environment. This idea of changes in suppression is called "adaptability" in the present thesis, and this idea was explored in Chapter 2. The younger adults did show significant adaptability, along with larger gain estimates and larger MOCR strength (gain reduction) estimates than the older adults. The older adults did not show adaptability. In Chapter 3, results from a small group of younger adults with self-reported noise exposure were compared to the results from the younger and older adults in Chapter 2. The younger adults had audiometric thresholds similar to or better than the younger adults in Chapter 2, yet they showed less gain, less gain reduction, less suppression, and a lack of adaptability. This provides evidence that changes in the auditory system can occur that are not captured in the traditional audiological examination. To examine whether suppression, MOCR strength, and/or adaptability correlate with speech perception in noise, the younger and older adults were tested on a measure of consonant perception in speech-shaped noise. The older adults performed poorer than the younger adults on conditions at 0 dB SNR and -5 dB SNR. Speech perception was correlated with the interaction of suppression and the MOCR (adaptability), but not with MOCR strength alone. Finally, individual differences in amount of suppression and gain were explored using an auditory-nerve model. Three different methods of estimating thresholds were used, and outer and inner hair cell gain were manipulated slightly to mimic a decrease in the endocochlear potential and cochlear synaptopathy. One method, where thresholds were based upon driven rate, produced adequate results, suggesting individual differences may be due to changes in cochlear health that may not be captured by the audiogram.

Degree

Ph.D.

Advisors

Strickland, Purdue University.

Subject Area

Audiology|Aging

COinS