The effects of cochlear hearing loss on the perception of pitch and temporal fine structure

Hearing impairment is a major health issue, affecting roughly 1 in 6 people. The disability is a barrier to communication with friends and family, makes everyday human contact difficult and embarrassing, and impairs enjoyment of music. With an increasingly aged population, the problem is likely to get worse. However, current hearing aids are limited in their effectiveness. In particular, they do not provide much help in understanding speech in the noisy environments we often encounter in our everyday lives.
Pitch is one of the primary auditory sensations. It underlies musical melody and some important speech information such as prosody (the stress pattern of an utterance). It is also important in helping us to separate out sounds that occur together, such as speech in a noisy background. Pitch is represented by nerve cells in the brain in part by the synchronized firing of electrical impulses.
Recently it has been suggested that this ?temporal code? may be disrupted in people with hearing loss. This may have an adverse effect, not only on pitch perception, but also on the ability to hear out the sound of interest from background noise (for example, a single speaker at a noisy party). Since this latter deficit is the main problem experienced by hearing-impaired listeners, it is important that we understand these limitations. In addition, hearing-impaired people often experience abnormal shifts in pitch that may greatly impair the enjoyment of music. These shifts may also be related to the temporal deficit.
Our research will investigate the causes and nature of these deficits. We will use behavioural tasks, in which human participants with normal and impaired hearing will make decisions regarding sounds presented to them over headphones. These tasks will measure perceptual limitations. We will also use a technique that involves recording the electrical activity of nerve cells in the brain as the participants listen to sounds, using electrodes attached to the head. This technique will measure the coding of pitch information in the brain, and will allow us to investigate the physiological basis of the deficits experienced by impaired listeners. It is hoped that the results will suggest new diagnostic techniques for determining the nature of the temporal coding impairment on an individual basis, and allow us to design more effective hearing aids that compensate for these deficits.

Sound waves that repeat over time are often heard as having a distinct pitch corresponding to the repetition rate. It is thought that pitch may be represented in part by a temporal code, based on the ability of auditory nerve fibres to synchronize their firing to a particular phase in the temporal fine structure (the individual pressure variations) of the sound. Recently, evidence has been presented suggesting that listeners with cochlear hearing loss (CHL, the most common form of hearing impairment) have a deficit in the processing of the fine structure information, and that this may contribute to their inability to understand speech in noisy environments. In addition, listeners with CHL often experience abnormal shifts in the pitch of tones, which can interfere with their enjoyment of music, and may also be related to deficits in temporal coding.
The experiments in the proposal are designed to investigate the underlying causes and nature of these deficits. During the investigation, we will address the following questions: What is the basis for the impaired performance of CHL listeners in tasks designed to measure fine structure processing? What is the upper rate limit for the use of temporal pitch information by CHL listeners? What are the underlying causes of the abnormal pitch shifts experienced by CHL listeners?
The experiments will test normally hearing listeners and listeners with mild-to-moderate CHL. A combination of behavioural and electrophysiological techniques will be employed. The behavioural techniques will use standard psychophysical procedures including discrimination tasks and pitch matching tasks. The electrophysiological measure is the ?frequency-following response? (FFR), recorded by attaching electrodes to the scalp. Periodic sounds presented to the ear generate a periodic pattern of fluctuations in the FFR that is related to phase locking at the level of the upper brainstem. Hence the FFR provides a unique measure of the neural temporal pitch code in humans.
The experiments are designed to distinguish between temporal coding deficits and other deficits associated with CHL, including poor frequency selectivity. The results will inform the development of diagnostic tests that measure temporal coding, and the design of the next generation of hearing prostheses.