Cortical determinants of human auditory cognition

Hearing loss due to ear damage eventually affects half the population, is a major obstacle to work and home life, and a drain on the UK economy. If you ask anyone with common hearing loss what is wrong, they will NOT say 'I have lost 40 decibels at 2 kilohertz': the hearing-test result. They WILL say that they cannot hear colleagues at work in the canteen, or their family at dinner. Listening to speech in noise is much more complicated than the simple detection of sounds measured with hearing tests. We have to separate overlapping speech sounds and background noise that are louder than the sounds in hearing tests, group together speech elements into words and sentences, and associate those with meaning, which depends on our language exposure and ability. It is not surprising, then, that the hearing test, measuring the detection of tones in quiet, is not a good predictor of speech-in-noise ability, or the benefit from hearing devices. Two patients with the same hearing loss who are given the same hearing aids with the same improvement in their hearing tests can have strikingly different speech in noise ability after: so that one is happy with the result and the other throws the hearing aid away. In this programme we will develop tests of the ability of subjects to group together and retain sounds that have a complex structure, like speech, and separate these from a noisy background. Unlike speech, the sounds are not associated with meaning, and their detection can be measured precisely in patients that speak different languages and have different language skills. These tests allow us to measure an important process that is more closely related to speech-in-noise analysis than simple hearing tests. The first tests we have used predict how well people hear speech in noise independently of simple hearing tests. We aim to develop further tests for the hearing clinic to predict how well people will manage in the real world: real world listening tests. Although the tests we will develop assess what might be considered a simple process based on grouping and separation of sound elements, the analysis of these requires extensive processing in the brain, which we want to understand better. This analysis requires the auditory part of the brain, but also parts of the brain that are not usually considered parts of the auditory brain, including key structures usually associated with memory. Perhaps this is not surprising: sound stimuli, unlike static visual stimuli, evolve over time, and the separation of mixtures of sound requires us to remember or hold in mind the elements that form the foreground and background. In this work, we will measure brain activity that allows us to separate mixtures of sounds like speech in noise. We will use functional magnetic resonance imaging carried out on normal listeners when they carry out these tasks as an indirect measure of brain activity. We can also measure brain activity directly, in patients with epilepsy who have electrodes placed in the brain for a few days in order to work out where the epilepsy starts. As well as the localising their epilepsy, we can measure brain activity during listening tasks. The outcome of these brain experiments will be the definition of brain systems for separating sounds like speech in noise. This will, firstly, provide other measures (in addition to the new listening tests) tests) of the success of interventions like hearing aids, cochlear implants, or hearing training. Secondly, the work will suggest possible interventions that might help patients to understand speech in noise. Although it may seem far fetched to try to improve speech-in-noise detection by interfering with the brain, there is already evidence that minimally invasive electrical stimulation can do this, and the work could identify brain targets for drug treatments.