Brain Mechanisms underlying Performance in ITD Processing: Biophysics to Behaviour

Hearing with two ears (?binaural hearing?) is a critical factor in every-day listening tasks. The brain is able to compare information about the sound arriving at each ear to determine the location of sound sources and to perform ?cocktail-party listening? ? the ability to follow a conversation in a noisy room. The key to these abilities lies in detecting small differences in the time of arrival of the sound at each ear. Sensitivity to these ?interaural? time differences (ITDs) - in the order of a few tens of millionths of a second - requires some of the fastest brain mechanisms that are known to exist. The current research proposal will examine how the brain is able to operate at such fast temporal limits to detect ITDs, and will seek to determine how binaural hearing can be re-established in those who, having lost their hearing, rely on devices such as cochlear implants (electrical devices that stimulate the auditory nerves directly) to hear and understand speech. The programme of work will also use new, non-invasive brain imaging technologies in both animals and humans to test directly the relevance of physiological findings from the brains of small animals to human hearing. Building on outcomes of my previous research, the programme will establish a coherent and integrated view of binaural hearing in the mammalian brain, including humans, and will suggest ways in which new technologies for the deaf and hard-of-hearing can be enhanced for better listening in acoustically cluttered or noisy environments.

Binaural hearing, particularly the ability to detect small differences in the timing of sound at the two ears (interaural time differences, ITDs) underpins the ability to localize sound sources, and is important for decoding complex spatial listening environments into separate objects ? a critical factor in ?cocktail-party listening?. The current proposal will address critical questions as to how the brain generates sensitivity to ITDs, assesses performance limits in ITD processing for a range of relevant listening conditions, including the use of ITDs carried in the ?envelope? structure of modulated sounds (critical to cochlear implantation) and demonstrates how a general principal of neural coding ? that of efficiency ? is evident in the biophysical properties of binaural neurons. The programme of worll will exploit in vivo and in vitro physiological recordings in small animals to examine basic mechanisms of binaural hearing, including biophysical and physiological limits to binaural hearing that potentially impact on the development of future hearing technologies. Human psychophysics, brain imaging (magneto-encephalography ? MEG ? and functional MRI) and electrophysiology (EEG) studies will seek to establish mechanisms and performance limits in processing binaural cues for spatial processing. Experiments are also proposed employing non-invasive techniques (MEG) to test more directly the relevance of physiological findings in small animals to the human brain. Theoretical and modeling studies will seek to establish how current models of ITD processing based on psychophysical and neural sensitivity to low-level acoustic cues can be used to inform perceptual and cognitive models of auditory spatial processing. Building on outcomes of my previous research, the proposed programme will establish a coherent and integrated view of binaural hearing in the mammalian brain, one that can be exploited in the development of hearing devices and audio technologies.