The neurophysiological studies in this laboratory aim to deepen our understanding of how the nervous system analyses complex sounds. Our ability to identify particular sounds against a noisy background is dependent in part on being able to determine where the source of the sound is located. This is in turn dependent on the processing of the complex differences in the sounds at each ear.
Our studies examine ways in which this information is combined from the two ears and has important implications, not just for our understanding of the processes of sound localization but for the analysis of other relevant auditory signals such as speech.
Clearly the localization of the source of a sound is an important component in predatory/prey interactions but it is also crucially important in human communication: particularly in the separation of "foreground" sounds of interest from "background" noise. Traditionally the difference in the time of arrival and sound level between the two ears have been identified as the principal cues to a sounds location.
More recent work has shown that the outer ear (the pinna) provides potent and unambiguous cues to a sounds location by modifying the spectral content of the incoming sounds in a manner which is dependent on the location of the source. These monaural changes in the spectra also result in location specific interaural differences.
How the auditory system processes these spectral cues is a principal research area of this laboratory. The overall aim of these studies is to examine spectral shape selectivity within the auditory system, particularly in the context of location of sounds in space.
The inner ear (the cochlea) codes frequency which is represented in the primary ascending auditory system as an ordered tonotopic arrangement in each nuclei. In the superior colliculus (SC), a mammalian midbrain nucleus, there exists a topographic representation of auditory space which can be demonstrated under both monaural and binaural listening condition. No tonotopic sequence of frequency sensitivity is evident within this nucleus. The specific hypotheses were are examining are
Understanding the rules by which this frequency information is integrated has important implications, not just for the processing of spectral cues, but for our understanding of other biologically relevant auditory signals.
The applications of these rules, derived from a fairly stereotyped system like the superior colliculus will give us insights and approaches to understanding how more complex and dynamic sound patterns like speech are analysed by the auditory system.
A number of different coding strategies are being investigated. Traditionally, auditory neurophysiologists examining central auditory processing have focussed on the rate at which action potentials are elicited by a particular stimulus. While this coding scheme has considerable support and theoretical merit, attention has recently turned to other possible coding schemes such as those based on the timing of action potentials produced by a different stimuli (e.g. Hopfield delay line networks). One advantage of these schemes is that they are relatively level independent. We are interested in examining the ways in which such strategies might be employed in the analysis of spectrally complex stimuli.
Return to Index