Belliveau, L; (2017) The role of spatial cues for processing speech in noise. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

How can we understand speech in difficult listening conditions? This question, centered on the ‘cocktail party problem’, has been studied for decades with psychophysical, physiological and modelling studies, but the answer remains elusive. In the cochlea, sounds are processed through a filter bank which separates them in frequency bands that are then sensed through different sensory neurons. All the sounds coming from a single source must be combined together again in the brain to create a unified speech percept. One of the strategies to achieve this grouping is to use common sound source location. The location of sound sources in the frequency range of human speech in the azimuthal plane is mainly perceived through interaural time differences (ITDs). We studied the mechanisms of ITD processing by comparing vowel discrimination performance in noise with coherent or incoherent ITDs across auditory filters. We showed that coherent ITD cues within one auditory filter were necessary for human subjects to take advantage of spatial unmasking, but that one sound source could have different ITDs across auditory filters. We showed that these psychophysical results are best represented in the gerbil inferior colliculus when using large neuronal populations optimized for natural spatial unmasking to discriminate the vowels in all the spatial conditions. Our results establish a parallel between human behavior and neuronal computations in the IC, highlighting the potential importance of the IC for discriminating sounds in complex spatial environments.

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