Studies on auditory processing of spatial sound and speech by neuromagnetic measurements and computational modeling

This thesis addresses the auditory processing of spatial sound and speech. The thesis consists of two research branches: one, magnetoencephalographic (MEG) brain measurements on spatial localization and speech perception, and two, construction of computational auditory scene analysis models, which exploit spatial cues and other cues that are robust in reverberant environments. In the MEG research branch, we have addressed the processing of the spatial stimuli in the auditory cortex through studies concentrating to the following issues: processing of sound source location with realistic spatial stimuli, spatial processing of speech vs. non-speech stimuli, and finally processing of range of spatial location cues in the auditory cortex. Our main findings are as follows: Both auditory cortices respond more vigorously to contralaterally presented sound, whereby responses exhibit systematic tuning to the sound source direction. Responses and response dynamics are generally larger in the right hemisphere, which indicates right hemispheric specialization in the spatial processing. These observations hold over the range of speech and non-speech stimuli. The responses to speech sounds are decreased markedly if the natural periodic speech excitation is changed to random noise sequence. Moreover, the activation strength of the right auditory cortex seems to reflect processing of spatial cues, so that the dynamical differences are larger and the angular organization is more orderly for realistic spatial stimuli compared to impoverished spatial stimuli (e.g. isolated interaural time and level difference cues). In the auditory modeling part, we constructed models for the recognition of speech in the presence of interference. Firstly, we constructed a system using binaural cues in order to segregate target speech from spatially separated interference, and showed that the system outperforms a conventional approach at low signal-to-noise ratios. Secondly, we constructed a single channel system that is robust in room reverberation using strong speech modulations as robust cues, and showed that it outperforms a baseline approach in the most reverberant test conditions. In this case, the baseline approach was specifically optimized for recognition of speech in reverberation. In summary, this thesis addresses the auditory processing of spatial sound and speech in both brain measurement and auditory modeling. The studies aim to clarify cortical processes of sound localization, and to construct computational auditory models for sound segregation exploiting spatial cues, and strong speech modulations as robust cues in reverberation.reviewe