Ph.D. Dissertation Defense: Krishna Puvvada
Wednesday, September 6, 2017
301 405 3681
Professor Jonathan Z. Simon, Chair
Professor Carol Espy-Wilson
Assistant Professor Behtash Babadi
Assistant Professor Samira Anderson
Professor Sandra Gordon-Salant, Dean’s Representative
Date/time: Wednesday, September 06, 2017 at 10.00 am
Place: AVW 2168
Title: Cortical Representation of Speech in Complex Auditory Environments and Applications
This dissertation pertains to cortical representation of speech in adverse listening conditions in healthy humans and how the neural representations of sounds are altered in patients suffering from psychiatric disorders.
Being able to attend and recognize speech or a particular sound in complex listening environments is a feat performed by humans effortlessly. The underlying neural mechanisms, however, remain unclear and cannot yet be emulated by artificial systems. Understanding the internal (cortical) representation of external acoustic world is a key step in deciphering the mechanisms of human auditory processing. Further, understanding neural representation of sound finds numerous applications in clinical research for psychiatric disorders with auditory processing deficits such as schizophrenia.
In the first part of this dissertation, cortical activity from normal hearing human subjects is recorded, non-invasively, using magnetoencephalography in two different real-life listening scenarios. First, when natural speech is distorted by reverberation as well as stationary additive noise. Second, when the attended speech is degraded by the presence of multiple additional talkers in the background, simulating a cocktail party. Using natural speech affected by reverberation and noise, it was demonstrated that the auditory cortex maintains both distorted as well as distortion-free representations of speech. Additionally, we show that, while the neural representation of speech remained robust to additive noise in absence of reverberation, noise had detrimental effect in presence of reverberation, suggesting differential mechanisms of speech processing for additive and reverberation distortions, with implications for distortion robust speech perception observed in humans. In the cocktail party paradigm, elaborating on the important mechanistic questions that remain open since Cherry’s 1953 seminal cocktail party work, we examined how speech streams in a multi-talker auditory scene are represented, parsed and attended in different hierarchical levels of auditory cortex. We demonstrated that primary like areas represent the external auditory world in terms of acoustics, whereas higher-order areas maintained an object based representation. The results suggest that object based representation of auditory scene emerge in higher-order auditory cortices. Further, it was demonstrated that background speech streams were represented as an unsegregated auditory object, rather than separate individual objects. The results imply that the auditory object segregation is influenced by attention rather than pre-attentive – an important debate in the field, with implications for downstream processing of speech.
In the second part of this dissertation, using electroencephalographic recordings from normal human subjects and patients suffering from schizophrenia, it was demonstrated, for the first time, that delta band steady state responses are more deficient in schizophrenia patients compared with healthy individuals, contrary to the prevailing dominance of gamma band studies in literature. Interestingly, the observed reduction in delta band responses was better associated with cognitive deficits observed in patients than gamma band deficits, thus extending and shifting the focus of neural response impairments away from what has been a relatively narrow focus on gamma band responses in the community and highlighting the potential importance of slow wave activity in studying the etiology of schizophrenia.
Overall, this dissertation furthers current understanding of cortical representation of speech in complex listening environments and how auditory representation of sounds is affected in psychiatric disorders involving aberrant auditory processing.