FUNCTIONAL NEUROIMAGING OF VENTRAL AND DORSAL STREAM PATHWAYS IN THE MACAQUE AUDITORY SYSTEM

Abstract

One fundamental goal of the brain is to predict sensory events in the environment in order to spatially direct actions. In vision, the ability to identify and locate objects depends on two cortical pathways: a ventral “what” stream supporting object recognition and a dorsal “where” stream supporting object localization. While this hierarchical model received strong support in vision, in audition the analogues functional roles have remained rather elusive, particularly for the dorsal “where” stream. Thus, the objective of this thesis was to explore the functional roles of auditory ventral and dorsal stream pathways in the macaque brain. We first explored the representational structure of natural sounds in early regions of the ventral pathway utilizing single-unit electrophysiology. We then used functional magnetic resonance imaging (fMRI) to map the representation of natural sounds along the ventral pathway including regions outside auditory cortex. Finally, using high-field fMRI we examined the functional representation of acoustic space in auditory cortical regions. Overall, our work confirms the role of the ventral stream in decoding sound identity and extends the evidence suggesting that vocalizations carry information that is represented outside auditory cortex. Moreover, our work in the dorsal stream also confirms the role of a posterior dorsal cortical region specialized in processing spatial information and reconciles competitive theories of spatial coding in auditory cortex. However, our space work also indicates a fundamental difference in the representation format for acoustic space in auditory cortex as compared to visual cortex. Taken together, our work confirms the functional roles of the ventral and dorsal streams and suggests incorporating subcortical level processes in the cortical model for a more integrated framework of acoustic processing in the primate brain.