Decoding auditory motion direction and location in hMT+/V5 and Planum Temporale of sighted and blind individuals

Abstract

The research presented in this thesis addresses the neural mechanisms of auditory motion processing and the impact of early visual deprivation on motion-responsive brain regions, by using functional magnetic resonance imaging. Visual motion, and in particular direction selectivity, is one of the most investigated aspects of mammalian brain function. In comparison, little is known about how the brain processes moving sounds. More precisely, we have a poor understanding of how the human brain codes for the direction of auditory motion and how this process differs from auditory sound-source localization. In the first study, we characterized the neural representations of auditory motion within the Planum Temporale (PT), and how motion direction and sound source location are represented within this auditory motion responsive region. We further explore if the distribution of orientation responsive neurons (topographic representations) within the PT shares similar organizational features to what is observed within the visual motion area MT/V5. The spatial representations would, therefore, be more systematic for axis of motion/space, rather than for within-axis direction/location. Despite the shared representations between auditory spatial conditions, we show that motion directions and sound source locations generate highly distinct patterns of activity. The second study focused on the impact of early visual deprivation on auditory motion processing. Studying visual deprivation-induced plasticity sheds light on how sensory experience alters the functional organization of motion processing areas, and exploits intrinsic computational bias implemented in cortical regions. In addition to enhanced auditory motion responses within the hMT+/V5, we demonstrate that this region maintains direction selectivity tuning, but enhances its modality preference to auditory input in case of early blindness. Crucially, the enhanced computational role of hMT+/V5 is followed by a reduced role of PT for processing both motion direction and sound source location. These results suggest that early blindness triggers interplay between visual and auditory motion areas, and their computational roles could be re-distributed for effective processing of auditory spatial tasks. Overall, our findings suggest (1) auditory motion-specific processing in the typically developed auditory cortex, and (2) interplay between cross- and intra-modal plasticity to compute auditory motion and space in early blind individuals.