Auditory Distance Perception and Its Neuronal Correlates

Abstract Spatial hearing is important in many everyday situations when humans interact with other humans or with machines. For example, perceiving the distance of sound sources is of key importance when avoiding approaching cars or when reaching for a ringing phone. Despite the clear importance of auditory distance perception in such situations, it has been studied much less extensively than horizontal or vertical localization. Similarly, in terms of cortical representation and processing, the mechanisms underlying horizontal and vertical localization have been extensively studied, while the neural mechanisms of auditory distance perception remain poorly understood. The main reasons for this lack of studies are that 1) it is difficult to distinguish distance processing from intensity processing, 2) multiple intensity-independent distance cues are frequently available, and 3) the cues are combined in a complex contextdependent manner. This PhD thesis aims to advance our understanding of the behavioral mechanisms and cortical structures underlying the level-independent auditory distance perception. It reports the results of a series of behavioral and brain imaging experiments performed in a virtual reverberant environment. It uses advanced computational tools to analyze the experimental data and computational modeling to describe them. A previous fMRI study identified a human auditory cortical area representing intensityindependent distance for nearby sources presented along the interaural axis (Kopčo et al., 2012). For these sources, two intensity-independent cues were available: the interaural level difference (ILD) and the direct-to-reverberant energy ratio (DRR). The current thesis consists of three studies following up on the previous results. The first study used a setup similar to (Kopčo et al., 2012) but with sounds simulated in front of the listener. It combined a behavioral experiment, an fMRI experiment and computational modeling. Its results suggest that posterior human auditory cortex areas contain neuron populations that are sensitive to distance independent of intensity and of binaural cues relevant for directional hearing. However, based on these results it is still not clear whether the identified areas represent integrated percepts of distance, per se, or the discrete cues based on which it is created. To address this, the second study introduced artificially manipulated stimuli and advanced fMRI data analysis. Its behavioral results showed that distance percepts were stronger when both ILD and DRR cues were available, confirming that both cues are used when listeners judge distance. Then, a univariate fMRI analysis identified areas in the PT+pSTG as encoding the DRR cue. Finally, an ROI-based multi-voxel pattern analysis (MVPA) over whole PT+pSTG region found representations of the distance percept. These results suggest that the PT+pSTG region encodes both the distance cues and percepts. However, while the cue encoding is local, the percepts are encoded in a distributed network. In the third study of this thesis, the relative weight of the ILD and DRR cues was examined behaviorally and computationally. The across-subject average results showed that, on average, listeners are more sensitive to ILD. However, a quarter of the subjects were more sensitive to the DRR. Thus, the result suggests that while the ILD is dominant among normal-hearing listeners, a large proportion of them prefer the DRR cue. Overall, the results of the studies in the thesis advance our understanding of how listeners with normal hearing perceive distance, which cues they use, and which brain regions are involved in level independent distance perception. The results can be used in the development of prosthetic devices for the hearing-impaired listeners as well as for the creation of auditory displays, virtual and augmented reality, intelligent robotics, as well as new neuroimaging computational tools. Keywords: Spatial Hearing; Sound Localization; Computational Modeling; Auditory Distance Perception; Behavior; Neural Coding; Auditory Cortex; What and Where Pathways; fMRI; MVPA.