Role of spectral cues in sound externalization

Sound sources are naturally perceived externalized, i.e., as coming from outside the head. This natural auditory perception can be disrupted when listening via headphones or hearing-assistive devices, and instead sounds are heard inside the head. An important reason for the loss of externalization is the reduction of high-frequency spectral cues naturally induced by the acoustic filtering of the incoming sound by the listener`s outer ears. However, it is unclear how this spectral information triggers sound externalization exactly, and thus, it is hard to design electronic filters to recover the naturally externalized spatial hearing. The goal of this project is to establish a model for the role of spectral cues in sound externalization. In order to gain insight into the auditory processing of spectral cues and to obtain reliable estimates of externalization, subjective psychoacoustic estimates will be related to objective behavioral (reaction time) and physiological (auditory-evoked potentials) measures. Increasing salience of listener-specific spectral cues is expected to increase the degree of externalization accompanied by a speed-up and intensification of auditory processing. The derived objective measures will then be used to extend an already existing model explaining the role of spectral cues in directional localization (Baumgartner et al., 2014, JASA 136:791802) to sound externalization while assuming similar processing of the cues for both aspects.

Spatial hearing is important to monitor the environment for interesting or hazardous sounds and to selectively attend to them. The spatial separation between the two ears and the complex geometry of the human body provide auditory cues about the location of a sound source. Depending on where a sound is coming from, the pinna (or auricle) changes the sound spectrum before the sound reaches the eardrum. Since the shape of a pinna is highly individual (even more so than a finger print) it also affects the spectral cues in a very individual manner. In order to produce realistic auditory perception artificially, this individuality needs to be reflected as precisely as required, whereby the actual requirements are currently unclear. That is why SpExCue was about finding electrophysiological measures and prediction models of how spatially realistic (externalized) a virtual sound source is perceived to be. Because artificial sources are mainly perceived inside the head, our investigations of spectral cues also served to study the phenomenon of auditory looming bias: sounds approaching the listener are perceived more intensely than those that are receding from the listener. Previous studies demonstrated auditory looming bias exclusively by loudness changes (increasing/decreasing loudness used to simulate approaching/receding sounds). Hence, it was not clear whether this bias truly reflects perceptual differences in sensitivity to motion direction rather than changes in loudness. Our spectral cue changes were perceived as either approaching or receding at steady loudness and evoked auditory looming bias both on a behavioral level (approaching sounds easier to recognize than receding sounds) and an electrophysiological level (larger neural activity in response to approaching sounds). Therefore, our study demonstrated that the bias is truly about perceived motion in distance, not loudness changes. Further, SpExCue investigated how the combination of different auditory spatial cues affects attentional control in a speech recognition task with simultaneous talkers, which requires spatial selective attention like in a cocktail party. We found that natural combinations of auditory spatial cues caused larger neural activity in preparation to the test signal and optimized the neural processing of the attended speech. SpExCue also compared different computational modeling approaches that aim to predict the effect of spectral cue changes on how spatially realistic a sound is perceived. Although many previous experimental results could be predicted by at least one of the models, none of them alone could explain these results. In order to assist the future design of more general computational models for spatial hearing, we finally created a conceptual cognitive model for the formation of auditory space.