The role of temporal coherence in auditory stream segregation

The ability to perceptually segregate concurrent sound sources and focus one’s attention on a single source at a time is essential for the ability to use acoustic information. While perceptual experiments have determined a range of acoustic cues that help facilitate auditory stream segregation, it is not clear how the auditory system realizes the task. This thesis presents a study of the mechanisms involved in auditory stream segregation. Through a combination of psychoacoustic experiments, designed to characterize the influence of acoustic cues on auditory stream formation, and computational models of auditory processing, the role of auditory preprocessing and temporal coherence in auditory stream formation was evaluated. The computational model presented in this study assumes that auditory stream segregation occurs when sounds stimulate non-overlapping neural populations in a temporally incoherent manner. In the presented model, a physiologically inspired model of auditory preprocessing and perception was used to transform a sound signal into an auditory representation, and a subsequent temporal coherence analysis grouped frequencychannels of the model together if they were stimulated in a temporally coherent manner. Based on this framework, the model was able to quantitatively predict perceptual experiments on stream segregation based on frequency separation and tone repetition rate, and onset and offset synchrony. Through the model framework, the influence of various processing stages on the stream segregation process was analysed. The model analysis showed that auditory frequency selectivity and physiological forward masking play a significant role in stream segregation based on frequency separation and tone rate. Secondly, the model analysis suggested that neural adaptation, and the resulting enhancement of neural responses to onsets, increases the sensitivity to onset synchrony for auditory stream formation. The effect of sound intensity on auditory stream formation was investigated, under the assumption that the wider auditory filters at high sound pressure levels should lead to a decreased ability to perceptually segregate sounds presented at high intensities. The results of listening experiments confirmed this hypothesis, showing that the minimum frequency separation required for stream segregation increases with increases in sound intensity. The computational model results also showed an increased tendency to group sounds presented at high intensities, but the size of the effect was overestimated relative to the experimental data, suggesting that the computational model does not fully reflect the auditory stream formation process. Lastly, an experimental paradigm designed to measure perceptual organization through an indirect, performance-based measure was investigated. This measure used comodulation masking release (CMR) to assess the conditions under which a loss of temporal coherence across frequency can lead to auditory stream segregation. The study indicated that CMR may be used as an indirect measure of stream segregation, and further supports the hypothesis that temporal coherence acts as a strong grouping cue. Overall, the findings of this thesis suggest that temporal coherence plays a significant role in the grouping of sounds into a single stream, and more generally, that a temporal coherence analysis may provide the framework for determining the perceptual organization of sounds into streams.