Mehta, AH; (2015) Neural correlates of attention in auditory stream segregation. Doctoral thesis , UCL (University College London).

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Abstract

The work in this thesis illustrates how directed attention can modulate multistable, ambiguous auditory percepts, and how these percepts are reflected in the stimulus-driven cortical EEG responses of human listeners. Natural auditory environments require listeners to parse out an acoustic signal of interest amidst auditory sources constantly overlapping or competing for salience. Listeners need to simultaneously use both sequential and synchronous sound segregation to focus on a target as most sounds both overlap and unfold over time. Additionally, understanding how ambiguous stimuli are perceived as well as represented in brain activity can be useful in dissociating the neural responses to physical stimuli from the correlates of perception. The first set of experiments explored the effect of attention on sequential sound segregation using a perceptually ambiguous stimulus described by van Noorden (1975). Following on from these findings, a novel stimulus based on a variant of Deutsch’s ‘octave illusion’ (Deutsch, 1974), which involved ambiguous stimuli that engaged both synchronous and sequential sound segregation, was investigated. The experiments using this new stimulus paradigm demonstrated that the octave illusion was subserved by the same mechanisms that govern auditory streaming. Furthermore, directed attention could alter the percept of this stimulus and these changes could be observed in the corresponding cortical brain activity. Subsequent experiments were carried out to further understand the mechanisms underlying the octave illusion. Results from psychophysics, cortical EEG and modeling consistently suggested that the perceived illusory percept results from a misattribution of time across perceptual streams of synchronous sounds. Overall, the results highlight the key role of attention in complex auditory stream segregation involving both alternating as well as synchronous sound segregation. This body of work also introduces a stimulus, typically associated with an auditory illusion that has not previously been studied with performance-based behavioral measures as a versatile and experimentally valuable stimulus to study stream segregation.

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