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Title: The role of acoustic periodicity in perceiving speech
Author: Steinmetzger, K.
ISNI:       0000 0004 8498 7788
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2017
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This thesis investigated the role of one important acoustic feature, periodicity, in the perception of speech. In the context of this thesis, periodicity denotes that a speech sound is voiced, giving rise to a sonorous sound quality sharply opposed to that of noisy unvoiced sounds. In a series of behavioural and electroencephalography (EEG) experiments, it was tested how the presence and absence of periodicity in both target speech and background noise affects the ability to understand speech, and its cortical representation. Firstly, in quiet listening conditions, speech with a natural amount of periodicity and completely aperiodic speech were equally intelligible, while completely periodic speech was much harder to understand. In the presence of a masker, however, periodicity in the target speech mattered little. In contrast, listeners substantially benefitted from periodicity in the masker and this socalled masker-periodicity benefit (MPB) was about twice as large as the fluctuatingmasker benefit (FMB) obtained from masker amplitude modulations. Next, cortical EEG responses to the same three target speech conditions were recorded. In an attempt to isolate effects of periodicity and intelligibility, the trials were sorted according to the correctness of the listeners' spoken responses. More periodicity rendered the event-related potentials more negative during the first second after sentence onset, while a slow negativity was observed when the sentences were more intelligible. Additionally, EEG alpha power (7-10 Hz) was markedly increased before the least intelligible sentences. This finding is taken to indicate that the listeners have not been fully focussed on the task before these trials. The same EEG data were also analysed in the frequency domain, which revealed a distinct response pattern, with more theta power (5-6.3 Hz) and a trend for less beta power (11-18 Hz), in the fully periodic condition, but again no differences between the other two conditions. This pattern may indicate that the subjects internally rehearsed the sentences in the periodic condition before they verbally responded. Crucially, EEG power in the delta range (1.7-2.7 Hz) was substantially increased during the second half of intelligible sentences, when compared to their unintelligible counterparts. Lastly, effects of periodicity in the perception of speech in noise were examined in simulations of cochlear implants (CIs). Although both were substantially reduced, the MPB was still about twice as large as the FMB, highlighting the robustness of periodicity cues, even with the limited access to spectral information provided by simulated CIs. On the other hand, the larger absolute reduction of the MBP compared to normal-hearing also suggests that the inability to exploit periodicity cues may be an even more important factor in explaining the poor performance of CI users than the inability to benefit from masker fluctuations.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available