Purpose: To investigate the effects of vocabulary, working memory, age, semantic context, and signal-to-noise-ratio (SNR) on speech recognition in adverse conditions (multitalker babble) in normal-hearing listeners aged 18-35. First, a general hypothesis was tested that listeners with larger receptive vocabularies would be more accurate at recognising speech in noise than listeners with more limited receptive vocabularies, even when target stimuli are words with high lexical frequency. A second more specific hypothesis was that the vocabulary would be predictive of speech recognition accuracy when the signal was moderately degraded, but not mildly or severely degraded. Method: 80 sentences with a high (HP) or low (LP) degree of semantic predictability (40 HP and 40 LP) were recorded from a male speaker of NZ English. These sentences were used as experimental target stimuli, and presented in multitalker babble at four SNRs: -8, -4, 0 and 4 dB SNR. Thirty-five participants (11 males and 24 females, aged 18 to 35), with puretone hearing thresholds of 15 dB HL or better, completed the Peabody Picture Vocabulary Test (PPVT) and the Wechsler Adult Intelligence Scale (WAIS) vocabulary subtest, the WAIS working memory subtests, and the experimental listening task in which they were required to repeat back the target sentences. Results: There was considerable variability between listeners in speech recognition performance, in terms of percent words accurately recognised overall (M = 45.8%; SD = 7.4) and for both HP (M = 54.4%; SD = 9.8) and LP (M = 35%; SD = 8.9) conditions. Hierarchical multiple regression analyses revealed that receptive (PPVT) and productive (WAIS) vocabulary knowledge, but not working memory, contributed 8 significant variance to listeners’ speech recognition scores overall and in both the HP and LP conditions. Further regression analyses at individual SNR levels showed that receptive vocabulary contributed significant variance to listening recognition scores in all predictability and SNR conditions except the most favourable (HP stimuli at 4 dB SNR) and least favourable (LP stimuli at -8 dB SNR) listening conditions. Working memory and age were not significantly related to overall listening score, HP listening score, or LP listening score, but age did contribute significant variance in the - 4dB SNR LP condition. Conclusion: The results provide further evidence that greater vocabulary knowledge is associated with improved speech recognition in adverse conditions. This effect was salient in mid-range adverse listening conditions, but was not apparent in highly favourable and extremely poor listening conditions. The results were interpreted to suggest that in moderately adverse listening conditions listeners with larger lexicons may be better able to exploit redundancies and/or intelligible ‘glimpses’ in the speech signal.

Purpose: This thesis examines the effect of listener characteristics (i.e., cognition and vocabulary) and language-based factors (i.e., lexical frequency and phonological similarity) on speech recognition accuracy in adverse listening conditions. Method: Fifty listeners (40 females and 10 males) aged 18-33 years and with normal hearing (puretone thresholds ≤ 20 dB HL, 0.25-8 kHz) participated. They completed a speech perception experiment, which required listeners to repeat back non-sensical English phrases presented at a variety of signal-to-noise ratios (-5, -2, +1, and +4 dB SNRs). In addition, all listeners undertook assessments of vocabulary knowledge (PPVT-IV) and cognition (WAIS -IV). The primary dependent variable was individual content word recognition accuracy, and results were analysed using binomial mixed effects modelling. Results: Listeners demonstrated variability in their speech recognition abilities, and their vocabulary and cognitive scores. Statistical analysis revealed that listener-based factors affected word recognition. Listeners with faster processing speed and larger working memories exhibited higher word recognition accuracy. Surprisingly, listeners with higher non-verbal intelligence scores exhibited lower word recognition accuracy. Vocabulary knowledge interacted with SNR, such that as the listening conditions became more favourable, listeners with larger receptive vocabularies identified more words correctly. Similarly, main effects were also present for language-based factors. The more phonologically distinct a word was, the more likely it was to be correctly identified; higher frequency words were more likely to be accurately recognised. In addition, higher frequency words were identified more accurately at higher SNR levels. Finally, listener- and language-based factors interacted. The positive effect of working memory on word recognition was reversed as word frequency increased; on the other hand non-verbal intelligence’s negative influence on word recognition was reversed as word frequency increased. Conclusion: In the current cohort, listener and language-based factors interacted in the process of word recognition in noise. These results provide an insight into the underlying speech recognition mechanisms in adverse conditions. Further understanding of how these listener differences affect an individual’s speech processing may lead to the development of improved signal processing techniques and rehabilitation strategies.