Speech masking speech in everyday communication : The role of inhibitory control and working memory capacity

Abstract: Age affects hearing and cognitive abilities. Older people, with and without hearing impairment (HI), exhibit difficulties in hearing speech in noise. Elderly individuals show greater difficulty in segregating target speech from distracting background noise, especially if the noise is competing speech with meaningful contents, so called informational maskers. Working memory capacity (WMC) has proven to be a crucial factor in comprehending speech in noise, especially for people with hearing loss. In auditory scenes where speech is disrupted by competing speech, high WMC has proven to facilitate the ability to segregate target speech and inhibit responses to irrelevant information. People with low WMC are more prone to be disrupted by competing speech and exhibit more difficulties in hearing target speech in complex listening environments. Furthermore, elderly individuals with a HI experience more difficulties in switching attention between wanted and irrelevant stimuli, and they employ more resources and time to attend to the stimuli than do normally - hearing (NH) younger adults.This thesis investigated the importance of inhibitory control and WMC for speech recognition in noise, and perceived listening effort. Four studies were conducted. In the first study, the aim was to develop a test of inhibitory control for verbal content, and to investigate the relation between inhibitory control and WMC, and how these two abilities related to speech recognition in noise, in young normally – hearing (YNH) individuals.In the second study we aimed to investigate the same relationship as in the first study to further strengthen the validity of the inhibitory test developed, as well as the importance of lexical access. It was also an aim to investigate the influence of age and hearing status on lexical access and WMC, and their respective roles for speech recognition in noise in both YNH and elderly HI (EHI) individuals.Study one and two showed that, for YNH, inhibitory control was related to speech recognition in noise, indicating that inhibitory control can help to predict speech recognition in noise performance. The relationship between WMC and speech recognition in noise in YNH shifted in the studies, suggesting that this relationship is multifaceted and varying. Lexical access was of little importance for YNH, although for EHI individuals, both WMC and lexical access was of importance for speech recognition in noise, suggesting that different cognitive abilities were of importance for the YNH and EHI individuals Study three investigated the relationship between inhibitory control, WMC, speech recognition in noise, and perceived listening effort, in YNH and elderly, for their age, NH, individuals (ENH). In study four the same relationships as in study three were investigated, albeit in EHI individuals. Two speech materials with different characteristics, masked with four background noises were used. The results in study three showed that less favourable SNRs were needed for informational maskers than for maskers without semantic content. ENH individuals were more susceptible to informational maskers than YNH individuals. In contrast, in study four, more favourable SNRs were needed for informational maskers. In both studies, results showed that speech recognition in noise performance differed depending on the characteristics of the speech material.The studies showed that high WMC, compared to low WMC, was beneficial for speech recognition in noise, especially for informational maskers, and resulted in lower ratings of perceived effort. Varying results were found in study three and four regarding perceived effort and inhibitory control. In study three good inhibitory control was associated with lower effort rating, while in study four, individuals with a HI and good inhibitory control rated effort as higher.The results suggest that hearing status, age, and cognitive abilities, contribute to the differences in performance between YNH, ENH, and EHI individuals in speech – recognition – in – noise - and cognitive tasks.This thesis has, for the first time, demonstrated that a measure of inhibitory control of verbal content, is related to speech recognition in noise performance in YNH, ENH and EHI individuals. Results presented in this thesis also show that both WMC and inhibitory control are related to an individuals’ perception of how effortful a listening task is. It also adds to the literature that WMC is related to speech recognition in noise performance for ENH and EHI individuals, but that this relationship is not as robust in YNH individuals.