The effects of background sound on communication : speech intelligibility and reading

Speech intelligibility can vary depending on the characteristics of background sound in which it is presented. Along the auditory pathway interference may occur due to a physically degraded representation of speech at the peripheral level and/or a perceptually degraded representation at higher central cognitive levels. By manipulating background sounds the level at which interference occurs can be considered. In the presence of an interfering single-talker speech background, intelligibility is sometimes improved compared to performance in a stationary noise background thought to be due to an improved physical representation of the target speech. In children, this improvement is often found to be smaller than that compared to adults, although such findings have not always been reported. What this could suggest however, is that children may be more detrimentally affected by speech backgrounds than adults. This generally has been understood to reflect the maturation of central cognitive processes in children, where speech backgrounds may interfere with speech intelligibility at higher cognitive levels. Recent research however, proposes that this measured improvement may be subject to a signal-to-noise-ratio (SNR) confound which relates to differences in performance with the baseline stationary noise condition from which such improvement is compared. Thus previous findings could have been misinterpreted. The first of three experiments within this thesis aimed therefore to quantify such speech intelligibility improvement amongst children and adults and to investigate the developmental trajectory over a one year period. Contributing data from large samples, it was found that the intelligibility improvement with speech backgrounds was significantly smaller in children aged 5-6 years compared to adults, and although performance got better amongst children one year later the improvement remained significantly smaller. The second experiment aimed to further understand the effect speech backgrounds have on speech intelligibility in children by exploring the validity of found child/adult differences. This concerned the SNR confound and used analyses inspired by previous research to take into account baseline stationary noise differences. It was concluded that the difference between children and adults’ intelligibility improvement with speech backgrounds lessened, yet a difference remained suggesting fundamental differences in the effects speech backgrounds have on children compared to adults. Such results may have implications for children listening in noisy classrooms. Since speech backgrounds may interfere with speech intelligibility at higher cognitive levels, the final experiment attempted to further understand the mechanisms involved by aiming to tap into cognitive demands arising from communication in realistic listening situations. In order to do this, a reading paradigm was utilised to track eye movements during reading in the presence of differing background sounds. Whilst input from visual and auditory modalities may not cause either any physical degradation at peripheral levels, interference might occur at higher cognitive levels when processing language. Only one other study to date has used eye tracking technology to provide online insights into cognitive processing difficulty when reading amongst background speech. Therefore the final aim was to determine how different background sounds disrupt the reading process. It was found in stark contrast to speech intelligibility findings, a single-talker speech background caused more interference to the reading task compared to a stationary noise background in adult participants, suggesting that speech intelligibility measurements may not provide any information about cognitive load in everyday communication conditions. It is concluded that children are more detrimentally affected by speech backgrounds in speech intelligibility tasks compared to adults and that speech backgrounds interfere at higher cognitive levels invoking complex cognitive processes. Further research is needed to establish how speech backgrounds affect children during reading which could have important implications for noise levels in classroom settings.