Interaural differences in sensory processing at lower levels of the auditory system and their association with the right ear advantage for dichotic listening and speech perception in noise among older adults : an exploratory study

A functional bias between the right and left ears has been widely documented, with a right-ear advantage (REA) in perceptual tasks, particularly those employing verbal processing. Complex sounds such as speech are more accurately perceived in the right ear (RE) compared to the left ear (LE) for dichotic listening (DL) and speech perception in noise (SIN) tasks. Although previous studies have shown that aging is associated with an overall decline in DL performance in both ears among older adults, the left ear (LE) performance often decreases more dramatically relative to the right ear (RE) performance, causing an increased REA for speech stimuli. This greater LE deficit may be the result of age-related changes in (A) cognitive functions; (B) functions of the right hemisphere (RH); and /or (C) the efficiency of interhemispheric transfer of auditory information via the corpus callosum. However, evidence in normal hearing young adults suggest that interaural difference (ID) in sensory processing at lower portions of the auditory system might also explain the magnitude of the REA for DL. The main aim of this thesis is to investigate whether ID in sensory processing at lower levels of the auditory system are associated with the magnitude of the REA for DL among older adults. In addition, this project aimed to investigate whether ID in sensory processing at lower levels of the auditory system is associated with SIN performance in older adults. A total of 70 older adults participated in this study. To assess sensory processing at lower levels of the auditory system, hearing thresholds, transient evoked otoacoustic emission (TEOAE), contralateral suppression of TEOAE, a proxy measure of medial olivocochlear (MOC) activation and click-and-speech auditory brainstem response to speech stimuli were measured in both ears separately. The ID in sensory processing was derived by calculating the difference between the right and left ears for each auditory measure. The mobile device app version for iPhone of the Bergen Dichotic Listening test, the iDichotic, was used to evaluate DL. In addition, SIN performance was evaluated with the hearing-in-noise test (HINT). With the aim to control for cognitive function, cognitive abilities such as speed of processing, cognitive flexibility and working memory were also evaluated. Bivariate and multivariate regression models were performed, and cognitive measures were accounted for in the regression models. Results revealed that ID in pure-tone thresholds and ID in MOC-induced TEOAE suppression were significantly associated with the magnitude of the REA for DL among older adults. Similarly, ID in TEOAE and ID in speech-ABR measures explained SIN performance in older adults. In addition, results revealed that cognitive measures significantly contributed to the magnitude of the REA for DL and SIN performance in older adults. The present thesis provides evidence indicating that ID in sensory processing at lower levels of the auditory system partially explain the magnitude of the REA for DL as well as SIN performance among older adults.

Keyphrases: Differences sensorielles interaurales, Interaural sensory differences, Dichotic listening, Right ear advantage, Aging, Speech perception in noise