The Specificity of the Medial Olivocochlear Effect on Frequency and Word Discrimination in Noise

Abstract

Ipsilateral or contralateral activation of the MOC pathway is known to result in modifications of OHC mobility. These changes impact on the cochlear active process and the ability to detect low intensity sound and can be observed through OAE measurements. Previous studies have shown that the presence of this effect in situations where sound is masked by noise can result in an improvement of discriminatory ability. The tonotopical nature of the MOC has been observed anatomically, but its physiological relevance remains uncertain. This thesis aimed to investigate the possible changes in MOC effect when contralaterally activated by noise of varying frequency spectrum. It was hypothesised that when attempting to distinguish tonal patterns of 4 or 5 kHz presented in a BBN, participants would perform better when contralaterally stimulated with noise that contained the same frequency range. Using DPOAE measurements in the presence of contralateral noise, it was also hypothesised that suppression of emissions caused by OHC modification could be used as relative measures of MOC strength. These measures were then referenced against performance in psychoacoustic discrimination tasks looking at the effect of varying frequency contralateral stimulation on word and frequency pattern discrimination. Results showed a significant relationship between the presence of contralateral noise and discrimination ability in the psychoacoustic tests. However, no change in DPOAE suppression was observed when tested in the presence of different contralateral stimulation (F (30, 600) = 0.742, p=0.840). No significant interaction was found between DPOAE measurements and psychoacoustic results. Despite steps to remove its influence, activation of the acoustic reflex was statistically significant in several findings which complicated the interpretation of the study‟s results. The benefit observed with contralateral stimulation in psychoacoustic testing suggested the presence and activation of the MOC pathway, despite the inability of DPOAE findings to confirm its suppression of the cochlear active process. The proportional benefit of using frequency relevant noise as an activator of contralateral MOC effect were potentially observed but unable to be confirmed by physiological data.