Auditory Processing and the Medial Olivocochlear Efferent System

The aim of this thesis is to investigate the role of the medial olivocochlear (MaC) efferent system in auditory processing. This was achieved by 'upgradi~g' an existing computer model of the mammalian auditory periphery (Meddis, 2006) to include the effect of stimulating the MaC system. The improved model (Ferry and Meddis, 2007) was evaluated against published physiological data including the responses of the basilar membrane, auditory nerve, and compound action potential. In all cases the model was able to replicate the basic effects ofMaC stimulation at each stage of the auditory periphery as demonstrated physiologically. The model was then used to investigate the suggestion that the MaC system is involved in improving the discriminability of transient sounds such as speech in noise. An improvement in the model auditory nerve response to speech when presented in noise was demonstrated as a result of MaC stimulation. This improvement was then quantified using a series of automatic speech recognition experiments. These experiments demonstrated that MaC stimulation in the model could be used to increase speech recognition performance at all signal-to-noise ratios tested. The temporal effect (or 'overshoot' phenomenon) was then used to characterise the time course of the MaC system psychoacoustically, and also to investigate the role of the MaC system in this phenomenon. The time course measured is in agreement with both temporal effect and MaC system literature and this time course will be added to the model at a future date. The temporal effect was demonstrated in the model only when MaC stimulation is used. This is consistent with the suggestion that the MaC system is an underlying mechanism of the temporal effect phenomenon. The model is proposed as a platform with which it possible to study the complex effects ofMaC stimulation in auditory perception.