Elicitation of the medial olivocochlear reflex (MOCR) causes a reduction in the amount of gain (amplification) applied by the cochlear amplifier. This gain-control function is thought to play an important role in speech-in-noise perception. Otoacoustic emissions (OAEs) offer a qualitative measure of the effect of the MOCR on cochlear gain, but a quantitative measure is lacking. The aim of this thesis was to test whether any of the putative perceptual correlates of MOCR-induced cochlear gain reduction might provide such a measure. The first study (Chapter 2) is concerned with the mechanism of the overshoot effect, in which a brief signal presented at the onset of a masker is harder to detect when the masker is preceded by silence than when it is preceded by a “precursor” sound. It has been suggested that, in overshoot, the precursor might reduce cochlear gain by eliciting the MOCR and thereby cause a reduction in suppressive masking of the signal (adaptation of suppression). Overshoot, suppression, and adaptation of suppression were measured in the same participants. While the precursor yielded strong overshoot, and the masker produced strong suppression, the precursor did not appear to cause any adaptation of suppression. Predictions based on an established model of the cochlear input-output function indicate that the failure to obtain any adaptation of suppression is unlikely to represent a false negative outcome. It is argued that overshoot may be due to higher-order perceptual factors such as transient masking or attentional diversion. Overshoot was therefore not pursued as a quantitative measure of the MOCR. The second study (Chapter 3) aimed to develop a quantitative measure of the MOCR by modifying the established temporal masking curve (TMC) method for estimating cochlear gain psychophysically. The TMC method involves measuring the lowest masker level needed to just render inaudible a weak signal as a function of the temporal gap between the masker and signal. Here, the masker’s duration was shortened so that the masker would not itself elicit the MOCR in time to affect the signal’s audibility. A new way of estimating cochlear gain from TMC data by fitting the entire data set with a generic model of the cochlear response function was also developed. Using this approach, the effect on cochlear gain of a broadband-noise elicitor presented to the contralateral ear was measured. The TMCs suggest that the elicitor reduced cochlear gain by 4 dB, on average. OAE suppression measurements in the same participants suggested that this gain reduction was mediated by the MOCR. The approach developed in this chapter provides a quantitative estimate of MOCR-induced cochlear-gain reduction caused by a contralateral elicitor. The third study (Chapter 4) aimed to assess the validity of recent findings by Yasin et al. (2014), who reported an MOCR-induced cochlear-gain reduction by an ipsilateral elicitor that was four times larger than that found in the second study using a contralateral elicitor. Yasin et al. (2014) estimated cochlear gain reduction using the fixed-duration masking curve (FDMC) method, which is similar to the TMC method used in Chapter 3. In Chapter 4, the FDMC method was used to estimate the amount of gain reduction caused by a long ipsilateral elicitor, like the one used by Yasin et al. (2014). This was compared to the amount of gain reduction caused by a much shorter ipsilateral elicitor, which was presented at a level that produced the same amount of masking of the signal as the long elicitor, but was too short to have activated the MOCR in time to affect the signal detectability. The long and short elicitors both caused large psychophysical effects, indicating either that the MOCR acts more quickly than previously thought, or that the effect was not due to MOCR-induced cochlear gain reduction. OAE suppression was also found for both the long and short elicitors. It is argued that both the OAE and psychophysical effects of the short and long elicitors may, at least in part, be the result of nonlinear interactions between the elicitor and the masker resulting from direct temporal overlap of their cochlear responses. This thesis provides evidence against the idea that MOCR-induced cochlear-gain reduction plays a major role in either overshoot or in a recently reported large psychophysical masking effect by an ipsilateral noise, both of which have previously been attributed to the MOCR. This thesis has also contributed towards the refinement of an approach for quantitatively measuring cochlear gain and MOCR-induced cochlear gain reduction by a contralateral noise. In future, this approach could become a valuable audiometric profiling tool, and may give insight into the individual differences that underlie hearing problems in audiometrically normal listeners. Parametric exploration of the MOCR using this approach may also allow the functional importance of the MOCR in humans to be better understood.

Saving Alicia is a creative thesis written to explore the possibility of incorporating some non-fictional concepts of neurophysiology into a work of fiction. The initial component presents the historical and contemporary context in which such a work is written along with an analysis of the writing techniques employed by other writers in the field. It sets out the aim of the subsequent creative composition. The second, and major, component of this thesis is a work of fiction. A story is developed in which the protagonist, a young woman, revives her deceased mother's neurophysiological research work in the hope that it will help her brain-damaged niece, Alicia, recover. For this she is dependent on two men who were her mother's colleagues. As they compete for her attention, while pursuing their own conflicting goals, the protagonist maintains her determination to keep her mother's work going. She has no prior knowledge of neurophysiology and, so that she can understand the research, she is keen to learn some of its basic concepts. Woven through the story of Saving Alicia are descriptions of neurons and their physiology. This is presented to the protagonist through the mouths of the two researchers. In this way, the non-fiction is interspersed with the fiction.

Thesis (Ph. D.)--University of Chicago, Committee on Conceptual and Historical Studies of Science, June 2001. / Includes bibliographical references. Also available on the Internet.

Studies investigating human information processing have provided evidence that in some cases, movements can be prepared in advance. Although evidence for motor preparation has been shown at cortical and spinal levels, motor preparation at a subcortical level is not well described. One line of inquiry has involved the use of a startling acoustic stimulus (115-124 dB) that can act as an early trigger for pre-programmed actions in reaction time (RT) tasks. In light of this new research paradigm, the startle reflex may be used as a tool to investigate motor preparation. Here, six experiments were conducted that work towards the goals of understanding the mechanism of RT shortening due to startle, and motor preparation at a subcortical level. The first section (2 experiments) of this dissertation provides evidence that when a motor action can be prepared in advance, it is pre-programmed and stored subcortically awaiting the normal cortical “go” signal. A startle appears to activate structures directly that are involved with the voluntary response channel leading to early triggering of the pre-programmed response, and dramatically reduced RT. In the current dissertation we investigated alternative mechanisms to explain startle RT facilitation, including the stimulus intensity effect, and a fast transcortical route, with results supporting the original subcortical storage hypothesis. The second section (4 experiments) presents data which together provide insight into motor programming processes, and the circumstances under which a response is pre-programmed. For example, when the possibility of not having to make the response existed, a known response was not pre-programmed. Similarly, no pre-programming occurred when certainty existed regarding when to respond. However, while a previous experiment showed that having to make a choice between several response alternatives precluded pre-programming, this dissertation shows that if possible response alternatives are not in conflict with one another, multiple responses can be prepared in parallel. Finally, the complexity of a response such as one involving multiple sequenced sub-components may limit the ability to pre-program in a simple RT task. Taken together, these results suggest that pre-programming is dependent on the task characteristics and appears to involve implementation of strategies to increase programming efficiency.

Thesis (Ph. D.)--University of Queensland, 2002. / Includes bibliographical references.

An important feature of the auditory system is its ability to distinguish many simultaneous sound sources. The primary goal of this work was to understand how a robust, preattentive analysis of the auditory scene is accomplished by the subcortical auditory system. Reasonably accurate modelling of the morphology and organisation of the relevant auditory nuclei, was seen as being of great importance. The formulation of plausible models and their subsequent simulation was found to be invaluable in elucidating biological processes and in highlighting areas of uncertainty. In the thesis, a review of important aspects of mammalian auditory processing is presented and used as a basis for the subsequent modelling work. For each aspect of auditory processing modelled, psychophysical results are described and existing models reviewed, before the models used here are described and simulated. Auditory processes which are modelled include the peripheral system, and the production of tonotopic maps of the spectral content of complex acoustic stimuli, and of modulation frequency or periodicity. A model of the formation of sequential associations between successive sounds is described, and the model is shown to be capable of emulating a wide range of psychophysical behaviour. The grouping of related spectral components and the development of pitch perception is also investigated. Finally a critical assessment of the work and ideas for future developments are presented. The principal contributions of this work are the further development of a model for pitch perception and the development of a novel architecture for the sequential association of those groups. In the process of developing these ideas, further insights into subcortical auditory processing were gained, and explanations for a number of puzzling psychophysical characteristics suggested.

published_or_final_version / Medicine / Doctoral / Doctor of Philosophy

Understanding how odors are encoded in the brain is of fundamental importance to neurobiology. The first two stages of olfactory information processing have been relatively well studied in both vertebrates and invertebrates. However, the organizational principles of higher order olfactory representations remain poorly understood. Neurons in the first relay of the olfactory system segregate into glomeruli, each corresponding to an odorant receptor. Higher-order neurons can receive input from multiple glomeruli, but it is not clear how they integrate their inputs and generate stimulus selectivity.

published_or_final_version / Physiology / Master / Master of Philosophy

No description available.