Cortical Representation of Tympanic Membrane Movements due to Pressure Variation: An fMRI Study

Middle ear sensory information has never been localized in the homunculus of the somatosensory cortex (S1). We investigated the somatosensory representation of the middle ear in 15 normal hearing subjects. We applied small air pressure variations to the tympanic membrane while performing a 3T-fMRI study. Unilateral stimulations of the right ear triggered bilateral activations in the caudal part of the postcentral gyrus in Brodmann area 43 (BA 43) and in the auditory associative areas 42 (BA 42) and 22 (BA 22). BA 43 has been found to be involved in activities accompanying oral intake and could be more largely involved in pressure activities in the oropharynx area. The tympanic membrane is indirectly related to the pharynx area through the action of tensor tympani, which is a Eustachian tube muscle. The Eustachian tube muscles have a role in pressure equalization in the middle ear and also have a role in the pharyngeal phase of swallowing. Activation of BA 42 and BA 22 could reflect activations associated with the bilateral acoustic reflex triggered prior to self-vocalization to adjust air pressure in the oropharynx during speech. We propose that BA 43, 42, and 22 are the cortical areas associated with middle ear function. We did not find representation of tympanic membrane movements due to pressure in S1, but its representation in the postcentral gyrus in BA 43 seems to suggest that at least part of this area conveys pure somatosensory information

Mathematical modelling and electrophysiological monitoring of the regulation of cochlear amplification

Thesis presented in partial fulfilment of the requirements of the degree of Master of Clinical Audiology / Doctor of Philosophy of The University of Western AustraliaThe cochlea presumably possesses a number of regulatory mechanisms to maintain cochlear sensitivity in the face of disturbances to its function. Evidence for such mechanisms can be found in the time-course of the recovery of CAP thresholds during experimental manipulations, and in observations of slow oscillations in cochlear micromechanics following exposure to low-frequency tones (the “bounce phenomenon”) and other perturbations. To increase our understanding of the these oscillatory processes within the cochlea, and OHCs in particular, investigations into cochlear regulation were carried out using a combination of mathematical modelling of the ionic and mechanical interactions likely to exist within the OHCs, and electrophysiological experiments conducted in guinea pigs. The electrophysiological experiments consisted of electrocochleographic recordings and, in some cases, measurement of otoacoustic emissions, during a variety of experimental perturbations, including the application of force to the cochlear wall, exposure to very-low-frequency tones, injection of direct current into scala tympani, and intracochlear perfusions of artificial perilymph containing altered concentrations of potassium, sodium, and sucrose. To obtain a panoramic view of cochlear regulation under these conditions, software was written to enable the interleaved and near-simultaneous measurement of multiple indicators of cochlear function, including the compound action potential (CAP) threshold, amplitude and waveshape at multiple frequencies, the OHC transfer curves derived from low-frequency cochlear microphonic (CM) waveforms, distortion-product otoacoustic emissions (DPOAEs), the spectrum of the round-window neural noise (SNN), and the endocochlear potential (EP). The mathematical model takes into account the known electrical properties of OHC, and includes the effect of fast and slow-motility of the cell body on transducer operating point and apical conductance. Central to the operation of the model is a putative intracellular 2nd-messenger system based on cytosolic calcium, which is involved in regulation of i) the operating point of OHC MET channels via slow motility and axial stiffness; ii) the permeability of the basolateral wall to potassium (via calcium-sensitive potassium channels); and iii) the cytosolic concentration of calcium itself, via modulation of its own sequestration into (and release from) intracellular storage organelles, and extrusion from the cell. The model was constructed in a manner which allowed simulation of different cochlear perturbations, and the comparison of results from these simulations to experimental data. The mathematical model we have developed provided a physiologically-plausible and internally-consistent explanation for the time-courses of the cochlear changes observed during a number of different perturbations. We show that much of the oscillatory behaviour within the cochlea is consistent with underlying oscillations in cytosolic calcium concentration. We conclude that a number of the discrepancies between the simulation results and the experimental data can be resolved if the cytosolic calcium functions as two distinct pools: one which controls basolateral permeability and one which controls slow motility. This two-calcium-pool model is discussed

The post-auricular muscle reflex (PAMR): Its detection, analysis, and use as an objective hearing test

A number of fundamental characteristics of the post-auricular muscle response (PAMR) have been examined in adult and infant human subjects using an automated computer-based measurement system. This system allowed simultaneous examination of the changes in background electrical activity of the PAM, and extraction of information regarding the sound-evoked PAMR waveform, such as response amplitude and peak latency. It was found that the PAMR was best recorded using an active electrode located directly over the body of the muscle, and a reference electrode located on the dorsal surface of the pinna. In addition, it was found that during lateral rotation of the eyes towards the recording electrodes the peak-to-peak amplitude of the PAMR increased by an average of 525%. The increase in response amplitude was highly correlated with the increase in EMG observed during this manoeuvre, suggesting that the mechanisms that increase both EMG and PAMR amplitude probably occur at a common point. The voltage spectrum of the PAMR was also measured. Contrary to previous findings (Thornton, 1975), the voltage spectrum of the PAMR extended from 10 Hz to approximately 550 Hz, with a broad spectral peak centred between 70 Hz and 115 Hz. Finally, a cheap, efficient and reliable objective hearing test was developed, using the correlation measure of the PAMR. The availability of such a device has the potential to vastly increase the number of children that are screened for hearing disorders, especially in poorer communities who do not have the funds or the expertise to establish screening programs using the currently available objective techniques of ABR and oto-acoustic emission measurement

An update on the University of Canterbury Master of Audiology programme

The Master of Audiology programme at the University of Canterbury was established in 2005 to address the shortage of Audiologists in New Zealand, and has so far graduated 72 Audiologists. Our core teaching staff consists of Dr Rebecca Kelly-Campbell, Associate Professor Greg O’Beirne, Dr Donal Sinex, and our clinical educators Dr Bekah Gathercole and Ms Fiona Yip. In addition, 20 distinguished international scholars have taught courses to our students as part of the Erskine fellowship programme. Following our successful NZAS Endorsement in 2013, this year sees the introduction of a reorganised and updated curriculum and a streamlined course structure. While the second year thesis component remains unchanged, the 16 postgraduate and two undergraduate courses of our old curriculum have been replaced by eight team-taught courses that are entirely at the postgraduate level. In addition to placements at our own Speech and Hearing Clinic, our close working relationships with NZAS members allow us to arrange supervised clinical practica for our students in a variety of settings (both public and private) within the greater Christchurch area and throughout New Zealand. Despite the challenges faced by academic programmes in New Zealand, the UC Master of Audiology is a thriving programme that continues to produce graduate Audiologists of high quality