Compensating for Basilar Membrane Mechanics When Obtaining the Auditory Brainstem Response

Abstract

The auditory brainstem response (ABR) is a scalp-recorded electrical response elicited from the brain by acoustic stimuli. It consists of several waves (peaks), each representing synchronous neural activity elicited from the auditory nerve and/or auditory nuclei within the brainstem. The ABR is most commonly used for neonatal hearing screening, hearing threshold estimation, long term monitoring of intensive care unit patients with traumatic brain injury (TBI), and intra-operative monitoring (IOM) for any skull base surgery. When recording the ABR the signal-to-noise ratio (SNR) is extremely poor, due to the omnipresent electrophysiological noise between the neural generators and the surface recording electrodes. Theoretically, the SNR can be improved by stimulating a greater number of auditory nerve fibres. Hence, eff ective acoustic stimuli are those that stimulate many frequency-speci c hair cells simultaneously. Broadband 'click' stimuli were originally thought to be such ideal stimuli, as they contain many frequency components - all of which are presented to the outer ear at the same instant. However, the physiology of the inner ear suggests that click stimuli undergo temporal distortion when entering the cochlea, and no longer achieve synchronous ring at all. To elicit ABR waveforms of maximal amplitude, recent studies have suggested that 'click' stimuli be replaced by rising-frequency 'chirp' stimuli. Chirp stimuli theoretically compensate for temporal dispersions that occur down the basilar membrane by delaying the higher frequency content of the stimulus until the lower frequency traveling waves are closer to the cochlea apex. This compensation results in simultaneous membrane maxima and allows all regions of the basilar membrane to contribute to the ABR. The final product is an ABR waveform that is larger, can be recorded in less time, and potentially has more diagnostic power. However, one aspect of this paradigm that is not yet clear is how to ensure that the temporal compensation off ered by a chirp is suited to the cochleae under test. Previous chirp studies have based their parameters on population averages, but normal variation in physiologies across male and female populations may limit the applicability of a 'one-size- fits-all' design for compensating chirps. This thesis investigated the utility of several 'population-based' chirp stimuli from the literature, with regard to synchronous discharges and ABR amplitudes. Until now, synchrony in chirp-evoked responses have been largely quanti fied using observed improvements in wave V amplitude. However, measuring wave V amplitude alone is not necessarily a robust measure of neural synchrony. This research quantifi ed the synchrony of basilar membrane displacement, and hence neural synchrony, in a series of studies involving click and chirp-evoked responses from normally hearing individuals. A range of surrogate measures were employed to quantify neural synchrony, such as wave V amplitudes, latency variances, rates of SNR increase, Fourier-domain magnitudes, and Fourier-domain phase variances. The results of one study, at high presentation intensities, indicated that the chirp-evoked responses were less synchronous than for click stimuli, despite evoking larger wave V amplitudes. In a subsequent study, at lower presentation intensities, the surrogate measures for synchrony indicated that the chirp-evoked responses were more synchronous than for click stimuli. Taken together, these results suggest that synchronous displacement on the basilar membrane can be impaired by a broadening of excitation at higher intensities and that enhanced wave V amplitudes can sometimes be more attributable to increased neural recruitment rather than neural synchrony. Additionally, this thesis also examined the relationship between chirp sweep rate and response synchrony by varying the delay between high- and low-frequency portions of chirp stimuli. Two approaches to tailoring chirp parameters to individual cochleae (in order to maximize neural synchrony) were described and investigated. Such research is of interest both to enhance the ABR and as a potential way to map the physiology of the basilar membrane. The results of these investigations con firm that gender-based di fferences exist in the synchrony of chirp-evoked ABRs, but that the described methodologies of tailoring chirp parameters to individual cochleae are not yet tenable. The thesis concludes that a tailoring of chirp parameters to gender may be bene cial in pathologies that severely aff ect neural synchrony, but that such a customization may not be necessary in routine clinical applications.