Objective Measurement Optimization in Early Hearing Detection and Intervention
Abstract
Following the fitting of amplification devices, outcomes need to be evaluated to determine benefit of intervention. Objective measurements, which generally require rapid acoustic or electrophysiological measurement without active participation of the individual, are one subset of evaluation methods.
This thesis aimed to improve the reliability, accuracy, and efficiency of a subset of objective outcome measurements: the speech-evoked Envelope Following Response (EFR), and the wideband real-ear-to-coupler-difference (wRECD), which are used in hearing aid validation and verification respectively.
The speech-evoked EFR, a neural response reflecting phase-locked activity to the envelope of a speech stimulus, can be detected using a variety of statistical indicators. Chapter 1 focused on improving speech-evoked EFR detection by comparing the sensitivity and efficiency of statistical indicators in adults and infants. Results show that indicators using phase information tend to outperform those that do not. Accuracy and speed of speech-evoked EFR detection was also found to differ between infants and adults.
The main contribution of this thesis is the proposal and validation of a clinically viable test paradigm for wRECD measurement which does not require a probe-tube microphone and is not affected by reflected wave interference. The projects evaluated the measurement of the integrated pressure level (IPL) wRECD using a Thevenin-equivalent source parameter calibrated transducer. Calibration of the transducer was found to be reliable across time. IPL wRECD improved wRECD reliability, high-frequency performance, and simultaneously assessed middle ear function using wideband acoustic immittance. Below 5 kHz, the IPL wRECD was not clinically significantly different from probe-tube microphone measurements when a generic coupling method was used. An individual’s earmold significantly impact resulting wRECD measurements due to variable lengths of tubing associated with the earmold. The current thesis proposed a method to acoustically determine the tubing length to create accurate and reproducible generic-tip-to-earmold transforms, which improved the estimation of the earmold wRECD.
In summary, IPL wRECD measurement shows promise as an alternative to probe-tube microphone wRECD measurement procedures and is expected to improve validity of hearing aid fittings, especially in the high frequencies. Similarly, speech-evoked EFRs can be used as an objective measurement to validate hearing aid fittings accurately and efficiently.