Simultaneous electrophysiological measurements with auditory narrow-band stimuli

Infants are the primary target group for objective hearing threshold evaluations using electrophysiological measurements, with the auditory brainstem response (ABR) serving as the gold standard. A significant challenge in estimating hearing thresholds in infants is that current ABR measurements often exceed the duration of an infant’s natural sleep cycle. Given the limited time available for testing, audiologists are in need of a time-efficient method to evaluate hearing across a range of frequencies.
This thesis explored a new method for recording ABR responses to four frequency-specific stimuli simultaneously in both ears. In an initial investigation, ABRs recorded simultaneously were compared to those recorded individually using the current clinical method. Experimental work with normal-hearing young adults showed that it is feasible to record four frequency-specific ABRs in each ear simultaneously at clinically relevant stimulation levels. Moreover, the findings suggested that simultaneous presentation improved the frequency specificity of the ABR at low frequencies. A computational auditory nerve modeling framework was applied to understand how cochlear processing influences ABR waveform morphology. The model captured changes in waveform morphology and confirmed the enhanced frequency specificity of simultaneously recorded ABRs.
Additionally, the experimental and model-based evaluations explored the effects of systematically increasing the stimulus presentation level of one frequency. The results indicated interactions among the stimulus responses in the cochlea, leading to altered ABR waveform morphology. These findings led to a suggested maximum presentation level difference among the frequencies presented simultaneously. This criterion was subsequently incorporated into the development of a protocol for evaluating simultaneous ABR in infants.
Furthermore, the thesis explored a new method for visually presenting grand average ABRs from a group of subjects. The new approach significantly improved the representation of ABR waveform morphology, better reflecting the underlying individual waveforms.
Overall, the thesis demonstrated that recording ABRs to four frequency-specific stimuli simultaneously holds significant clinical potential for reducing test-time in objective hearing evaluations.