Bone conducted responses using the parallel auditory brainstem response (pABR) paradigm
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Objectives
Auditory brainstem responses (ABRs) are used to diagnose hearing loss across multiple frequencies in infants and others who cannot participate in behavioral testing. The parallel ABR (pABR) paradigm measures response waveforms to all frequencies from 500– 8000 Hz in both ears all at once, rather than serially in one ear at a time. Work so far has been with air conducted stimuli through insert earphones, but bone conducted stimuli are also an essential part of diagnosing the type of hearing loss by determining the relative contributions of conductive and sensorineural hearing loss. This study aimed to confirm the feasibility of bone conduction pABR.
Design
We recruited a cohort of young adults with normal hearing. An intensity series of two-channel ABRs were measured to pABR stimuli played through ER-2 insert earphones as well as with a B-71 bone vibrator with contralateral masking. Behavioral thresholds were also measured to compare dB nHL correction factors. To keep sessions under 3.5 hours only one ear was tested with bone conduction. The ear with bone conduction and the order of transducers was counterbalanced across participants. Waveform morphologies for each transducer were compared by their wave V latencies and amplitudes and residual noise.
Results
For the same dB nHL levels, responses to both transducers aligned well and wave V amplitudes and latencies were similar. While stimulus artifacts can be an issue with bone conduction, the steps taken to mitigate it in air conduction pABR were also effective for bone conduction: clear bone conducted responses were measured and residual noise was similar for both transducers. Bone conducted responses to 8 kHz were weak, which was not surprising given the physics of bone conduction and the output capabilities of the bone vibrator. As expected, behavioral thresholds were higher for pABR than pure tone stimuli, and by the same offset for both transducers. The dB nHL values differed due to the difference in reference equivalent threshold levels for inserts and the bone vibrator.
Conclusions
Air and bone conduction provided similar waveforms, indicating that bone conduction pABR is feasible. Together with establishing dB nHL values, this work represents an important step towards translating the pABR for diagnosing all types of hearing loss.
