Can the auditory late response indicate audibility of speech sounds from hearing aids with different digital processing strategies

Abstract

Auditory late responses (ALR) have been proposed as a hearing aid (HA) evaluation tool but there is limited data exploring alterations to the waveform morphology from using digital HAs. The research had two phases: an adult normal hearing phase and an infant hearing impaired clinical feasibility phase. The adult normal hearing study investigated how different HA strategies and stimuli may influence the ALR. ALRs were recorded from 20 normally hearing young adults. Test sounds, /m/, /g/, /t/, processed in four HA conditions (unaided, linear, wide dynamic range compression (WDRC), non linear frequency compression (NLFC)) were presented at 65 dB nHL.Stimuli were 100 ms duration with a 3 second inter-stimulus interval.
An Fsp measure of ALR quality was calculated and its significance determined using bootstrap analysis to objectively indicate response presence from background noise. Data from 16 subjects was included in the statistical analysis. ALRs were present in 96% of conditions and there was good repeatability between unaided ALRs. Unaided amplitude was significantly larger than all aided amplitudes and unaided latencies were significantly earlier than aided latencies in most conditions. There was no significant effect of NLFC on the ALR waveforms. Stimulus type had a significant effect on amplitude but not latency. The results showed that ALRs can be recorded reliably through a digital HA. There was an overall effect of aiding on the response likely due to the delay, compression characteristics and frequency shaping introduced by the HA. Type of HA strategy did not significantly alter the ALR waveform. The differences found in ALR amplitude due to stimulus type may be due to tonotopic organisation of the auditory cortex.

The infant hearing impaired study was conducted to explore the feasibility of using ALRs as a means of indicating audibility of sound from HA’s in a clinical population. ALRs were recorded from 5 infants aged between 5-6 months with bilateral sensori neural hearing loss and wearing their customised HA’s. The speech sounds /m/ and /t/ from the adult study were presented at an rms level of 65 dB SPL in 3 conditions: unaided; WDRC; NLFC. Bootstrap analysis of Fsp was again used to determine response presence and probe microphone measures were recorded in the aided conditions to confirm audibility of the test sounds. ALRs were recordable in young infants wearing HAs. 85% of aided responses were present where only 10% of unaided were present. NLFC active improved aided response presence to the high frequency speech sound /t/ for 1 infant. There were no clear differences in the aided waveforms between the speech sounds. The results showed that it is feasible to record ALRs in an infant clinical population. The response appeared more sensitive to improved audibility than frequency alterations.

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