Procedure

Participants gave informed consent prior to the experiment. Otoscopy was performed on both ears. Participants with wax presence that contraindicated the use of foam insert earphones in either ear were excluded from immediate participation in this study. Tympanometry was conducted in both ears using an Interacoustics Titan tympanometer (Middelfart, Denmark). Participants were reimbursed for travel costs with a gratuity of 30 Australian dollars.

All participants underwent pure tone audiometry for each ear using Etymotic Research ER-3A (Elk Grove Village, IL) foam insert earphones. Frequencies were tested at 0.25, 0.5, 1, 2, 4, and 8 kHz with an audiometer (Diagnostic Audiometer AD28, Interacoustics, Middelfart, Denmark). Stimuli were calibrated at 70 dB HL using a IEC126 HA2 2-cc coupler, incorporating a 1-inch 4144 microphone, a 1-to-½ inch DB0375 adaptor, and a 2231 sound level meter (all Brüel & Kjær) using the relevant International Organization for Standardization 389–2 norms.⁴³

For normal hearers, programs 1, 2, and 3 were equivalent to an overall flat +0, +10, and +20 dB insertion gain response from 250 to 6,000 Hz, respectively. The maximum gain of +20 dB was matched to the hearing aid gain of +20 dB used for normal hearers in Billings et al to allow replication of their data.³⁰ For participants with a hearing loss, the three programs were fitted to the National Acoustic Laboratories - Non Linear (NAL-NL2) correct prescribed gain (+0 dB, program 1), to the prescribed gain minus 10 dB (−10 dB gain, program 2) and the prescribed gain plus 10 dB (+10 dB gain, program 3). This gain variation was therefore similar to that provided to the normal hearers. Real ear insertion gain measures were verified with an insert probe tube into the subject's ear and compared with 55, 65, and 75 dB SPL fitting targets. Attack and release times were measured in a hearing aid test box. Tables 1 and ​and22 present the relevant hearing aid parameters for both populations.

The participants were kept awake and attentive during testing by allowing them to watch a silent movie with closed captions. An earplug was inserted into the subject's nontest ear. Electrode sites were prepared using a cotton applicator and electrode gel. Single-use Ambu Blue Sensor N (Copenhagen, Denmark) self-adhesive electrodes were used. The active electrode was placed on Cz, the reference electrode on the mastoid (randomized), and the common electrode on the high forehead.⁴⁴ Electrode impedance was checked before and after each recording. If necessary the preparation was repeated to achieve an impedance under 5 kOhm between active and common, and between reference and common. During recording using the HEARLab system (Frye Electronics, Tigard, Oregon), the electroencephalogram activity was amplified by a factor of 1,210, and band-pass filtered between 0.3 and 30 Hz. The recording window consisted of a 200-millisecond prestimulus and a 600-millisecond poststimulus. Artifact rejection was set at ± 150 μV. Baseline correction was applied to each individual epoch based on the average over 100 milliseconds prior to stimulus onset.

A total of 12 recordings were conducted, comprising a test and retest of six different conditions. The conditions included three unaided presentations at 55, 65, and 75 dB SPL in the free field and three aided recordings at 55 dB SPL in the free field with three different gains. The presentation order was randomized. Each recording consisted of three speech sounds being rotated in blocks of 25, presented with a stimulus onset asynchrony of 1,125 milliseconds. Data acquisition stopped after 70 accepted epochs for each speech sound.

For the N1 component, the mean amplitude over a latency interval of 75 to 135 milliseconds was calculated. For the P2 component, the amplitude was averaged over the latency interval from 150 to 210 milliseconds. This approach was used to minimize the effect of residual noise in the averaged waveform on cortical response amplitude. The difference between P2 and N1 mean amplitudes was analyzed as the main dependent variable.