Material and methods

9 subjects, 8 male and 1 female, aged between 35 and 73 years, with high-frequency cochlear hearing loss, participated in this study. Informed consent was obtained and the study was approved by the Leon Bérard Centre (Lyon, France) ethical committee. Subjects were tested in the ear in which the hearing-loss slope was steeper: i.e., 8 in the right ear and 1 in the left. All subjects were initially candidates for HA fitting and all were tested before auditory rehabilitation and then 1 month, 3 months and 6 months after it had begun. 6 subjects had binaural and three had monaural HA fitting (see Tables 1 and 2). All tests were conducted without the HA.

Table 1 : Data on patients with high-frequency hearing loss.
Table 1 : Data on patients with high-frequency hearing loss.

Fc is the lesion-edge frequency; fref is the frequency located 1 octave before Fc which is not amplified by the hearing aid; bDLF corresponds to the frequency with the best frequency discrimination limen.

The stimuli were pure tones of 350 ms length with a20 ms rise/fall time, generated via a Roland UA3 soundcard with a 44.1 kHz sampling rate. The tones were then delivered monaurally via SennheiserHD 265 linear headphones. Subjects were tested during 1 whole day (approximately 8 hours) per session, in a soundproof room.

The experimental procedure was carried out in three steps: first, absolute hearing threshold measurement; next, a loudness matching procedure; and finally, the frequency discrimination procedure.

Table 2 : Aided gains brought by the hearing aid at various frequencies at the end of the period of hearing aid adjustment.
Table 2 : Aided gains brought by the hearing aid at various frequencies at the end of the period of hearing aid adjustment.

Absolute hearing threshold: Each subject’s absolute hearing threshold was measured, in order to target the hearing-loss slope precisely. Thresholds were measured between 250 and 8,000 Hz, first by octaves and then down to 1/8 octave around the hearing loss. These measurements enabled the audiogram edge of the hearing loss, or cut-off frequency (Fc), to be located. Fc was defined as the test frequency that corresponded to the audiogram edge and as the highest with a hearing threshold not more than 5 dB above that of the best hearing frequency. Once the Fc was precisely located, the hearing thresholds of 9 neighbouring frequencies were determined: eight spaced at intervals of 1/8 octave around the Fc and one located 1 octave below Fc (the reference frequency).

The absolute threshold paradigm was the same as that of Thai Van et al. (2003): a one-interval, yes/no procedure in which the subject’s task was to signal when he or she perceived a tone. The stimulus level was initially set at 50 dBHL, and was increased after a ‘no’ response and decreased after a ‘yes’ response (one-down, one-up adaptive tracking rule). The stimulus level varied by 6dB steps until the fourth reversal, and by 2 dB steps thereafter. After a total of eight reversals, the procedure stopped and the absolute threshold as measured was the arithmetic mean of the levels of the last four reversals.

Each frequency was tested in random order and the final absolute threshold was the average of two measurements.

Loudness matching: The loudness matching procedure used a two-down, two-up tracking rule. Two tones were presented successively with a 300 ms silence in between. One of the tones was the reference and was fixed at 30 dBSL and the other was the comparison frequency, of variable level. The reference was the frequency located one octave below Fc, whereas the comparison frequency was one of the 9 frequencies neighbouring the Fc.

The order of presentation of the two tones was randomised, each tone having an equal probability of being presented first. The subject had to indicate in which interval the tone was louder. The level of the comparison tone was decreased when the subject judged it louder than the reference tone on two consecutive trials and increased when it was judged less loud on two consecutive trials. Steps of 5 dB were used until the second level reversal and 2 dB steps thereafter. The arithmetic mean of the levels at the last four reversals was computed. The various frequencies were tested in random order.