Material and methods

The six subjects, 5 male and 1 female, who participated in this experiment were aged between 46 and 68 years old. All had high-frequency cochlear hearing loss. Informed consent was obtained from the participants prior to the study, which was approved by an ethics committee in Lyon (France). All subjects were initially applicants for HA fitting and were tested first before and then 3 months after their auditory rehabilitation began. 3 subjects had binaural and 2 monaural hearing-aid (HA) fitting. The last subject gave back his hearing aids during the first week of rehabilitation, but was also tested at three months in order to serve as control. All were tested in the ear in which the hearing-loss slope was steeper: i.e., 3 in the right ear and 3 in the left (see Tables 1 and 2). All tests were conducted without the HA.

Fc is the lesion-edge frequency. fref is the reference frequency, located 1 octave before Fc and not amplified by the HA. bDLF: frequency with the best frequency discrimination limens. Data for the control subject S.G. are in italics.

Stimuli were the same throughout: i.e., pure tones of 350 ms length with a 20 ms rise/fall time, generated via a Roland UA3 soundcard with a 44.1 kHz sampling rate. Tones were delivered monaurally via Sennheiser HD 265 linear headphones. Subjects were tested during 1 day and a half (approximately 10 hours) in each session, in a soundproof room.

The experiment comprised four assessments:

1. absolute threshold,
2. loudness matching,
3. frequency discrimination, and
4. reaction time.

The first three steps were carried out as in Thai Van et al.’s (2003) study. The experimental procedure was the same before rehabilitation and at three months.

Absolute hearing threshold: Each subject’s absolute hearing threshold was measured, in order to target the hearing-loss slope precisely. Thresholds were measured at 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 procedure: 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.

Frequency discrimination procedure: DLFs for pure tones were then measured, using all of the frequencies around the Fc used for the loudness-scaling task except the highest. In order to prevent subjects exploiting any residual loudness differences as the frequency changed in the DLF procedure, the level of each stimulus was varied by a random value selected from a uniform distribution ranging between -6 and +6 dB around the loudness-matching value. The DLFs were measured using a three-interval, two-alternative forced-choice procedure with a two-down, one-up decision rule. On each trial, two of three successive observation intervals contained pure tones at a reference frequency; the remaining interval, which had a 50% chance of coming second and a 50% chance of coming third, contained a pure tone of a frequency that was variable but always higher than the reference frequency. The intervals were separated by 300 ms silent gaps. The subject’s task was to identify the interval that contained the highest tone. The difference between the reference and the variable frequency was initially set at 20%. After two consecutive correct responses, the difference in frequency was decreased, and was increased after each incorrect response. Ten reversals were obtained in each run. The frequency difference was varied by a factor of 2 until the fourth reversal occurred and by a factor of 1.41 thereafter. After each run, the geometric mean for the last six reversals was computed. The order of testing was determined by random selection from among the set of frequencies, spaced 1/8 octave apart, for which the absolute hearing threshold had been previously measured. Three runs were collected at each of these test frequencies.

Reaction time: The present study employed simple reaction time measurements: i.e. subjects had to press a button as quickly as possible when they heard a tone. Subjects were tested on the reference frequency and on six frequencies surrounding the hearing-loss cut-off frequency and the best discrimination limen for frequency. For each frequency, measurement was repeated 200 times; consequently, the total number of recorded RTs was 1,400 per subject. The tone level was set at 30 dB in the loudness matching procedure. A recording box was specially designed in the laboratory in order to measure reaction time as accurately as possible.

The reaction time measurement procedure was as follows: on each trial, a dim red 50 ms flash indicated the occurrence of a stimulus. This was followed by an interval of between 600 and 1,000 ms.. A monaural tone of pseudorandomly varying frequency was then presented. As soon as the subject pressed the button response, a new trial was set off. If no response was made after 1 second, the trial was automatically terminated. The inter-trial interval was between 100 ms and 1,000 ms. Every 35 trials, there was a pause to allow the subject to rest. Analysis was performed on RTs lasting more than 100 ms and less than 800 ms. 3.4 % of the data were eliminated on this criterion.

Statistical analysis: Interaction between the influence of the auditory rehabilitation (before and 3 months after) and the various frequencies tested was analysed by 2-way repeated measures ANOVA. With reaction time as dependent variable, four frequencies around Fc (fref, Fc-1/8 octave, Fc, Fc+1/8 octave) and four around the bDLF (fref, bDLF-1/8 octave, bDLF, bDLF+1/8 octave) were tested. With auditory threshold and with loudness perception as dependent variables, ten frequencies around Fc were tested (fref and 9 frequencies from Fc-1/2 to Fc+1/2 octave). Results were regarded as statistically significant when p <0.05. Post-hoc comparisons were made by Tukey test.

The control subject (S.G.) was excluded from all statistical analysis on means. His performances were studied individually for the analysis of RT measurements at Fc and at bDLF and for the linear regression measurements.