}ft: \ s n
Figure 3.2: Spontaneous otoacoustic emissions in a normal subject with multiple frequency component spectrum
The presence of SOAEs was observed as spectral peaks of amplitude of -30 dB SPL or greater, and at least 5 dB above the noise floor, repeatable in two consecutive scans. SOAEs at frequencies < 500 Hz were not considered due to the higher susceptibility to noise contamination in this frequency region.
3.33.4 Medial olivocochlear suppression test
This test consists of recording of TEOAEs with and without contralateral stimulation and the difference in responses is considered, at least in part, to be the MOC effect. A dual channel OAE analyser was used, one channel (A) for ipsi- and the other (B) for contralateral acoustic stimulation. Both, ipsi- and contralateral, stimuli were delivered
through the identical B-type IL088 probes (section 3.3.3, Equipment). For ipsilateral
stimulation a click in linear mode (4 clicks of identical intensity and polarity) was used. An intrameatal peak stimulus level of 60±3 dB SPL was used, being sufficiently low to be within “compressive” range of the cochlear activity (described in the section 1.3.5) and to reduce artifacts - middle ear ringing (as linear click is not noise-protected), and sufficiently high to elicit TEOAE response in all subjects. For contralateral stimulation a 5ms burst of white noise ( 0 . 5 - 6 kHz) was used, as this was known to be the most effective for suppressing OAE (Collet et al., 1990b) at 40 dB SL. This level was low enough not to trigger the acoustic reflex and avoid “cross-talk” (sections 1.2.1.1 and 1.3.5), but sufficiently high to elicit contralateral effect, through the activation the medial olivocochlear reflex. An alternating technique, a “Difference B on/off” mode, from the IL092 software was used. This mode allows alternating recording of TEOAE responses with and without contralateral stimulation. A total of 600 sweeps were recorded, in 10 groups of 60 sweeps. The average responses were directly computed and the difference obtained by their subtraction, represented the suppression effect.
3.3.3.5 Calibration
Calibration of the IL088 system was performed using a 1 cm3 - test cavity supplied by the manufacturer, on average once a week. Synthetic stimuli, three tones at different frequencies,
provided by the IL088 software, were delivered through the probe tip inserted into the test cavity. Additional, “biological”, calibration was carried out by the author, on average twice a month. The inter-session performance of the probe remained stable (within ±ldB) throughout the project execution. Figure 3.3 illustrates the stability of the probe output intensity.
c B jl* 5 p o m T n |ISE LEVEL 15,5 JEETIOK ft rlJ .J S tiM lu s ILOvS ME A nalyser 114. 2BB8d
Pa ti e n t: Bcrkji E a r . r i g h t C a se :ito rm l l a t e . . . . 05/18/1994 Response H O I5I.LvaJ3.61 sr47,81 BBJIWUXT t 4.SHM
StiM ulus IL088 OAE A nalgser (14.288©
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Response Ua»efor* PCNSE 7 .6 1
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HtiLin CLIKM 4 .5 It NX NonLin CLIKNSTIMULUS d£ CA ,H «tt h irtf*» -8,11 18,11 Response litaeefciit rtm3Tfl PONSE 9.3 1 Response liiavefor* RESPONSE 9 .8 1
upvefew 93 SMC R S V O /9 ® ! :AUMi.:s.o<t 51 94 85 <6 68 x 101 4 9 3 3 1 1 8 ,5it?a 8 5*P a (2 dB) S T IH H U IS IW fg k ; ir m A T ittxW Six tfsr n« oFj£i TEST T IP * DM 4M EC MTC DWCOV W Z d i w c t c c v C ‘iC H X F T ft C : \ D C M FILLED 1V1» IC V IW OIHCCTOVW C:>ECHOORTR\B2C\B M SCP&M BATA 5C tH C f ?.r*i P re se t P re s e t
Figure 3.3: TEOAE responses recorded from the right ear o f a normal subject (the author) over the period 1994-1997, demonstrating the output stability o f the IL088 system, as well as a high intrasubject reproducibility o f the response amplitude (< ±1 dB) and power spectra.
3.4 STATISTICAL ANALYSIS
All data were analysed using SPSS (Statistical Package for Social Sciences), including routine statistical procedures, at the significance level p<0.05.
Measures o f variability
The spread of variation was expressed by the mean ± standard deviation and range.
Additionally, for graphic presentation, a 95% confidence interval (Cl) was used, deducting a 95% probability that the true mean of the population was within ± 2 standard errors of the sample mean.
Tests for statistical significance
i.) Numerical data
The significance of the data was analysed applying different tests, depending on distribution. The normality of distribution of a variable was estimated by inspection of the normality plots (e.g. histogram distribution), with normal distribution being in the shape of the Gaussian curve.
For the comparison between two groups:
- Independent t-test (Levene’s) for parametric data, with normal distribution, was used to compare the means of the two independent groups of subjects, or
- Mann-Whitney U-test, for non-parametric data, when the data were not normally distributed.
For the comparison between three or more groups:
- One-way analysis of variance (ANOVA), for normal distribution
Post hoc Bonferroni multiple comparison test was used to determine which group means are significantly different from others, or
- Kruskal-Wallis test, for the data without normal distribution, was employed.
ii) Nominal data
X2 - test was applied to analyse the statistical significance of nominal (frequency) data of subjects assigned to one of two different categories (e.g. gender: male or female). This test analyses the distribution of a categorical variable against the hypothesis that each category has a specified proportion of cases in the population.