|Year : 2014 | Volume
| Issue : 1 | Page : 12-17
Fast psychophysical tuning curves of the cochlea in normal hearing individuals
Mohamed Shabana1, Brian CJ Moore2, Mostafa el-Khosht1, Mona H Selim1, Mona Dokla1
1 Department of Audiology, ENT, Cairo University, Cairo, United Kingdom
2 Department of Experimental Psychology, University of Cambridge, Cambridge, United Kingdom
|Date of Submission||28-Feb-2014|
|Date of Acceptance||28-Mar-2014|
|Date of Web Publication||28-Jul-2014|
Brian CJ Moore
Emeritus Professor of Auditory Perception, Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge CB2 3EB
Source of Support: None, Conflict of Interest: None
Fast psychophysical tuning curve (fPTC) test is a fast computer-based method that aims to assess the frequency selectivity of the cochlea and to detect the dead regions. It can quickly identify tip frequency and Q10 of psychophysical tuning curves (PTCs) derived by using a band of noise that changes in center frequency and a Bιkιsy method to adjust the masker level required for threshold of the noise. We applied this method in normal hearing individuals in the presence of threshold equalizing noises at three signal levels. The sharpness of the PTCs (Q10) and the typical shift of tips of the PTCs for 16 normal hearing individuals, when the tip frequency is estimated for the average of a forward and reverse sweep, were obtained. The results were used to determine the mean, SD, and 95% confidence interval of the shifts in normal hearing individuals.
The purpose of this experiment was to estimate the typical shift of tips of the PTCs for normal hearing individuals. The results were used to determine the mean, SD, and 95% confidence interval of the shifts. The sharpness of the PTCs change with signal level under conditions where off-frequency listening is restricted, using a background noise, was also assessed. This was performed to allow a comparison with the results of hearing-impaired patients tested at the same level (but without background noise).
Sixteen adults of both sexes (eight male individuals and eight female individuals) were randomly selected to establish normative data for the fPTC test. They were selected with age ranging from 18 to 45 years. All individuals had normal middle ear function as indicated by tympanometry and acoustic reflex measurement and by hearing threshold equal to or better than 20 dB at octave frequencies in the frequency range (250-8000 Hz) (as defined by ANSI S3.6-2004).
Keywords: fast psychophysical tuning curves, Q10, tip frequency
|How to cite this article:|
Shabana M, Moore BC, el-Khosht M, Selim MH, Dokla M. Fast psychophysical tuning curves of the cochlea in normal hearing individuals. Adv Arab Acad Audio-Vestibul J 2014;1:12-7
|How to cite this URL:|
Shabana M, Moore BC, el-Khosht M, Selim MH, Dokla M. Fast psychophysical tuning curves of the cochlea in normal hearing individuals. Adv Arab Acad Audio-Vestibul J [serial online] 2014 [cited 2017 Aug 18];1:12-7. Available from: http://www.aaj.eg.net/text.asp?2014/1/1/12/137559
| Introduction|| |
Normal hearing process has two stages: a mechanical stage and a second electrochemical stage. Initially, the sound enters the ear, reaches the cochlea, and vibrates the basilar membrane. The electrochemical stage involves transmission of signal along the auditory pathway. Interference with the acoustic signal at any stage will result in hearing loss .
The role of the outer and middle ear mechanisms in collecting sound energy and conveying it to the cochlea is well understood. Pathology in these areas affects hearing through stimulus attenuation, which can be severe but never profound. Unlike sensory loss, conductive loss introduces no distortion; hence, it can be very effectively corrected by amplification.
The cochlea is the principal hearing organ of the mammalian auditory system. Inner and outer cells within the cochlea are arranged in rows, which extend from one end of the cochlea to the other. The IHCs transduce information to the brain about the presence of acoustic vibrations at a specific place in the cochlea, whereas outer hair cells (OHCs) influence mechanical vibrations before they reach the inner hair cells (IHCs).
A psychophysical tuning curve (PTC) is a curve plotting the level of a narrow-band masker needed to mask a fixed sinusoidal signal, as a function of masker frequency. PTC demonstrates auditory nerve fibers best response, and hence better thresholds at the fibers' characteristic frequency and at frequencies immediately surrounding it .
Testing PTCs of cochlea involves measuring the threshold of a masker changing in its center frequency needed to mask a fixed narrow-band signal near threshold (for sharpest tuning). For normally hearing listeners, the tip of the PTC (i.e. the frequency at which the masker level is lowest) always lies close to the signal frequency . If the tip of the curve is shifted away from the signal frequency, then this frequency is detected by an area on the basilar membrane other than the area that it should be tuned to because of deficiency in the inner hair cells at this point. This is defined as off-frequency listening . The edge frequency of the PTC is the boundary of the healthy inner hair cells. When the tip of the PTC is shifted downward in frequency, this indicates a high-frequency dead region beginning at the frequency of the shifted tip. When the tip is shifted upward in frequency, this indicates a low-frequency dead region, whose upper boundary lies at the tip frequency of the PTC .
PTCs potentially provide the most accurate method for determining the frequency limits of dead regions [2,5]. In principle, measurement of PTCs provides a more precise method than the threshold equalizing noise (TEN) test of estimating the edge frequency of a dead region.
PTCs of the cochlea is a lengthy procedure; the diagnosis of a dead region typically takes at least 2 h, and the application in kids or elderly people may take even longer time. Thus, PTCs measured in the traditional way are not suitable for use in routine clinical practice .
Sek et al.  developed a fast method for determining PTCs of the cochlea, using a band of noise that sweeps in center frequency and a Bιkιsy method to adjust the masker level required for threshold. Sek et al.  studied the shapes of the PTCs and concluded that they were similar for the fast and traditional methods, for both normally hearing and hearing-impaired individuals.
The fast psychophysical tuning curves (fPTCs) test takes only 4-5 min/frequency to apply. Signal frequencies from 500 to 4000 Hz in half-octave steps can be used. Even if PTCs are measured for all octave and interoctave signal frequencies, the total testing time should not exceed about 30-40 min. The fPTC makes the PTCs faster and more clinic friendly.
This study was part of an extended study designed to establish normative data of fPTC regarding the tip frequencies and the Q10 and to study the variables affecting it. In other parts of the study, we also used the fPTC test to evaluate the prevalence of cochlear dead regions within the population of people with sensorineural hearing loss and to examine the relationship between presence/absence of dead regions and slope of the audiogram, severity of the thresholds, age, and sex. Q10 was evaluated in a third experiment to assess frequency selectivity among a group of hearing-impaired patients who suffered from sensorineural hearing loss and those who did not.
| Participants and methods|| |
This study was designed to evaluate 16 normal hearing individuals. They were volunteers examined at Audiology Unit, Out-Patient Unit, Cairo University during the period from May 2010 to October 2012.
Sixteen adults of both sexes (eight male individuals and eight female individuals) were randomly selected to establish normative data for the fPTC test. They were selected with age ranging from 18 to 45 years. All participants had normal middle ear function as indicated by tympanometry and acoustic reflex measurement and by hearing threshold equal to or better than 20 dB at octave frequencies in the frequency range (250-8000 Hz) (as defined by ANSI S3.6-2004). In addition, they had excellent speech discrimination (as defined by Soliman et al. [6,7]).
Details of the fast psychophysical tuning curve test procedure
Calibration of the output was performed to ensure proper stimulus level. Details of the procedure are elaborated in the help file of the program.
For the fast method, PTCs were determined using a pure tone signal at frequency (f) and as narrow-band noise masker. The signal was presented at a sensation level of 10 dB and was pulsed on and off in a regular manner. Each tone pulse lasted for 500 ms (including rise and decay times of 20 ms each) and the gap between successive pulses was 200 ms. The center frequency of the masker swept from fmax to fmin for a forward sweep or from fmin to fmax for a reverse sweep, over a 4-min period. The rate of change of the masker center frequency was constant on a logarithmic s frequency scale . For the fast method, the entire masker waveform was presynthesized, using MATLAB (Cambridge, UK), and stored on the hard drive of the PC.
The measurement method was similar to that used in Békésy audiometry. However, here, the noise level required just to mask the signal was determined as a function of the changing masker center frequency. The measurement of a PTC started with several pulses of the signal without the masker, so that the participant knew what to listen for. After those initial pulses, the masker was turned on. Participants were requested to press a button when the signal was audible and to release the button when the signal was inaudible. When the button was pressed, the level of the noise increased at a rate that was chosen to be 2 dB/s. When the button was released, the level decreased at the same rate. The level was changed in 0.1-dB steps.
The signals were presented monaurally by TDH39 headphones. Participants were seated in a sound-attenuating booth [Figure 1].
Testing was performed according to the following rules:
(1) Signal frequencies of 0.5, 1, 2, and 4 kHz were used.
(2) The PTCs were measured in the presence of a TEN with level of 20, 35, and 50 SPL/ERB N.
(3) The signal level was set at 15 dB above the TEN level.
(4) For each signal frequency and level, a forward and a reverse sweep were performed. The estimated tip frequency was averaged for the two cases.
(5) For each participant, the total number of PTCs to be measured was: four signal frequencies×three levels × two sweeps × two ears = 48.
Testing started with the lowest TEN levels and ended with the highest TEN. Testing for all signal frequencies at a given TEN level will be completed before the next TEN level is used. The signal and noise levels were checked with the participant first and the aforementioned levels were chosen on the basis of trials with several participants' tolerability. Eight participants were tested with signal frequencies in the order 0.5, 1, 2, and 4 kHz. Of these eight, four were tested for each signal frequency using a forward sweep and then a reverse sweep, and the other four were tested using a reverse sweep and then a forward sweep.
Eight participants were tested with signal frequencies in the order 4, 2, 1, and 0.5 kHz. Of these eight, four were tested for each signal frequency using a forward sweep and then a reverse sweep and the other four were tested using a reverse sweep and then a forward sweep.
The fPTC test allows the raw PTC [Figure 2] material to be analyzed automatically using an option of five statistical methods [low-pass filtering (at three frequencies: 0.15, 0.2, and 0.25 Nq), moving averages (2 and 4 points), quadratic function, double regression, and ROEX].
The fPTCs were described in terms of tip frequency and sharpness of tuning represented by Q10 dB [Figure 3]. Q10 equals the frequency of the response peak divided by the bandwidth 10 dB below the peak .
|Figure 3: Results of fast psychophysical tuning curve (analyzed by low-pass filtering).|
Click here to view
This study complies with Declaration of Helsinki, Ethical Principles for Medical Research Involving Human Participants, 2008 amendment.
Normative values were estimated, and upper and lower 95% confidence limits were estimated using linear regression model and were represented by continuous lines. Quantitative (numerical) data were presented as mean and SD values. Student's t-test was used in testing significance for the comparison between means of two groups. Analysis of variance (ANOVA) test was used to compare between means of more than two groups. Qualitative (categorical) data were presented as numbers and percentages. The χ2 -test was used for comparison between qualitative data. Pearson's correlation coefficient (r) was used to determine significant correlations between the different quantitative variables. The significance level was set at P value less than 0.05 and it was highly significant when P value was less than 0.01.
| Results and discussion|| |
Normative data for the fPTC test were derived from 16 normal hearing adults (32 ears) (eight male individuals and eight female individuals) tested at four frequencies (500, 1000, 2000, and 4000 Hz) at three signal levels (35, 50, and 65 dB) to decide normal tip frequency and Q10 ranges and to compare with results later in this study when dealing with hearing-impaired individuals for the presence or absence of dead regions [Table 1].
At all times, the forward sweeps tip frequency values were above reversed sweeps in the same individual. The forward sweeps tip frequency values were usually above the signal frequency, whereas the reverse sweeps tip frequency values were below it. This is consistent with results obtained by Sek et al. . Deviation of the tip frequency from the signal frequency is summarized in [Table 2]. It is noted that the deviation becomes less when the level of the signal is lower; however, this was not statistically significant [Table 3]. The 500 Hz frequency showed the largest deviation percentage than other frequencies.
Absolute values for means, SD, ranges, and 95% confidence interval of tip frequencies were yielded from forward and reverse sweeps and then averaged to rule out the effect of hysteresis and are displayed in [Table 4].
As the auditory filters are not linear, their shapes would be affected by the level of the stimulus used to measure it. Accordingly, a regression model was applied to derive the Q10 according to the level of the stimulus and yielded the formulae to predict Q10 at 500, 1000, 2000, and 4000 Hz as shown in [Table 3].
[Table 3] shows within-individuals ANOVA conducted on the tip frequency proportions with factors ear (left or right), level (35, 50, or 60 dB SPL), and signal frequency (500, 1000, 2000, or 4000 Hz). No main effects were significant (P > 0.05).
The interaction of ear and frequency was significant [d.f. (3.45) variance ratio (v.r.) = 2.99, P = 0.041]. The nature of the interaction can be seen in [Table 3]. For frequencies of 500, 1000, and 2000 Hz, the proportion was slightly above 1.0 for both ears. For the frequency of 4000 Hz, the proportion was above 1.0 for the left ear but slightly below 1.0 for the right ear. However, all mean proportions were very close to 1.
[Table 5] shows within-individuals ANOVA conducted on the Q10 values with factors ear (left or right), level (35, 50, or 60 dB SPL), and signal frequency (500, 1000, 2000, or 4000 Hz). The data were averaged across forward and reverse sweeps. This analysis showed:
(1) A significant effect of ear: Q10 greater for the left than for the right ear.
(2) A significant effect of frequency: Q10 increases with increasing frequency.
(3) A significant effect of level: Q10 decreases with increasing level.
(4) A significant interaction of ear and frequency: the difference across ears is large at 4000 Hz and small for other frequencies.
(5) A significant interaction of ear, frequency, and level: the change in Q10 with level at 4000 Hz is greater for the left ear (change from 4.9 at 35 dB to 3.3 at 65 dB) than for the right ear (change from 3.0 at 35 dB to 2.7 at 65 dB). Alternatively, one might say that the difference across ears at 4000 Hz was present mainly for the lowest level.
Effects 2 and 3 could be expected from previous data. The difference across ears is slightly unexpected. However, a multiple linear regression model was applied fitting the effect of ear, frequency, and level, and revealed results that were only significant for 4000 Hz average percent and nonsignificant for other frequencies. However, the goodness of fit, expressed as r2 , was very small (12.8%). This means that 87.5% of the variance can be attributed to factor/s other than the ear (right/left).
| Conclusion|| |
This experiment aimed to gather baseline data for normal hearing individuals for comparison with data obtained from hearing-impaired patients. The typical shift of tips of the PTCs for normal hearing individuals, when the tip frequency is estimated for the average of a forward and reverse sweep, was estimated and the results will be used to determine the mean, SD, and 95% confidence interval of the shifts. This information is to be used to determine when the PTC for a hearing-impaired patient has a shifted tip (when the shift falls outside the 95% confidence interval for normal hearing individuals).
| Acknowledgements|| |
| References|| |
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|4.||Sek A, Alcantara J, Moore BC. Development of a fast method for determining psychophysical tuning curves. Int J Audiol 2005; 44 :408-420. |
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|7.||Soliman S, Fathallah A, Shehata W. Development of the Arabic Staggered Spondaic Words (SSW) test. Proceedings of the 8 th AinShams Medical Congress 1985; 2 :1220-1246. |
|8.||Robles L, Ruggero MA. Mechanics of the mammalian cochlea. Physiol Rev 2001; 81 :1305-1352. |
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]