PSYCHOACOUSTIC ANALYSIS OF THE N0RM7U.

AUDIOMETRIC REGION IN NOISE-INDUCED

HEARING LOSS SUBJECTS

by

LESLIE ANN SELENT, B.S.

A THESIS

IN

SPEECH AND HEARING SCIENCES

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for

the Degree of

MASTER OF SCIENCE

Approved

Accepted

May, 1983

7-

,~ .-OL /--r

ACKNOWLEDGEMENTS

I would like to express my sincere thanks to Dr. Clifford C.

Olsen and to Dr. William K. lekes for their guidance in the develop­

ment of this thesis. Also, I would like to thank my fellow graduate

students for assisting me in the identification of possible subjects

and for the help and moral support they gave me.

11

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ii

LIST OF TABLES iv

LIST OF FIGURES v

CHAPTER

I.' INTRODUCTION 1

II. METHODS AND PROCEDURES 8

III. RESULTS AND DISCUSSION 13

IV. SUMMARY AND CONCLUSIONS 43

REFERENCES 46

APPENDICES 50

111

LIST OF TABLES

1. Mean percentage SISI increment detected for each Al level as a function of sensation level and subject group for 500 Hz 14

2. Mean percentage SISI increment detected for each Al level as a function of sensation level and subject group for 2000 Hz 15

3. Mean percentage SISI increment detected for each Al level as a function of sensation level and subject group at 4000 Hz 16

4. DL Difference scores between 10 and 40dB SL for normal and NIHL subjects at 500, 2000, and 4000 Hz 18

5. Confusion matrix for initial phoneme identification for normal listeners 24

6. Confusion matrix for initial phoneme identification for NIHL listeners 25

7. Confusion matrix for final phoneme identification for normal listeners 26

8. Confusion matrix for final phoneme identification for NIHL listeners 27

9. Initial and final position phoneme discrimination performance for normal and NIHL subjects 29

10. Total number of words correctly identified across normal and NIHL subjects (N = 7 for each group) 30

11. Initial and final position plosive identification performance for normal and NIHL listeners 32

12. Initial and final position fricative indentification performance for normal and NIHL subjects 33

IV

13. Aural overload thresholds (sound pressure level) at the fundamental frequencies of 250, 500, and 1000 Hz for normal and NIHL subjects 35

14. Mean aural overload thresholds (dB SPL) and standard deviation from present study, Humes (1977) and from Fausti (1971) 37

15. Mean increment and standard deviation in masked threshold for every lOdB increase in masker intensity for normal and NIHL subjects 40

LIST OF FIGURES

1. Mean DL Difference score (40-10) at 500 Hz as a function of each Al increment level for normal (0) and NIHL (A) subjects 19

2. Mean DL Difference score (40-10) at 2000 Hz as a function of each Al increment level for normal (0) and NIHL (A) subjects 20

3. Mean DL Difference score (40-10 or 20-10) at 4000 Hz as a function of each Al increment level for nommal (O) and NIHL (A) subjects 21

4. Mean masked threshold of a 2000 Hz sine-wave as a function of masker intensity of a 2000 Hz NBN masker for normal (0) and NIHL (A) subjects 39

VI

CHAPTER I

INTRODUCTION

We have observed an interesting enigma in our clinical practice

at the Texas Tech University Speech and Hearing Clinic. Occasionally

we will find young adults who display normal pure tone hearing thresh­

olds but who complain that they cannot understand speech. Most other

individuals with normal pure tone audiometrie thresholds make no such

comment about not being able to understand speech. Can we conclude

from this observation that the audiogram may be a poor indicator of the

social adequacy of one's hearing? Since this observation is not uncom­

mon, it has led us to speculate that there may be some undetected audi­

tory dysfunction that may not always be manifest in the basic pure-tone

audiogram.

For subjects with noise-induced hearing loss, depressed hearing

sensitivity is typically evident in the high frequencies with normal

hearing in the low-to-mid frequency range. However, as mentioned above,

it may be possible, and perhaps probable, that the audiometrie region

displaying normal threshold sensitivity (250 to 2000 Hz) may include

some underlying abnormality that is not evident in the basic battery

of audiometrie tests.

Although research has shown that people who work in similar noise

environments have varying degrees of hearing loss (Ward, 1965; Burns,

1973), noise-induced hearing loss (NIHL) progresses in a well recognized

pattern with a decrease in hearing sensitivity between 3000 and 6000 Hz

(Maas, 1972; Robinson, 1976; Melnick, 1978). The question of why this

cochlear region is primarily affected has been researched for years with

various proposed explanations. Boehn (1976) suggested that pillar cells,

which are damaged at the basal end of the cochlea, lead to the destruc­

tion of inner hair cells. This portion of the cochlea (8-10 mm from the

base) has been described by Schuknecht (1974) as "that area which tono-

topieally corresponds to the 4000 Hz region." He adds that this area

may be vulnerable to excessive noise because of a weakness in this sec­

tion of the cochlea or from the addition of surplus mechanical stress

during stimulation. Other possible explanations include low supply of

blood at this region (Crow, 1934), the vulnerability of this area to

hydromeehanical forces generated by the stapedial movement (Hilding,

1953), and the possibility that the traveling wave is accelerated high­

est at this region (Schuknecht and Tonndorf, 196).

While the physiological basis for reduced high frequency sensitiv­

ity in NIHL is well documented (Kryter, 1970; Burns, 1973; Bilger, 1976),

very few studies have demonstrated damage to the low-to-mid frequency

region. Since low frequency thresholds are normal in the typical NIHL

patient, it is assumed that low frequency auditory fiinction is normal.

The assumption however, that normal threshold sensitivity is that index

of auditory sensory capacity most likely to reflect any dysfunction, is

not borne out by clinical experience.

Mention of low frequency involvement in NIHL is difficult to find in

the literature. Findlay (1976) found in NIHL subjects whose audiometrie

thresholds were different by at least 20dB between 2000 and 4000 Hz,

approximately equal incidence and magnitude of threshold adaptation on

fixed-frequency Bekesy audiometry at these frequencies. These results

led him to suggest that "pure tone audiometry may fail to identify cer­

tain frequency regions for which auditory function is abnormal." Evidence

of a low-frequency notch was found in 27 of the 1800 subjects (1.5%) ex­

amined who also displayed noise notches at 4000 Hz (Chung, 1980). Chung

ascribes the possible cause of low-frequency notches to the variability

of noise to which each subject was exposed.

The Research Question

Although only a few studies have shown evidence of damage to the

lower audiometrie region in subjects who have permanent hearing loss as

a result of noise, these efforts have not involved discriminating psy-

ehoaeoustie tasks. Thus, the present research was undertaken to assess

low-to-mid frequency auditory function in NIHL subjects. Specifically,

four indices of audition more sensitive to auditory perceptive function

were utilized to address the question: Does auditory dysfunction occur

in the normal audiometrie region in subjects displaying noise-induced

hearing loss?

Review of Test Used

SISI Test. The Short Increment Sensitivity Index or SISI Test is

probably the most familiar test of the four measures used in this study.

Introduced in 1959 by Jerger, Shedd, and Harford, the SISI test deter­

mines the ear's ability to detect small changes in intensity, the in­

creased capacity for which is often associated with cochlear pathology.

Prior to the development of the SISI test, research had fairly well

established the difference limen for intensity or DLL The DLI, which

is defined as the minimum change in intensity that can just be detected

or judged different from an initial intensity value, was incorporated in

a monaural method for assessing recruitment by Luscher and Zwislocki

(1948). This procedure proved to be easy to administer and required

simple instrumentation, but subjects exhibited wide variation in their

responses. In Denes and Naughton's (1950) test of loudness memory,

patient variability was overcome by comparing DLs for intensity at two

sensation levels. However, this method, too, had its disadvantages. It

was cumbersome to administer and it did not attempt to compare the DLs

to a norm.

It was not until 1953 that Jerger combined the best of these two

methods in his DL Difference Test. By obtaining DLs at lOdB SL and 40dB

SL for each subject, Jerger found that for the normal ear the DL at lOdB

was greater than the DL at 40dB. For the sensory impaired ear the dif­

ference between DL 10 and DL 40 was reduced to a clinically significant

degree.

Although the SISI test is usually administered at a sensation level

of 20dB, studies have suggested the test to be more sensitive '/.hen admin­

istered at different sensation levels. Luscher and Ermanni (1950) sug­

gested that SLs of 40dB yield "exceptionally good" sensitivity. Meu-

berger (1950) agreed with Luscher and Ermanni's conclusion after testing

subjects at 10, 20, 30, 40, and 50dB SL. However, as stated earlier, it

was Jerger who integrated Denes and Naughton's method of testirg at two

sensation levels to obtain more information rather than testing only at

one level.

By using the composite ideas of these researchers and the basic

clinical procedure of the SISI test, the present study sought to deter­

mine if the SISI test administered at two sensation levels across a

range of intensity increments would be sensitive to subclinical dysfunc­

tion at test frequencies considered normal in NIHL subjects (500 and

2000 Hz).

California Consonant Test. Developed in 1977 by Owens and Schubert,

the California Consonant Test (CCT) has been used with subjects who dis­

play high-frequency cochlear hearing loss as an aid in determining their

phonemic recognition difficulties. The CCT, which tests for 39 fricatives,

43 stops, and 18 affricates, has received recognition for its ability to

identify phonemic difficulties (Schwartz and Surr, 1979). By comparing

the CCT with the Northwestern University Word Test #6 on subjects with

high-frequency losses above 1000 Hz, Schwartz and Surr found that 86% of

their subjects scored better than 80% on the Northwestern University Word

Test, while only 18% scored higher than 80% on the CCT.

To permit clear isolation of the phonemes tested, the CCT was de­

signed as a closed set or multiple-choice test. By doing so, Owens and

Schubert felt that the number of response alternatives were equal for all

subjects (Miller, Heise, and Lichten, 1951) and that word familiarity was

controlled (Pollack, Rubenstein, and Decker, 1959). For each stimulus,

four response foils are provided with one being the correct response.

The design of the CCT allows for the analysis of initial and final con­

sonant identification.

In the present study, the CCT was low-pass filtered at 1000 Hz. The

rationale for low-pass filtering is related to the encoded nature of

speech (Liberman, 1977). It has been demonstrated that in a consonant-

vowel-consonant utterance, for instance, the acoustic cues that transmit

the initial auid final consonants are inbedded or encoded in the formant

frequency modulations of the vocalic nucleus. The cues, therefore, for

consonant identification are not solely high frequency in nature. Since

vowels have their greatest energy below 1000 Hz, the acoustic cues

necessary for consonant discrimination should be equally available to

normal and NIHL subjects. It is reasoned that while both subject groups

have equivalent low frequency threshold sensitivity, there may be phoneme

discrimination differences between the two groups due to a possible un­

detected abnormality in the low-to-mid frequency region of the NIHL

subjects.

Aural Overload Test. The aural overload test (Lawrence and Yantis,

1956) has recently made its way back into the clinical literature as a

measure that appears promising. With modifications made by Fausti (1971),

thresholds obtained with the new aural overload procedure are in excellent

agreement with the original technique.

Aural overload, which is a "distortion process that occurs in the

inner ear at moderate stimulus intensities," can be determined electro-

physiologically by measures of the cochlear microphonic or indirectly

through psychoacoustic methods (Humes, 1977). By electrophysiologically

evaluating the threshold of aural overload in guinea pigs and psycho-

acoustically measuring this threshold in h\amans (Lawrence and Blanchard,

1954) , it is reasoned (Humes, 1977) that susceptibility to acoustic over­

stimulation could be evaluated through the aural overload test. In a

study measuring auditory fatigue (Humes and Schwartz, 1977), it was found

that lower aural harmonic thresholds were associated with greater auditory

fatigue (TTS ). After comparing the aural overload method with other clin­

ical procedures (brief-tone audiometry, threshold of octave masking, and a

speech discrimination test in noise), Humes (1977) chose the aural over­

load method as the best predictor of ear susceptibility to acoustic

overstimulation.

Lawrence and Yantis (1957) , in determining alterations in sensory

cell function, found the shift in aural overload threshold to be a more

sensitive indicator of cochlear dysfunction than the temporary shift in

threshold. This finding supports Humes' conclusions. Humes (1977) found

similar results and felt that the measure of aural overload reflected more

precisely the amount of inner ear dysfunction after exposure to noise

than pure tone thresholds of the TTS measure. The aural overload test

has also been employed in determining site of lesion and as an aid in

diagnosing loudness recruitment in cochlear-impaired ears.

It is evident from the research literature that the aural overload

test is receiving recognition as a clinical procedure sensitive to sub­

clinical pathology. Since damage to the cochlea may not always be de­

tected in the basic pure tone audiogram, especially in the frequencies

below 2000 Hz, determination of the aural overload threshold in the

apparently normal audiometrie region was pursued in this study.

Masking. Masking, which is defined as the amount by which the thresh­

old of audibility of a sound is raised by the presence of another sound

(ASA Z24.1, 1951) , has received recognition as a clinical procedure which

can be used to distinguish among various kinds of hearing loss. Over the

years, the volume of masking research has allowed us to generate some

basic conclusions concerning the masking phenomenon. For tone-on-tone

masking, Wegel and Lane (1924) found that the greatest masking effect

occurs when the masker and the test tone are of similar frequency and in­

tensity, and that at higher intensities more masking occurs above the

masker than below it.

Numerous studies have compared masking functions in normal hearing

and sensorineural loss subjects. Clack and Bess (1969) and Chung (1981)

have concluded that the sensorineural impaired ear produces more aural

distortion and has lower aural distortion thresholds than the normal

hearing ear. By employing pure tone maskers that varied in intensity and

frequency, Chung (1981) designed a tone-on-tone simultaneous masking par­

adigm with Bekesy sweep audiometry and found that when masking was mea­

sured in terms of masked threshold, sensorineural impaired subjects

exhibited an abnormal spread of masking.

Jerger, Tillman, and Peterson (1960), Nelson and Bilger (1974), and

Olsen and Berry (1979) found masking in the sensorineural impaired ear to

be the same as the normal ear. In a study with 14 sensorineural impaired

listeners. Nelson and Bilger (1974) researched the hypothesis that the

sensory impaired ear exhibits abnormal harmonic distortion. By employ­

ing a pure-tone octave masking procedure, data computation of the sensa­

tion level of the test signal at masked thresholds resulted in the

conclusion that abnormal distortion is not apparent in the sensory

impaired ear.

Although these studies provide conflicting conclusions regarding the

nonnalcy of masking functions in the sensorineural impaired ear, it is

felt that masking remains a sensitive index of cochlear dysfunction when

the sensation level/ sound pressure level issue is incorporated in the

data analysis. As such, the present study sought to determine the noise-

on- tone masking functions for NIHL and normal hearing subjects.

In summary, the four separate experimental procedures detailed above

were designed to stress that portion of the cochlea that the audiogram

suggested was normal. It was anticipated that if si±)clinical auditory

dysfunction existed in the NIHL subject group, performance deterioration

would be found in one or several of the experiments administered.

CHAPTER II

METHODS AND PROCEDURES

Subjects

The experimental group consisted of seven male adults ranging in

age from 20-32 years (mean = 24.1 years). Each subject met the follow­

ing criteria:

1. Normal middle ear function as suggested by impedance audio­

metry, maximum compliance at +_ 50mm H O,

2. Air conduction thresholds no poorer than 15dB HL (ANSI S3.6-

1969) at 250, 500, 1000, and 2000 Hz, and

3. A high frequency audiometrie notch at 4000 Hz, poorer than or

equal to 30dB HL, and hearing thresholds at 3000, 6000, and

8000 Hz at least 5dB better than the threshold at 4000 Hz.

The control group consisted of seven male adults ranging in age from

19-26 years (mean = 21.7 years) who met the following criteria:

1. Normal middle ear function as suggested by impedance audiometry,

maximum compliance at +_ 50mm H O and

2. Air conduction thresholds no poorer than 15dB HL at octave

frequencies from 250 to 8000 Hz.

Each of the seven males in the experimental group had a history of

noise exposure (heavy machinery and/or loud music). No history of ex­

posure to noise was reported by any of the seven normal hearing subjects.

Age and audiometrie data across subjects may be found in Appendix A.

Throughout the entire experiment each subject listened with the same ear.

Instrumentation, Calibration, and Procedures

Prior to the administration of each of the four experimental condi­

tions, each subject was screened for threshold sensitivity and middle ear

function. A Maico 24 clinical audiometer was used to determine pure tone

thresholds for all subjects. Electroaeoustic impedance measures were

obtained with a Teledyne Impedance Meter (model TA-3D). Testing was con­

ducted in an Industrial Acoustics Company single-walled prefabricated

8

test booth. All subjects listened monaurally through a 10 ohm, TDH 39

earphone mounted in a MX 41/AR cushion. To control for auditory fatigue,

the experimental procedures were administered on three different days

with the CCT and masking experiments given on one date, and the SISI

test and aural overload procedure administered on subsequent dates. To

insure accurate sound pressure level readings, the Maico 24 audiometer

was calibrated daily throughout the data collection period.

SISI Test. The Maico 24 clinical audiometer was also used in the

administration of the SISI test. Both the steady-state tone and the

superimposed intensity increments were directed to the right earphone.

Subjects indicated detection of the increments by use of a hand-held

response button.

The basic experimental procedure used in the SISI experiment was

modeled after Jerger, Shedd, and Harford (1959). In the present design,

intensity increments of 200ms duration (+50ms rise time, -50ms decay time)

were superimposed on a carrier tone of constant sensation level. The

interstimulus interval was 5 seconds. The size of these increments was

varied in a descending fashion from 5.0 to .lOdB. The percentage of in­

crements detected constituted the SISI "score." The following procedure

was employed by the examiner:

1. For Al(s) of 5.0 to 1.25dB, if a subject detected all of the

first 5 presentations, proceed to the next increment in

descending order.

2. For Al(s) of 1.0 to .lOdB, if a subject detected all of the

first 10 presentations, descend to the next increment.

3. If a subject missed any of the increments in a trial, he was

given all 20 presentations at that intensity.

4. If a subject missed the first 10 presentations for any Al level,

proceed to the next descending increment.

5. When a subject missed the first 10 presentations at any two con­

secutive levels, stop test.

Differing from the original SISI procedure where 20 increments are

presented at each Al level, this modified procedure shortened the

10

presentation time and resulted in an easier listening task for the sub­

ject. Other lesearchers (Griffing and Tuck, 1963; Yantis and Decker,

1964; Owens, 1965) have also used shortened procedures when administer­

ing the SISI test.

At sensation levels of 10 and 40dB, SISI scores were determined for

500, 2000, and 4000 Hz. Due to depressed thresholds at 4000 Hz for the

NIHL subjects, a SISI score was obtained at 20dB SL rather than 40dB SL.

Subjects were instructed that they would hear a steady-state tone

that had several jumps in loudness. The subjects were instructed to push

the response button whenever they detected the loudness increment. Several

increments were deleted by the examiner to determine whether the subject

was responding to the change in intensity rather than to a learned time

interval.

California Consonant Test. The commercially available version

(Auditec) of the California Consonant Test (Owens and Schubert, 1977) con­

sisting of 100 items from List 1 was played over an Advent Cassette

Recording System (Model 201). The male speaker voice was routed to a

Starkey Hearing Science Laboratory where it was filtered by 3 cascaded

filters each set at 1000 Hz low-pass, resulting in a -36dB/octave roll-off.

The filtered speech signal was then sent through the hearing test booth

and delivered to the subject earphone. Subjects were given a response

sheet and asked to check the correct word (1 out of 4) for each item pre­

sented. The taped signal delivered to the earphone was calibrated with a

Bruel and Kjaer Sound Level Meter (type 2203/1613) prior to the adminis­

tration of the CCT to each subject.

The low-pass filtered CCT was administered to each subject with the

speech signal delivered at an average sound pressure level of 65dB. It

was explained to the subjects that they would hear the carrier phrase

"check the word" preceding each test item. Four practice words were

given prior to the start of the 100-item test.

Aural Overload Test. The modified aural overload method (Fausti,

1971) was used to determine the threshold of aural overload at the funda­

mental frequencies of 2 50, 500, and 1000 Hz. In this experimental

11

procedure, both a fundamental (f ) and exploring (f ) tone were mixed

and sent to the subject monaurally. The intensity relationship between

f and f was fixed with f set at lOdB greater than f . The frequency

relationship between f and f is held constant with the exploring tone

set at one octave plus 4 Hz above the fundamental tone. By combining

the fundamental and exploring tones in this fashion, a waxing and waning

of loudness known as "beats" was perceived at that intensity where the

primary tone introduced an aural distortion product.

A Bekesy tracking technique was employed to obtain the aural over­

load threshold for each of the fundamental tones. Subjects were in­

structed to push the response button until beats were no longer audible,

then release the switch with the disappearance of the beats. Subjects

continued tracking until the examiner observed stable tracings over 3

minutes.

The fundamental frequency tones were generated by a Hewlett-Packard

Audio Oscillator (model 200AB) with the exploring tones produced by a

Grason Stadler Bekesy Audiometer (Model E800). Both of these signals were

then routed to a Starkey Hearing Science Laboratory where they were mixed

and fed into a Grason Stadler Recording Attenuator (Model E3262A) and

ultimately sent through the wall of the test suite to the subject earphone,

A hand-held attenuator switch allowed the subjects to track the beating

sensation at an attenuation rate of 2.5dB/second.

To ensure a constant output from the Hearing Science Laboratory,

sound pressure level, frequency, and harmonic distortion of f and f were

monitored prior to each test session. Harmonic distortion was not ob­

served above the 25dB filtering limits of the Bruel and Kjaer octave set.

The stabilized portion of each subject's tracing was used to deter­

mine the threshold of aural overload. Threshold, which was defined as

the midpoint of the total pen excursions, was computed by dividing the

summed value of each pen reversal by the total number of reversals in the

appropriate time frame. For each of the 14 subjects, the f presentation

was counterbalanced in an attempt to avoid any possible order effects.

12

Masking. A 2000 Hz pulsed signal (200msec on, 200msec off) was

generated by one channel of the Grason Stadler 1701 Audiometer while

channel two emitted a narrow-band noise centered at 2000 Hz. Subjects

tracked monaural quiet and masked thresholds at an attenuation rate of

2.5dB/second. A Grason Stadler 1701 X-Y Recorder plotted tracking

behavior. Subjects tracked threshold in quiet and proceeded to track

threshold while lOdB increments of noise were added every 40-60 seconds.

Masking growth functions were thus generated over a range of masker in­

tensities from 10 to 80dB SPL. Masked threshold was determined by cal­

culating the midpoint of pen excursions of stabilized tracings.

CHAPTER III

RESULTS AND DISCUSSION

SISI Test

Tables 1, 2, and 3 depict the mean percentage SISI increment de­

tected at 500, 2000, and 4000 Hz for each Al level. Scores are listed

at sensation levels of 10 and 40dB for both normal and NIHL listeners.

Difference scores between the two groups are also shown at each Al

level with a positive score indicating that the normal hearing siibjects

had a higher mean percentage score than the NIHL subjects and a negative

score denoting the opposite.

Data from these tables were analyzed to determine if there was a

significant difference between the mean percentage SISI increment values

at each Al level for normal and NIHL listeners at each frequency. T-test

analyses were applied to the mean differences. Those differences signif­

icant at the .05 level of confidence are indicated by an asterick. At a

sensation level of lOdB the normal hearing subjects performed signifi­

cantly better at the 2dB Al for 500 Hz. At 4000 Hz however, the NIHL

group scored significantly better than the normal group at the 1.5dB

increment. At 4000 Hz, recall that the NIHL group received the test at

a greater sound pressure than the normals even though an equivalent sen­

sation level was employed. Thus at lOdB SL, the 2dB Al increment for 500

Hz separated the NIHL and normal subject groups.

At the 40dB SL presentation, differences were found between groups

at the 500 Hz .75dB increment and at the 2000 Hz l.OdB increment with

normal hearing subjects scoring significantly better than the NIHL sub­

jects. As with the lOdB SL SISI presentation, the NIHL group scored

significantly better than the normals at 40dB SL, 4000 Hz for the 1.25dB

and .75dB increments. The equal sensation level/differing sound pres­

sure level factor, confounds these data at 40dB SL/4000 Hz as it did at

lOdB SL/4000 Hz.

The t-test analyses have shown significant group SISI differences

at lOdB SL/500 Hz for the 2dB increment and at 40dB SL/500 Hz .75 Al;

13

14

TABLE 1. Mean percentage SISI increment detected for each Al level as a function of sensation level and subject group for 500 Hz

Al

5.0

4.0

3.0

2.0

1.5

1.25

1.0

.75

.50

.25

.10

Normal

100

100

85.7

34.3

12.9

0

0

0

0

0

0

lOdB

NIHL

100

88.6

49.3

8.6

0.7

0

0

0

0

0

0

SL

Difference

0

11.4

36.4

25.7*

12.2

—

—

--

—

—

—

Normal

100

100

85.7

85.7

60.0

48.6

23.6

7.9

0

0

0

40dB

NIHL

100

100

100

96.4

52.9

30.7

1.4

0

0

0

0

SL

Difference

0

0

-14.3

-10.7

7.1

17.9

22.2

7.9*

—

--

--

'significant at the .05 level of confidence

15

TABLE 2. Mean percentage SISI increment detected for each Al level as a function of sensation level and subject group for 2000 Hz

Al

5.0

4.0

3.0

2.0

1.5

1.25

1.0

.75

.50

.25

.10

Normal

100

100

65.7

25.7

7.1

2.9

0

0

0

0

0

lOdB

NIHL

85.7

82.1

70.7

5.7

1.4

0

0

0

0

0

0

SL

Difference

14.3

17.9

-5.0

20.0

5.7

2.9

—

—

—

—

—

Normal

98.6

100

100

100

58.6

44.3

30.0

12.1

0

0

0

40dB

NIHL

100

100

100

86.4

31.4

20.0

0.7

1.4

0

0

0

SL

Difference

-1.4

0

0

13.6

27.2

24.3

29.3*

10.7

—

—

^^

^Significant at the .05 level of confidence.

16

TABLE 3. Mean percentage SISI increment detected for each Al level as a function of sensation level and subject group at 4000 Hz

Al

5.0

4.0

3.0

2.0

1.5

1.25

1.0

.75

.50

9^

Normal

100

100

95.7

42.9

30.0

17.1

5.7

4.3

0

n

lOdB

NIHL

100

97.1

87.1

83.6

77.1

46.4

35.0

9.3

0

0

SL

Difference

0

2.9

8.6

-40.7

-47.1*

-29.3

-29.3

-5.0

—

_ _

40

Normal

100

100

100

88.6

70.0

55.7

25.7

11.4

5,7

0

or 20dB

NIHL

100

100

100

100

100

88.6

64.3

45.7

32.1

0

SL

Difference

0

0

0

-11.4

-30.0

-32.9*

-38.6

-34.3*

-26.4

—

10 0 0 0 0

^Significant at the .05 level of confidence.

17

and at the 40dB SL/2000 Hz 1.0 Al. Examination of these tables show

that these group differences occur at or toward the lowest intensity

increments detected (2.0, 1.0, and .75db). A frequency effect was

present, with the NIHL subjects displaying high SISI scores correspond­

ing to the audiometrie region of maximum loss (4000 Hz). These results

agree in part with Jerger (1962), who in reviewing the frequency depen­

dence of the classic SISI test which is administered at 2-dB SL, found

high scores (80-100%) between 2000 and 4000 Hz for cochlear impaired

subjects.

Higher group SISI scores were achieved at lower Al levels for the

40dB SL presentation as compared to lOdB SL across frequency with both

groups detecting intensity increments below the l.OdB increment. Hanley

and Utting (1965) also found normal hearing subjects who were able to

detect intensity increments as low as l.OdB in the high frequencies.

From their results, they suggested that the SISI test be scored at a Al

level of .75dB rather than l.OdB to maximize diagnostic utility. The

present results would suggest that the SISI may show promise as a mea­

sure of subclinical pathology but interpretation is confounded' by a

sensation level by frequency by Al interaction.

The mean DL Difference scores between the SISI presentation levels

of 10 and 40dB SL for normal and NIHL listeners across frequency are

listed in Table 4 with Figures 1, 2, and 3 graphically depicting these

results. In Table 4, a plus sign before each number indicates that a

larger SISI score was obtained at 40dB SL than that obtained at lOdB SL

for that frequency and Al level. From this table, it is evident that the

mean SISI scores at 40dB SL were greater than the mean SISI scores at

lOdB SL occurring especially at the Al levels from 3.0 to l.OdB.

In Figures 1-3, the mean DL Difference score is shown as a function

of each Al level for normal and NIHL subjects. The ordinate shows detec­

tion rate, with SISI increments shown on the abscissa. In Figure 1, the

DL Difference scores for both normal and NIHL subjects are maximized at

a SISI increment of 2.0dB. The greatest between group differences for

the DL Difference score occur at the 3dB increment where the DLD (40-10)

18

TABLE 4. DL Difference scores between 10 and 40dB SL for normal and NIHL subjects at 500, 2000, and 4000 Hz

Al

500 Hz

Normal NIHL

2000 Hz

Normal NIHL

4000 Hz

Normal NIHL

5.0

4.0

3.0

2.0

1.5

1.0

0

0

0

+ 51.4

+47.1

1.25 +48.6

75

+23.6

+7.9

0

+11.4

+50.7

+87.9

+52.6

+ 30.7

+1.4

1.4

0

+ 34.3

+ 74.3

+ 51.4

+41.4

+ 30.0

+12.1

+14.3

+17.9

+29.3

+80.7

+30.00

+20.00

+ .71

+1.43

0

0

0

+2.9

+4.3 +12.9

+45.7 +16.4

+40.00 +22.9

+38.6 +42.2

+20.0 +29.3

+7.1 +36.4

.50

.25

.10

+5.7 +32.1

10-

.90'

19

.8 0 -

?0-

.60-

LJLJ

a':.50-

o <~> LJ .4 0 -1 I— UJ CJ

.30'

.20'

.10'

/

/ To

! I ! i\

I I

O I I I ^ I

I

A tk

l<^»M«IW»l»M'^ - ^ W ^ -11»—'• • .• .«. .^.> WrwH..... « • . . « « ' A ^ " . = . « . - - < ^ * « . — • i . " ' ^

•1 -

N(R~-iEN^Lt v'^:.Sv:r:'

Figure 1. Mean DL Difference score (40-10) at 500 Hz as a function of each Al increment level for normal (0) and NIHL (A) subjects.

10'

.90"

2 0

A i ir INCREMENT LEVELS (dB)

Figure 2. Mean DL Difference score (40-10) at 2000 Hz as a function of each Al increment level for normal (0) and NIHL (A) subjects.

21

.1.0"

.90-

.<90-

.70-

60

;?.50'

Q I —

LU U (.—

LiJ Q

;0'

.2 0

10-

— 1 " '-T-BI.»I»-.MIW '•«'• i«X* . iW» •I'^U^'I

"T r* :5 .50 .7S 10 i

.^ ! VIU': M-: h

..' JV

Figure 3. Mean DL Difference score (40-10 or 20-10) at 4000 Hz as a function of each Al increment level for normal (0) and NIHL (A) subjects.

22

for normals is 50% and the DLD for NIHL subjects is 0%. A t-test re­

vealed no significant between group difference for the DLD at 500 Hz.

The DL Difference scores at 2000 Hz are shown in Figure 2, which

like Figure 1, displays the highest DLD score at 2.0dB for both groups.

The largest between group differences for the DL Difference occurs at

l.OdB with a DL Difference between groups of 29.29% (normals = 30%,

NIHL = .71%). A t-test was also performed at this frequency with no

significant difference evident for the DLD between groups. At 4000 Hz

(Figure 3), the DL Difference scores are greatest at 2.0dB for the normal

listeners and 1.25dB for the NIHL listeners with the largest between

group differences occurring at a Al level of 2.0dB (normals = 45.7%,

NIHL = 16.4%) and at a level of .75dB (normals = 7.1%, NIHL = 36.4%)

where the difference between groups was 29.3% at both levels. Since the

SISI test was administered at different sensation levels for the two

groups at this frequency, a t-test was not performed. In summary, the

DL Difference test was not as sensitive to between group differences as

was the SISI test presentation at a single sensation level.

The classic SISI test (Jerger et al., 1959) is typically scored at

a Al level of l.OdB. It was therefore of interest to study subject per­

formance at the Ai level of l.OdB when the SISI test was administered at

a sensation level of 40dB at 500 and 2000 Hz. At the 40dB SL/500 Hz

presentation, scores for the normal subjects ranged from 0-80% (mean =

23.6, S.D. = 28.4); with scores for the NIHL subjects ranging from 0-10%

(mean = 1.4, S.D. = 3.5). A t-test indicated a significant difference

only at p <.10 between subject groups. At the 40dB SL/2000 Hz condition

scores ranged from 0-80% (mean = 30.0, S.D. = 29.8) for the normal lis­

teners and from 0-5% (mean = .71, S.D. = 1.75) for the NIHL listeners.

A significant difference (p <.05) was found between the two groups in

this analysis. Findlay (1976) found no significant difference between

normal and noise-exposed listeners in the SISI test for 2000 Hz when

administered at 30dB SL. Similar to the present study, both groups of

subjects in Findlay's experiment had hearing thresholds no poorer than

20dB HL at the octave frequencies from 250 to 2000 Hz. It is possible

23

that the results from the present study and Findlay's study differ due

to the difference in the administration level of the SISI test.

From these results, it was found that both groups of listeners were

able to detect intensity increments at lower Al levels when the test was

administered at a sensation level of 40dB rather than at lOdB. As would

be expected, a frequency effect did occur with the NIHL subjects achiev­

ing higher mean SISI scores at 4000 Hz at both sensation levels. Al­

though differences between the two groups were shown at various Al

levels in the DL Difference scores, no significant differences were

revealed which separated the normals from the NIHL listeners at any fre­

quency. SISI test scores at lOdB and 40dB SL suggest that at lower Al

values (2.0dB and below) this test may be sensitive to subclinical sen­

sory dysfunction especially when administered at 40dB SL at 500 and

2000 Hz.

In future clinical use for subclinical diagnosis, it is recommended

that the SISI test be given at a high sensation level, such as 40dB, and

that it be presented at Al levels less than 2.0dB. Also, it is suggested

that the SISI test be administered to all subjects at least at two "nor­

mal" frequencies.

California Consonant Test

Results from the low-pass filtered (1000 Hz) CCT are summarized in

four confusion matrixes shown in Tables 5, 6, 7, and 8. Tables 5 and 6

show initial-position phoneme confusions for normal and NIHL subjects

respectively. Tables 7 and 8 show the final-position phoneme confusions

for normal (Table 7) and NIHL (Table 8) listeners. For each of the four

tables, the actual phonemes tested (by virtue of the available word

choices for each item) are indicated along the horizontal axes with the

perceived phonemes along the vertical axes. The numbers in the tables

quantify how many times the target phoneme was perceived correctly and

if not perceived correctly, which phoneme was substituted. For example,

in Table 5, the phoneme /p/ was perceived as a /p/ 21 times, as a /t/ 5

times, and as a /k/ 2 times in the initial position by normal listeners.

TABLE 5. Confusion matrix for initial phoneme identification for normal listeners

ACTUAL

24

P

t

21 10

22

10 33

0 / '} t/

7

Q

H 0 U W c;

/

V

t/

h

w

8

1

2

2 5 23

3

4

7

3

4

1 5

1

5 12

25

TABLE 6. Confusion matrix for initial phoneme identification for NIHL listeners

ACTUAL

P t k b d f G s / d r t /

P

^3 t/

w

23

8 36

26 6 1 8

8

g

Q f

5 0 u W

/

V

&

1

5

4

3

6

7

2

37

1

1

4

12

1

1

2

1

3

13

26

TABLE 7. Confusion matrix for final phoneme identification for normal listeners

ACTUAL

0 / V t/

Q

a s p.

/

^5 t/

m

n

45

10

11

3

1

15

12

4

2

1

1

3

20

17

14

2

1

6

1

2

1

22

1

1

^

7

4 4

1

20

3 13

25

1

1

5

2

12

31

1

1

2

4

15

9

4

11

1

4

1

14

20

7

3

2

10

1

13

27

TABLE 8. Confusion matrix for final phoneme identification for NIHL listeners

ACTUAL

0 / V 13 t/

p

t

k

b

d

g

f Q

H ^ ta

f

V

z

'} t /

1

m

n

+8

8

7

5

2

11

14

5

4

1

1

16

23

2

14

7

2

1 1

1

20

4

5 1

9

2

3

13

10

33

1

4

8 1

8

28

5

1

2

17

9

10

19

5

3

11

2

9

3

4

3

12

10

28

The number of correct responses for the initial and final phonemes

tested are shown in Table 9 for both subject groups. Chi square anal­

ysis (Wike, 1971) testing overall phoneme identification rates between

groups as a function of initial and final position was performed. For

initial phoneme identification, the NIHL group scored significantly

better (£ <.01) overall, 69% vs 58%. That is, the NIHL subjects had a

significantly higher number of correct initial phoneme identifications

than did the normal hearing subjects. For the phonemes tested in the

final position, no significant difference was found between the two

groups, 50% vs 51%.

The total number of correct word discriminations for each of the 14

subjects is shown in Table 10. Out of a possible 700 words (100-words

for each subject), the normal hearing siibjects correctly identified a

total of 363 words (mean = 51.9); while the NIHL subjects correctly iden­

tified 405 words (mean = 57.9). Thus the normal hearing subjects

achieved a mean CCT score of 52% while the NIHL subjects scored 58% as a

group. Chi square analysis, which tested the total number of correct

identifications between the two subject groups, revealed a significant

difference (£< .025) with the NIHL subjects scoring significantly better.

A test similar to the one described in this study was employed by

Sher and Owens (1974). In their study, one group of subjects with normal

hearing up to 2000 Hz accompanied by a high frequency loss, and a group

of normal hearing subjects were given 100 multiple-choice items. The

normal hearing listeners were administered the test through a low-pass

filter with a cut-off frequency of 2040 Hz. Although no significant

differences were found in three categories (kinds of error substitutions

made, probability of error for individual phonemes, or in overall score),

the authors concluded that the two subject groups may have performed the

same because subjects with high-tone losses may have "grown accustomed"

to their loss whereas, the normal listeners were "suddenly faced with an

added built-in distortion."

In the present study, a significant between group difference was

found for initial phoneme identification. However, the difference wa ;

29

TABLE 9. Initial and final position phoneme discrimination perfoinnance for normal and NIHL subjects

Phoneme

P

t

k

b

d

g

f

0

Initial

Normal

21/28 (.75)

22/24 (.52)

33/42 (.79)

4/7 (.57)

7/14 (.50)

5/7 (.71)

8/14 (.57)

NIHL

23/28 (.82)

26/42 (.62)

36/42 (.86)

3/7 (.43)

8/14 (.57)

4/7 (.57)

6/14 (.43)

F

Normal

45/70 (.64)

12/35 (.34)

17/42 (.40)

14/14 (1.0)

6/7 (.86)

22/28 (.79)

3/7 (.43)

inal

NIHL

48/70 (.69)

14/35 (.40)

23/42 (.55)

14/14 (1.0)

7/7 (1.0)

20/28 (.71)

4/7 (.57)

23/42 (.55) 37/42 (.88) 25/70 (.36) 33/70 (.47)

/ 7/21 (.33) 12/21 (.57) 12/21 (.57) 8/21 (.38)

31/35 (.89) 28/35 (.80)

z

^5 t /

Totals

4/7 (.57)

12/28 (.43)

146/252

(.58)

6/7 (.86)

13/28 (.46)

174/252

(.69)

9/28 (.32)

14/35 (.40)

13/56 (.23)

223/448

(.50)

9/28 (.32)

12/35 (.34)

10/56 (.18)

230/448

(.51)

30

TABLE 10. Total number of words correctly identified for normal and NIHL subjects (N = 7 for each group)

Normal Hearing NIHL

Subjects Subjects

55 58

43 59

69 56

60 67

41 50

43 58

52 57

Total 363 405

X 51.9 57.9

31

in an unexpected direction with the NIHL subjects achieving a higher

percentage of items correct. Perhaps the NIHL subjects had an advan­

tage listening to the filtered speech since they may have already been

relying on formant mudulations for consonant identification whereas the

normal subjects, who perhaps are accustomed to relying on spectral in­

formation (which was filtered), simply could not change their perceptual

strategy "on demand." It is not readily apparent why initial versus

final discriminations between groups differed although House, Williams,

Hecker, and Kryter (1962) also found evidence that the initial consonant

is easier to identify than the final consonant.

Error analysis was also performed across phonemes categorized by

"manner of articulation." For plosives (Table 11) and fricatives (Table

12) , the number of correct responses for initial and final consonants

are shown for each group of listeners. Three separate Chi square anal­

yses (Wike, 1971) were performed. For plosive identification in the

initial position, no significant difference was evident between the num­

ber of correctly identified words for normal hearing subjects (40%

correct) and the NIHL subjects (44% correct). However, when plosives

were tested in the final position, a significant difference (£<.025) was

shown with the NIHL subjects scoring 63% whereas the normal subjects

achieved 56%.

A significant difference (£<.025) was evident between subject groups

for fricatives tested in the initial phoneme position. In this category,

the NIHL subjects scored significantly greater with 70% correct as com­

pared to 51% correct for the normal hearing listeners. For the frica­

tives tested in the final position, the normal hearing and NIHL groups

each performed at 54%.

Again, unexpected results were obtained with the NIHL group scoring

significantly better than the normal listening group when plosives

occurred in the final position and when fricatives occurred in the ini­

tial position. It is not known why plosives were more readily identified

in the final position nor why fricatives were easier to detect in the

initial position, but as mentioned earlier, it is felt that under these

32

TABLE 11. Initial and final position plosive identification performance for normal and NIHL subjects

Phoneme

P

t

k

b

d

g

Totals

Initial

Normal

21/28 (.75)

22/24 (.52)

33/42 (.79)

4/7 (.57)

7/14 (.50)

87/220

(.40)

NIHL

23/28 (.82)

26/42 (.62)

36/42 (.86)

3/7 (.43)

8/14 (.57

96/220

(.44)

Final

Normal

45/70 (.64)

12/35 (.34)

17/42 (.40)

14/14 (1.0)

6/7 (.86)

94/168

(.56)

NIHL

48/70 (.69)

14/35 (.40)

23/42 (1.0)

14/14 (1.0)

7/7 (1.0)

106/168

(.63)

33

TABLE 12. Initial and final position fricative identification performance for normal and NIHL subjects

Phoneme

Initial

Normal NIHL

Final

Normal NIHL

f

0

s

/

V

5/7 (.71)

8/14 (.57)

23/42 (.55)

7/21 (.33)

Totals 43/84

(.51)

4/7 (.57)

6/14 (.43)

37/42 (.88)

12/21 (.57)

59/84

(.70)

22/28 (.79) 20.28 (.71)

3/7 (.43)

12/21 (.57)

9/28 (.32)

102/189

(.54)

4/7 (.57)

25.70 (.36) 33/70 (.47)

8/21 (.38)

31/35 (.89) 28/35 (.80)

9/28 (.32)

102/189

(.54)

34

experimental circumstances, the NIHL group may have been at a listening

advantage over the normal hearing subjects in that they are already

equipped with some of the characteristics of hearing via a low-pass

filter.

However, since the low-pass 1000 Hz CCT did not separate the sub­

ject groups based on performance in a direction that would assist in the

identification of subclinical auditory dysfunction, use of the test as

modified in this experiment cannot be recommended. That the NIHL sub­

jects performed better as a group than the normal hearing listeners has

interesting implications for perceptual acoustics.

Aural Overload Test

Recall that in the aural overload procedure, the subjects tracked

a beating sensation. This beating, or waxing and waning of loudness, is

the result of combining both a fundamental and exploring tone to the

listener. When the intense (85dB SPL) fundamental tone is introduced, an

overloading of the cochlea results. This, in turn, produces harmonics

at integral multiples of the fundamental. By adding an exploring tone

(75dB SPL) and mixing it with the fundamental tone, the listener is able

to perceive beats. These beats are a result of the interaction of the

exploring tone and the second aural harmonic of the fundamental.

In the present study, normal and NIHL listeners obtained aural over­

load thresholds at the fundamental frequencies of 250, 500, and 1000 Hz.

The corresponding exploring frequencies were 504, 1004, and 2004 Hz re­

spectively. The threshold of aural overload for each frequency is

defined by Humes (1977) as the "lowest possible intensity of the funda­

mental that will give rise to beats."

Results of the modified aural overload experiment for each of the

14 subjects at the fundamental frequencies of 250, 500, and 1000 Hz are

hown in Table 13. Mean aural overload thresholds between the two

qroups were subjected to t-tests (Spence, Cotton, Underwood, and Duncan,

1976) No significant difference was evident among the groups at any

of the f frequencies.

35

TABLE 13. Aural overload thresholds (sound pressure level) at the fundamental frequencies of 250, 500, and 1000 Hz for normal and NIHL subjects

Sub # Normal

250 Hz

Sub # NIHL

500 Hz

Normal NIHL

1000 Hz

Normal NIHL

1

2

3

4

5

6

7

61.89

57.80

54.19

53.90

34 .28

28 .88

62.32

1

2

3

4

5

6

7

49 .23

78.19

62.55

51.85

60 .87

36.85

50.30

62 .03 40.97

60.92 76.20

62.14 70.33

59.88 56.52

19.80 75.70

23.28 39.03

69 .31 50.80

66 .33 37.95

38.43 76 .61

42 .48 68 .43

34.26 48.70

8.26 48.50

21.26 26.96

69.05 72.55

X 5 0 . 4 7 X 55.69 51.10 58.51 40 .01 54.24

36

Table 14 depicts the mean aural overload thresholds and the stan­

dard deviations at each of the fundamental frequencies tested for both

groups. For comparison purposes, data from Humes (1977) and Fausti

(1971) for normal hearing subjects is also presented. The mean thresh­

old data from the present study show an elevation in threshold for 250

and 500 Hz for both groups with a lower threshold at the fundamental

frequency of 1000 Hz. As can be seen, the data from Humes and Fausti

show elevated aural overload thresholds at higher frequencies. Humes

concluded that less intensity is required for overload at low fre­

quencies than at high frequencies. The present findings were similar

with Hume's data for 250 and 500 Hz where an elevation in the aural over­

load threshold is indicated for both subject groups. However, a lower

aural overload threshold was found at an f of 1000 Hz when compared to

the threshold at 500 Hz. The aberrent data for 1000 Hz were due in part

to an inhomogenous experimental group of relatively few subjects. Note,

that at 1000 Hz, an extremely low threshold of 8.26dB was obtained from

a normal hearing listener.

In addition, the individual subject data, which is listed in Table

13, shows considerable variability between subjects at each of the three

frequencies. Very high standard deviations were found in the present

study (Table 14) as compared to the variability found by Humes and

Fausti. It is possible that the extreme amount of threshold variability

can be accounted for by the lack of subject training. Although each of

the subjects was given 3-4 practice runs before an aural overload thresh­

old was found, this evidently was not enough training to ensure the

accurate perception of beats. However, although the high subject vari­

ability may possibly be the result of inadequate subject training, this

aural overload procedure mirrored the clinical reality of not having the

time to adequately train in the clinical setting.

For cochlear impaired ears, research has shown that at frequencies

where cochlear damage has occurred, greater intensity is required to

reach aural overload thresholds (Wever and Lawrence, 1955; Lawrence and

Yantis, 1956; Humes, 1977). Termed the "injured hair-cell principle"

TABLE 14. Mean aural overload thresholds (dB SPL) and standard deviation from present study, Humes (1977), and from Fausti (1971)

37

Present Study

Normals NIHL

Humes

Normals

Fausti

Normals

250 50.5 (12.4) 55.7 (12.1)

500 Hz 51.1 (18.9) 58.5 (14.6) 49.15 (8.5) 55.4 (9.9)

1000 Hz 40.0 (20.6) 54.2 (17.4) 54.6 (12.6) 61.9 (7.9)

38

(Wever and Lawrence, 1955) it is hypothesized that the cochlear micro­

phonic relies on healthy hair-cells to produce excitement within the

organ of Corti. When damage to these hair-cells occurs, greater inten­

sity is needed to spread this excitement, or to state another way, to

pass the injured cells to an area where healthy hair-cells are con­

tained. When a region of the organ of Corti is damaged, more intensity

is required to obtain a cochlear response, and once this response has

stabilized, more intensity is needed to overload this response.

' By applying the injured hair-cell principle to subjects with NIHL,

greater aural overload thresholds should occur at those frequencies

(3000-6000 Hz) where hair-cell damage is evident. Although the present

study did not explore aural overload thresholds at those audiometrie

frequencies having elevated sensitivity, the thresholds that were

obtained in the normal audiometrie region for NIHL subjects were somewhat

higher than those obtained for normal listeners both in the present study

and in the studies by Humes (1977) and Fausti (1971).

In an attempt to employ an experimental procedure that was practical

for clinical use, inadequate subject training may have resulted. So

while the aural overload test shows promise as a subclinical test, per­

haps the listening task is so difficult that it is not clinically prac­

tical due to the excessive training time required.

Masking

Mean masked threshold as a function of masker intensity is shown in

Fiqure 4. The ordinate shows mean masked threshold of a 2000 Hz sine

wave and the abscissa displays the masker intensity of a 2000 Hz narrow­

band noise. As can be seen, the masking growth functions closely approxi­

mate one another. Both the experimental and control group functions

ascend in a linear fashion with functions nearly superimposed. For both

groups, .97dB of masking was obtained for every single decibel increase

masker intensity. Table 15 shows the mean increment in masked thresh­

old for every lOdB increase in masker intensity. Individual data may be

found in Appendix B and C.

39

<

90 1 _.J

CD S LiJ

i > UJ

10

'J. /^^ S ;

8

80

70 / </

60

:y\

CZ:

C' J . (./) L'J i-i_ X 1—

r. IxJ : ^ 00 <

:s

f

/. 0 -

"•• . ' ^ . N»

*.' V

7C-

0"

o

/ /

/ /

, /

•V

/ • •

w

,„—.^—,_„..J_,....— -p ..J ,p y

10 n V-

I A (, |v' /r- i \ :; . !A lENiSiT f ' ' i •

-, i-\r,r. iU : • :

/ y; 60

,\i '.-;

Figure 4. Mean masked threshold of a 2000 Hz sinewave as a function of masker intensity of a 2000 Hz NBN masker for normal (0) and NIHL (A) subjects.

40

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41

Recall that Chung (1981) and Clack and Bess (1969) concluded that

sensorineural impaired ears experienced an abnormal growth of masking.

In a previously cited study, Chung (1981) found abnormal masked thresh­

olds at normal audiometrie frequencies (500 and 1000 Hz) in subjects

with sensorineural hearing loss. In addition. Clack and Bess (1969)

found octave masking to occur at a reduced sensation level in the sen­

sorineural impaired ear. Both Chung and Clack and Bess conclude that

masking functions may assist in the early detection of cochlear

dysfunction.

Other experiments (Jerger et al., 1960; Nelson and Bilger, 1974;

Olsen and Berry, 1979) that employed masking in either tone-on-tone or

noise-on-tone paradigms reported no difference in masked functions be­

tween normal and sensorineural loss subjects. Jerger et al. (1960)

investigated the spread of masking with broa(3band thermal noise in

sensory-impaired ears. By comparing results to a study with normal

hearing subjects (Bilger and Hirsch, 1956), Jerger et al., found no dif­

ference in the amount of masking between subject groups when the signals

were presented at equivalent sensation levels.

Nelson and Bilger (1974) studied the notion that the sensory im­

paired ear displays abnormal harmonic distortion. In their pure-tone

octave masking experiment, f the masked threshold for the test signal,

was equal to twice the frequency of the masking signal, f . When the

absolute level of the f test signal at masked threshold as a function

of the absolute level of f was analyzed, data appeared to support the

notion that the sensorineural impaired ear performs differently than

the normal ear. However, when the sensation level of the test signal

at masked threshold was analyzed, the impaired ear did not display

abnormal masking patterns.

The present results support those studies finding similar growth

functions between the sensorineural impaired and normal ear. Perhaps

different masking growth functions between the two siabject groups were

not evident due to the similarity of their pure-tone averages at the

test frequency 2000 Hz (normals = 7.1dB, NIHL = 5.0dB). If

42

subclinical pathology exists at 2000 Hz in NIHL subjects, this particu­

lar noise-on-tone paradigm was not sensitive to it.

CHAPTER IV

SUMMARY AND CONCLUSIONS

Previous research concerning low-to-mid frequency involvement in

NIHL listeners has suggested that the audiogram may not be sensitive in

identifying auditory abnormalities in frequency regions that appear

normal by pure tone thresholds (Findlay 1976). The present study was

designed to determine if auditory dysfunction is present in the normal

audiometrie region of NIHL subjects. By employing four discriminating

psychoacoustic tasks (SISI Test, low-pass filtered California Consonant

Test, aural overload test, and noise-on-tone masking), the low-to-mid

frequency regions (250-2000 Hz) were assessed in seven normal hearing

and seven NIHL subjects.

For the Short Increment Sensitivity Index Test, which was admin­

istered at sensation levels of 10 and 40c3B across a range of intensity

increments, results showed that mean SISI scores were higher for the

normal listeners at 500 and 2000 Hz both at 10 and 40dB SL. Significant

group differences at lower SISI increments (2.0dB and less) suggest this

test may be sensitive to si±)clinical sensory dysfunction when adminis­

tered at 500 or 2000 Hz and especially at 40dB SL. The DL Difference

test (40-lOdB presentations) was found to be insensitive to group dif­

ferences and is therefore not considered a good prospect for further

study in subclinical dysfunction.

For the low-pass filtered CCT, unexpected results occurred with the

NIHL subjects exhibiting significantly higher scores in the following

categories: in initial phoneme identification (£< .01), in the total

number of correct identifications (£< .025), for plosives tested in the

final position (£< .025), and for fricatives tested in the initial

position (£<.025).

It is possible that the NIHL subjects were at a listening advantage

over the normal subjects. In a similar study (Sher and Owens, 1974),

Sher and Owens felt that the noise-exposed listeners may have "grown

accustomed" to their loss, whereas the normal listeners are "suddenly

43

44

faced with added built-in distortion." This was probably the case in

the present, also. In any event, the NIHL subjects' performance cer­

tainly did not support the use of the filtered CCT in a subclinical

diagnostic capacity.

The aural overload thresholds showed great amount of variability

even in the normal ears who have approximately the same degree of hear­

ing sensitivity in the low-to-mid frequency region as do the NIHL sub­

jects. T-tests, which tested the mean aural overload thresholds at each

fundamental frequency, revealed no significant difference between the two

groups. The NIHL group did however display elevated aural overload

thresholds across frequency as would be expected in a sensory disordered

sample of subjects. Humes (1977), in determining aural overload thresh­

olds for normal listeners, found that less intensity is required for

overload at low frequencies than at high frequencies. Results from the

present study agree with Humes' findings for the fundamental frequencies

of 250 and 500 Hz, but at an f of 1000 Hz, a lowered aural overload

threshold was found. This lower threshold perhaps can be accounted for

by the lack of subject training which resulted in high aural overload

threshold variability.

Although the aural overload test shows promise as a subclinical test,

it was perhaps too difficult a listening task to yield real differences

between subject groups due to the small number of subjects employed in

this study. Thus in an effort to simulate the subject training-time

practically available to audiologists in a clinical setting, we may have

obscured group differences that would perhaps be revealed with a larger

number of subjects.

The noise-on-tone masking functions for both the normal and NIHL

listeners closely approximate each other in the present study. It is

f It that these similar growth functions may be due to the similar pure

tone thresholds that are exhibited between groups.

Although some researchers have found differences between normal and

sensory-impaired ears in masking paradigms (Chung, 1981; Clack and Bess,

1969)/ other researchers, have not (Jerger et al., 1960; Nelson and

45

Bilger, 1974; Olsen and Berry, 1979). If subclinical pathology existed

at 2000 Hz in the NIHL subjects studied here, this particular noise-on-

tone paradigm was not sensitive to it.

Group differences were found in "normal" audiometrie regions of

NIHL subjects suggesting the investigation of subclinical sensory dys­

function has merit. Tests of particular promise appeared to be the

SISI test and the aural overload test. Results further supported the

common clinical finding that the audiogram is indeed a poor indicator

of the underlying sensory capacity of one's hearing.

REFERENCES

American Standards Association. 1951. American Standard Acoustical Terminology. AS Z24.1-1951.

Bilger, R. 1976. The audiometrie profile of noise-induced hearing loss. In Donald Hendersen, Roger Hamernick, Darshan Dosanyh, and John Mills, eds. Effects of Noise on Hearing. 457-465. Raven Press, New York.

Bilger, R. and Hirsch, I. 1956. Masking of tones by bands of noise. J. Acoust. Soc. Am. 28, 623-630.

Bohne, B. 1976. Mechanisms of noise damage in the inner ear. In Donald Hendersen, Roger Hamernick, Darshan Dosanyh, and John Mills, eds. Effects of Noise on Hearing. 41-68. Raven Press, New York.

Burns, W. 1973. Noise and Man. 2nd Ed. 189-251. J. B. Lippincott, Philadelphia.

Carhart, R. 1973. Updating special tests in otological diagnosis. Arch. Otolaryngol. 97, 88-91.

Chung, D. 1980. Meanings of a double-notch audiogram. Scand. Aud. 9, 29-32.

Chung, D. 1981. Tone-on-tone masking in subjects with normal hearing and with sensorineural hearing loss. J. Speech Hearing Research. 24, 506-513.

Clack, T. and Bess, F. 1969. Aural harmonics: The tone-on-tone masking vs. the best beat method in normal and abnormal listeners. Acta Otolaryngol. 67, 399.

Crow, S., Guild, S. and Polvogot, L. 1934. Observation on pathology of high-tone deafness. John-Hopkins Med. Journal. 54, 315.

Denes, P. and Naughton, R. F. 1950. The clinical detection of auditory recruitment. J. Laryngol. 65, 375-398.

Fausti, S. 1971. The relationship between aural harmonic thresholds and MCL for sppech. Doctoral dissertation, Univ. of Washington, Seattle

Findlay, R. 1976. Auditory dysfunction accompanying noise-induced hear­ing loss. J. Speech Hearing Disorders. 41, 374-380.

Griffing, T. and Tuck, A. 1963. Split-half reliability of the SISI. J. Aud. Research. 3, 159-164.

46

47

Hanley, C. and Utting, J. 1965. An examination of the normal hearer's response to the SISI. J. Speech Hearing Disorders. 30, 58-65.

Harbert, F. and Young, I. 1965. Spread of masking in ears showing abnormal adaptation and conductive deafness. Acta Otolaryngol 60, 49-57. •

House, A., Williams, C., Hecker, M. , and Kryter, K. 1962. Articulation-testing methods: Consonantal differentiation with a closed set-response set. J. Acoust. Soc. Amer. 37, 158-166.

Humes, L. 1977. Review of four new indices of susceptibility to noise-induced hearing loss. J. Occup. Med. 19, 116-118.

Humes, L. 1978. The aural-overload test: Twenty years later. J. Speech Hearing Disorders. 43, 34-46. "

Humes, L. and Schwartz, D. 1977. Influence of overstimulation on tem­porary threshold shift and the onset of aural overload. Scand. Aud. 6, 73-77.

Jerger, J. 1953. The DL Difference test; improved method for clinical measurement of recruitment. AMA Arch. Otol. 57, 490-500.

Jerger, J. 1962. The SISI test. Int. Audiology. 1, 246-247.

Jerger, J., Shedd, J., and Harford, E. 1959. On the detection of extremely small changes in sound intensity. Arch. Otolaryngol. 69, 200-211.

Jerger, J., Tillman, T., and Peterson, J. 1960. Masking by octave bands of noise in normal and impaired ears. J. Acoust. Soc. Am. 32, 385-390.

Kryter, K. D. 1970. The Effects of Noise on Man. Academic Press, New York.

Lawrence, M. and Blanchard, C. 1954. Prediction of susceptibility to acoustic trauma by determination of the threshold of distortion. Ind. Med. Surg. 23, 193-200.

Lawrence, M. and Yantis, P. 1956. Thresholds of overload in normal and Pathological ears. Arch. Otolaryngol. 63, 67-77.

Lawrence, M. and Yantis, P. 1957. Overstimulation, fatigue, and the on­set of overload in the normal ear. J. Acoust. Soc. Am. 29, 265-274.

Liberman, P. 1977. Speech Physiology and Acoustic Phonetics. 120-121. Macmillan Publishing Co., Inc., New York.

48

Luscher, E. and Zwislocki, J. 1949. A simple method for indirect monaural determination of the recruitment phenomenon (difference limen in intensity in different types of deafness). Acta Otolaryngol. 78, 156-168.

Maas, R. 1972. Industrial noise and hearing conservation. In Jack Katz, ed. Handbook of Clinical Audiology. 772-818. Williams and Wilkins, Baltimore.

Martin, E. and Pickett, J. 1970. Sensorineural hearing loss and upward spread of masking. J. Speech Hearing Research. 13, 426-436.

Melnick, W. 1978. Temporary and permanent threshold shift. In Donald Hendersen, Roger Hamernick, Darshan Dosanyh, and John Mills, eds. Effects of Noise on Hearing. 83-107. Raven Press, New York.

Miller, G. , Heise, G. and Lichten, W. 1951. The intelligibility of speech as a function of the context of the test materials. J. Exp. Psychol. 4, 329-335.

Nelson, D. and Bilger, R. 1974. Pure-tone octave masking in listeners with sensorineural hearing loss. J. Speech Hearing Research. 17, 252-269.

Olsen, C. and Berry, G. 1979. Further intrepretation of the threshold of octave masking (TOM) test. Scandinavian Audiology. 8, 217-223.

Owens, E. 1965. The SISI test and Vlllth nerve versus cochlear involve­ment. J. Speech Hearing Disorders. 30, 252-262.

Owens, E. and Schubert, E. 1968. The development of consonant items

for speech discrimination testing. J. Speech Hearing Research. 11,

656-667.

Owens, E. and Schubert, E. 1977. Development of the California Consonant Test. J. Speech Hearing Research. 20, 463-474.

Pollack, I., Rubenstein, H., and Decker, L. 1959. Intelligibility of known and unknown message sets. J. Acoust. Soc. Am. 31, 273-279.

Robinson, D. 1976. Characteristics of occupational noise-induced hear­ing 'loss. In Donald Hendersen, Roger Hamernick, Darshan Dosanyh, and John Mills, eds. Effect of Noise on Hearing. 383-405. Raven Press, New York.

Schuknecht, H. and Tonndorf, J. I960. Acoustic trauma of the cochlea from ear surgery. Laryngoscope. 70, 479.

Schwartz, D. and Surr, R. 1979. Three experiments on the California consonant Test. J. Speech Hearing Disorders. 44, 61-72.

49

Sher, A. and Owens, E. 1974. Consonant confusions associated with hearing loss above 2000 Hz. J. Speech Hearing Research. 17, 669-681.

Spence, J., Cotton, J., Underwood, B. and Duncan, C. 1976. Elementary Statistics. 4th Ed. Prentice-Hall, Inc., Englewood Cliffs, N. J.

Ward, W. 1965. The concept of susceptibility to hearing loss. J. Occup. Med. 7, 595-607. ~

Wegel, R. and Lane, C. 1924. The auditory masking of one pure tone by another and its probable relation to the dynamics of the inner ear. Physical Review. 21, 266-285.

Wever, E. and Lawrence, M. 1955. Patterns of injury produced by over­stimulation of the ear. J. Acoust. Soc. Am. 27, 853-858.

Wike, E. 1971. Data Analysis. A Statistical Primer for Psychology Students. 123-139, 224. Aldine-Atherton, Chicago and New York.

Yantis, P. and Decker, R. 1964. On the short increment sensitivity index (SISI test). J. Speech Hearing Disorders. 29, 231-246.

APPENDICES

A. AGE AND AUDIOMETRIC DATA ACROSS SUBJECTS

B. INDIVIDUAL MASKED DIFFERENCE SCORES OF A 2000 Hz SINE WAVE FOR NORMAL SUBJECTS

C. INDIVIDUAL MASKED DIFFERENCE SCORES OF A 2000 Hz SINE WAVE FOR NIHL SUBJECTS

50

51

APPENDIX A: AGE AND AUDIOMETRIC DATA

ACROSS SUBJECTS

_

Normal Subjects

# Age

1 23

2 23

3 26

4 23

5 19

6 19

7 19

X 21.7

Ear

L

R

R

L

L

L

R

.25k

5

5

0

5

10

10

5

5.7

.5k

10

5

5

10

10

15

5

8.6

Test

Ik

5

5

0

10

5

15

0

5.7

Frequencies

2k

10

5

5

0

10

15

5

7.1

3k

10

5

10

0

10

5

5

6.4

in kHz

4k

10

10

10

10

5

5

10

8.6

6k

10

10

10

10

5

5

10

8.6

8k

10

15

5

15

0

5

10

8.6

NIHL Subjects Test Frequencies in kHz

#

1

2

3

4

5

6

7

Age

20

20

24

24

32

26

23

Ear

L

R

R

L

R

R

L

.25k

10

5

10

5

10

10

10

.5k

5

5

10

0

15

10

5

Ik

5

5

10

0

15

10

5

2k

0

0

5

5

10

10

5

3k

55

0

40

35

25

75

25

4k

65

35

60

40

35

80

70

6k

50

20

50

15

25

75

65

8k

25

15

15

15

10

75

60

X 24.1 8.6 7.1 7.1 36.4 55 42.9 30.7

52

APPENDIX B: INDIVIDUAL MASKED DIFFERENCE SCORES

OF A 2000 Hz SINE WAVE FOR NORMAL SUBJECTS

Sub. Q-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80

1 3.91 10.96 11.79 8.35 8.68 11.71 7.73 7.54

2 5.05 10.4 8.77 11.82 9.61 11.91 9.11 5.63

3 4 .95 7.47 10 .78 7.19 10.46 7.43 10.99 13.68

4 13 .5 10 .93 13.28 9 .23 8.62 10.66 9 .28 8.83

5 1.8 8.76 8.68 11 .5 11 .3 9.23 6.58 11.25

6 0 .43 1 0 . 3 9.90 10.07 13.13 7.57 8.39 12.77

7 0 .58 4 .59 7.77 12.28 10.52 9.39 9.52 7.33

53

APPENDIX C: INDIVIDUAL MASKED DIFFERENCE SCORES

OF A 2000 Hz SINEWAVE FOR NIHL SUBJECTS

S u b . Q-10 1 0 - 2 0 2 0 - 3 0 3 0 - 4 0 40 -50 50 -60 60 -70 70 -80

1 1 1 . 9 6 1 0 . 3 6 1 0 . 5 6 1 0 . 9 5 6 .42 1 0 . 9 7 9 . 4 1 1 2 . 1

2 1 0 . 6 9 . 1 2 9 . 0 4 1 0 . 3 4 8 .20 1 1 . 0 6 1 0 . 1 0 9 . 8 4

3 5 . 1 7 1 1 . 3 9 . 6 9 1 0 . 8 1 8 .17 1 1 . 2 2 8 .44 9 . 6 9

4 2 . 1 1 3 . 5 8 9 . 5 7 7 . 3 9 1 1 . 0 2 6 .4 9 . 8 7 1 1 . 2 1

5 2 . 5 3 9 . 2 7 9 . 3 9 . 6 2 9 . 2 3 7 . 9 5 9 . 1 3 1 3 . 5 9

6 1 .78 8 . 3 9 7 . 4 1 0 . 0 6 9 . 6 3 1 1 . 9 4 9 . 9 1 9 . 6

7 7 .94 1 2 . 5 7 1 0 . 2 9 . 9 7 9 . 6 9 8 . 8 5 9 . 2 1 0 . 9 6

^^tttnatammmmmmmam