I no,36

ACOUSTIC MEASURES IN NORMAL AND

SENSORINEURAL HEARING-IMPAIRED

Bradley Carl Miller

A Dissertation

Submitted to the Graduate School of Bowling Green State University in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

August 1975

Approved by j^octoral Committee

’Advisor ii

ABSTRACT

Three temporal parameters of the acoustic reflex were compared in 30 normal and 15 sensorineural hearing- impaired subjects. Wide-band noise and octave-band noise centered at 500, 1000, 2000 and 4000 Hz were utilized as reflex producing stimuli.

The sensorineura1 group demonstrated significantly longer reflex offset times than the normal group. Estimates of probabilities of a subject having a sensori­ neural hearing loss were calculated from the frequency distribution of the reflex durations and it was found that weighted decisions about their hearing could be made with a high degree of accuracy. 1X1

ACKNOWLEDGEMENTS

I wish to express my sincere appreciation to Dr.

Michael Doherty, who served actively as a member of my committee and as my minor area advisor.

Thanks is also extended to Dr. George Herman and

Dr. Raymond Tucker who served supportively as members of my committee.

I will always appreciate the concerned help of Miss

Barbara Price of the Toledo School District, without whose efforts this study would have been delayed several months.

I feel a particular debt of gratitude toward my chairman, Dr. Herbert Greenberg, who has related to me as a teacher, advisor, colleague and friend. Academically, he more than anyone is responsible for making my graduate studies purposeful and rewarding.

My greatest debt is unquestionably to my loving wife, Nancy, who has made many personal sacrifices, but has persevered as my most vigorous supporter throughout my personal and academic endeavors.

Prayerful gratitude is humbly expressed for the abilities and blessings that enabled me to undertake and complete this study and degree. iv

TABLE OF CONTENTS

Page

INTRODUCTION...... 1

REVIEW OF THE LITERATURE...... 3

Threshold and Suprathresh©Id Properties of the Acoustic Reflex...... «..*...... • • 3

Temporal Response Measures of the Acoustic Reflex...... 4

Statement of the Problem...... *7

METHOD...... 3

Subjects...... 3

Stimuli...... 9

Instrumentation...... 11

Test Procedure...... 13

Response Analysis...... 14

RESULTS...... 15

Measurement Reliability.15

Reflex Amplitude Validity...... 15

Reflex Measurements...... 16

Estimate of Probabilities of Sensori­ neural Deficit...... 13

DISCUSSION...... 22

Future Research...... 26

REFERENCES...... 28

APPENDIX...... 31 V

LIST OF TABLES

Page Table

1. Mean Values and Standard Deviations of Acoustic Reflex Amplitude for Normal and Sensorineural Hearing-Impaired 16 Subjects...... 2. Mean Values (msec) and Standard Devia­ tions of Acoustic Reflex Onset, Dura­ tion and Offset Responses for Normal and Sensorineural Hearing-Impaired Subjects Using Various Noise Bands...... 17

3. Estimates of Probabilities of Sensori­ neural Deficit...... 20 VI

LIST OF FIGURES

Page Figure —

1, Block Diagram of Equipment Used for Signal Presentation and Reflex Recording...... 12 2. Distribution of Acoustic Reflex Dura­ tion for Nonna1 and Sensorineural Hearing-Impaired Subjects19 INTRODUCTION

Even though researchers have been studying the

acoustic reflex for almost a century many questions

about this middle muscle reflex still remain un­ resolved (Dallos, 1973).

Recent advances in equipment technology have en­ abled investigators to study many different properties of the acoustic reflex (Holst et al., 1963). The primary concern of researchers has been the measurement of re­

flex threshold, although there have been some investi­ gations into other reflex parameters (Anderson et al,,

1970j Alberti, 1972j Norris et al., 1974a, 1974bj

Colletti, 1974). There have been three main types of other reflex measurements discussed in the litera­ ture. The most commonly studied has been that of re­ flex onset where the investigators were concerned with the time interval between onset and reflex onset (Metz, 1946 j Klocfchoff, 1961> Dallos, 1964).

However, more recently researchers have been studying reflex adaptation and reflex offset. Anderson et al.

(1970) found that they were able to distinguish between normal and VIII nerve lesion ears by measuring the re­ flex adaptation to a stimulus of 10 seconds duration.

Norris et al. (1974a) and Colletti (1974) found they were able to differentiate normal from sensorineural 2 hearing-impaired individuals by observing reflex offset to a short duration pulsed stimulus. The most recent study to date (Norris et al., 1974b) examined several reflex parameters in normal and sensorineural hearing- impaired populations. They found differences between these two populations in reflex offset but not in re­ flex onset. However, only a 1 KHz pure tone of 250 msec duration was used as the stimulus to elicit the acoustic reflex. A more extensive stimulus plan should certainly be used to study and compare the temporal characteristics of the acoustic reflex in these groups.

This study is therefore designed to assess several temporal parameters of the acoustic reflex of normal and sensorineural hearing-impaired ears using various octave bands of white noise. 3 REVIEW OF THE LITERATURE

This literature review includes research in­ vestigations in two major areas: (1) threshold and

suprathreshold properties of the acoustic reflex, and

(2) temporal response measures of the acoustic reflex,

i.e., reflex onset, reflex adaptation and reflex offset.

Threshold and Suprathreshold Properties of the Acoustic Reflex

The acoustic reflex (AR) in man has been studied using acoustic impedance techniques since the early l930*s.

A thorough history and description of the acoustic re­ flex is presented in Moller (1972) and Jepsen (1963).

Deutsch (1972) measured the acoustic reflex in 30 normal hearing young adults. Pure tones of 250 Hz, 2

KHz, and bursts of narrow-band noise centered at 2 KHz and 4 KHz were employed to elicit an AR. He found that the mean reflex threshold for pure tones was 81 dBSL and the mean for noise was 62 dBSL. He reported that noise stimuli elicited a more stable reflex than did pure tones. Other investigators (Metz, 1946 > Djupes- land, 1964j Danaher and Pickett, 1974) also reported that noise stimuli were considerably more effective than pure tones in eliciting the AR in normal and sensorineural ears. 4

Very little information is available with regard to the measurement of reflex thresholds on a sensori­ neural hearing-impaired population. Keating et al.

(1972) used an audiometer with a maximum output of

110 dBHL to investigate the AR thresholds of students at a school for the deaf. They examined 133 children classified as otologically normal and obtained reflex thresholds on only 22 of these subjects. Danaher and

Pickett (1974) studied 15 congenitally deaf students with marked-to-severe sensorineural hearing-impair­ ments. They employed a signal source with maximum output of 130 dBSPL and found the range of reflex thresholds to be from 100 to 130 dBSPL. They suggested the use of broad-band noise rather than pure tones in eliciting the reflex since pure tones were not as effective a stimulus for this population. Since the

AR was elicited at lower levels with a noise stimulus, fewer cross-over problems were encountered.

Temporal Response Measures of the Acoustic Reflex

Reflex Onset

Reflex onset refers to the time interval between stimulus onset and initial reflex onset. For a given sound pressure level, the reflex onset is shorter for noise than for pure tones around l KHz, and longest 5

for pure tones lower than 300 Hz (Hung and Dallos, 1972).

This study also reported that the reflex onset was

shortened by increasing the stimulus intensity. Dallos

(1964) and Djupesland (1964) reported similar findings,

although Djupesland observed a more rapid change in

impedance for low frequency stimuli than for high fre­ quency stimuli when the stimulus intensity was increased.

Norris et al. (1974b) observed slight differences in reflex onset between a normal and a sensorineural hearing-

impaired group, however, no significant differences were

found.

Reflex Adaptation

Dallos (1964) pointed out that at high stimulus levels a sustained muscle reflex is observed due to the involvement of many neurons; at medium levels the re­ sponse rapidly achieves an initial peak and then steadily decreases; and, near threshold it peaks and then is rapidly extinguished. He observed these re­ sponses when stimuli of up to 120 seconds in duration were employed. Anderson et al. (1970) studied this adaptation phenomenon of the acoustic reflex with normal and VIII nerve lesion subjects. They used a

10 second stimulus at 10 dB above their reflex thres­ hold, Their results showed that VIII nerve lesion 6

subjects have much faster adaptation than the normals,

even in cases when they have normal audiometric

thresholds. Together, these two studies suggested

that a reflex elicited by a stimulus above the AR

threshold and of sufficient duration (at least 3

seconds) is influenced by neural adaptation. Habener

and Snyder (1974) studied reflex adaptation in a large

population of normal hearing subjects. They employed

pure tones of 10 second duration at 10 dB above the

AR threshold. At 500 Hz and l KHz there was no ap­

preciable adaptation, however, adaptation was ob­

served in the majority of the subjects at 2 KHz and

4 KHz.

Reflex Offset

Reflex offset refers to the time interval between

stimulus offset and the cessation of the reflex, Dallos

(1964) was the first to report that the reflex offset

was much slower than the onset, however, since then

little has been done in the area of reflex offset in

normal or sensorineural ears, Colletti (1974) and

Norris et al. (1974a) found that sensorineural ears have a slower reflex offset than normal ears when

rapidly pulsed pure tone stimuli are presented.

Norris et al. (1974b) were among the first investigators 7

to measure several temporal parameters of the acoustic

reflex including reflex offset on normal and sensori­

neural ears. They presented a l KHz pure tone stimulus

with 250 msec on-time at 10 dB above reflex threshold.

Significant differences in reflex offset between the

normal and sensorineural groups were observed with the

sensorineural group exhibiting longer offsets.

Statement of the Problem

It can be seen from the review of the literature

that only a few investigations of the AR have been per­

formed and only Norris et al. (1974b) has made a direct measure of temporal reflex parameters in normal and

sensorineural ears. However, since Norris et al. (1974b) used a signal duration of 250 msec there is a pos­ sibility that their results were contaminated by the differing effectsCof temporal summation in normal and sensorineural ears. Since there was no indication of reflex amplitude comparisons between the two groups, data validity again is questionable because of equip­ ment limitations. Many studies have reported that noise is the most sensitive and reliable reflex elicitor. Yet no investigations have been conducted to determine temporal parameters of the AR using broad or narrow-band noise in normal and sensorineural 8 hearing-impaired populations. In view of the growing

interest in the properties of the AR and the fact that the temporal aspects of the AR appear to possess some diagnostic potential, it seemed desirable to study and compare the effects of a noise stimulus on the reflex of normal hearing and sensorineural hearing-impaired subjects. This study was therefore designed to evaluate and compare the reflex onset, duration and offset in normal and sensorineural populations as an aid to diagnostic evaluation.

METHOD

Several octave bands of noise were presented to normal and sensorineural hearing-impaired subjects at

10 dB above their reflex threshold. The subjects* re­ flex and the stimulus occurrence were recorded for each observation interval. The AR temporal character­ istics were then measured and compared.

Sub iects

As a subject advances in age, the ability of the muscles (MEM) to contract may decrease and reach an upper limit of contractibility regardless of whether the ear is normal or hearing-impaired (Beedle and Harford, 1973). For this reason, teenage or young 9

adult subjects were used. The age difference between

the two populations studied was not considered to be a factor that would influence the results.

Normal Hearing

Fifteen normal hearing college students with a mean age of 20 years served as subjects. To be included

in this group, a subject had to have pure tone thres­ holds no poorer than 10 dBHTL (ANSI, 1969) for the frequencies 500 Hz, 1 KHz and 2 KHz, negative otologic findings, and normal tyrapanometric curves.

Hear ing-Impaired

Fifteen subjects with congenital marked-to^severe sensorineural hearing losses and a mean age of 12 years served as subjects in this group. All sensorineural hearing-impaired subjects were from the Hearing Impaired

Program of the Toledo School District. They had negative otologic findings, normal tympanometrie curves, and a loss of hearing sensitivity between 60-70 dB pure tone average,

Stimuli

Many investigators (Metz, 1946$ Dallos, 1964$

Djupesland, 1964$ Danaher and Pickett, 1974) have re­ ported that noise is a more sensitive and reliable 10

stimulus than pure tones for eliciting an acoustic reflex. For this reason, wide-band noise and octave bands of white noise centered at 500 Hz, 1 KHz, 2 KHz and 4 KHz were used in this study.

Singh (1975) has shown that a narrow-band stimulus of less than 300 msec duration can affect the reflex thresholds of normal and sensorineural hearing-impaired ears. Anderson et al. (1970) and Habener and Snyder

(1974) have reported that pure tone stimuli- at moderate intensity levels above the reflex threshold and greater than 3 seconds duration are affected by neural adaptation.

Therefore, a stimulus duration of 1 second was used in order to avoid contamination by either of these phenomena.

A three second inter-stimulus interval was used because a pilot study showed that the reflex totally relaxed within this time period.

A stimulus level of 10 dB above the reflex thres­ hold was employed to obtain the reflex. Most of the studies investigating reflex parameters used this level due to the stability of the AR at this point.

A 25 msec stimulus rise-fall time was employed to avoid an onset click (Djupesland, 1964 j Beedle and Har­ ford, 1973j Norris et al., 1974b). 11

Instrumentation

A simplified block diagram of the instrumentation

used for stimulus presentation is shown in Figure i.

The noise signals were produced by a noise generator

(Grason-Stadler, Model 455C), controlled by an electronic

switch (Grason-Stadler, Model 829G) and interval timer

(Grason-Stadler, Model 471-1), amplified (Fisher, Model

TX-50), filtered (Allison, Model 2BR), and then sent

to an earphone (Telephonies, TDH-49) whose level was

controlled by an attenuator (Hewlett-Packard, 350D).

The stimulus duration, repetition rate and rise-fall

time were controlled by the electronic switch and

interval timer. The acoustic output at the earphone

was calibrated daily on a sound level meter (Bruel

& Kjaer, Type 2203), The spectrum of each noise band

used was measured at the earphone using a wave analyzer

(General Radio 1523-P4) and recorder (General Radio,

1523) at the beginning and end of the study.

The technique used for recording the acoustic reflex is also shown in Figure 1, The reflex measure­ ment was made on the ear contralateral to the stimulus by monitoring the admittance changes accompanying

MEM contraction on a Grason-Stadler Otoadmittance

Meter, Model 172OB, It should be noted that a 20 msec Figure 1» Block Diagram of Equipment Used for Signal Presentation and Reflex Recording. 13 equipment latency is characteristic of the otoadmittance meter, however, since it is equally added to the reflex time of both groups it does not bias the results.

Feldman and Zwislocki (1965) found that the acoustic reflex mainly affects the stiffness reactance component of the impedance of the middle ear system, leaving re­ sistance almost unchanged, thus only reactance changes were measured in this investigation. For a permanent recording of the admittance changes, the output of the reactance meter was connected to one channel of a multi-channel pen recorder (Sanborn, Model 585-5460:).

To allow temporal reflex calculations, the time of stimulus presentation was recorded on another channel of the pen recorder.

Test Procedure

The subjects were seated in a sound-treated room

(IAC Model 402A) in which the ambient noise level was sufficiently low to permit threshold testing of normal hearing persons. Initially, routine pure tone air con­ duction thresholds and impedance measures were obtained.

Selection of the ear to receive the reflex activating signal was randomly alternated in the normal population, whereas the signal was presented to the better ear of 14

the sensorineural hearing-impaired group. The subject

was then instructed as to the test procedure and asked

to relax and sit quietly. In an ascending fashion, each

signal was presented in 10 dB steps, beginning at 60 dBSPL

until a reflex was observed. The stimulus intensity

was then decreased 10 dB and raised in 2 dB steps until

a clear recording of the reflex was obtained. The

stimulus was then presented at 10 dB above this level.

This procedure was carried out for each noise band in

Sequence (Wide band, 500 Hz, 1« KHz, 2 KHz, and 4 KHz).

If a reflex was not obtained to any of the noise bands,

the subject's data was discarded.

Response Analysis

Measurements of the acoustic reflex were performed using a metric ruler. Reflex onset response was defined as the time interval between stimulus onset and initial reflex onset (5 per cent upward movement from baseline)? reflex duration was described as the time interval between

initial reflex onset and cessation of the reflex (95 per cent return to baseline)? and, reflex offsetrefer­ red to the time interval between stimulus offset and cessation of the reflex. RESULTS

Measurement Reliability

A graduate student in audiology was asked to measure

20 randomly selected reflex durations. He was requested to measure (to the nearest millimeter) the area between initial reflex onset and cessation of the response.

Approximately 15 minutes were spent in training this individual. His measurements and those of the author were then statistically compared producing a product- moment correlation coefficient of 0,99. No significant difference was found between the two sets of measure­ ments ,

Reflex Amplitude Validity

The otoadmittance meter has varying equipment laten­ cies when reproducing waves of differing amplitudes. For this reason it is important that the: ref lex amplitudes be similar for the two groups being compared. In many studies this instrumental variable is not accounted for and thus a degree of artifact is included in their reflex data. For this reason, a t-test was used to compare the difference between the reflex amplitudes of the two groups and no significant difference

(p > 0.10) was found. These data are shown in Table

1. 16

Table 1

Mean Values and Standard Deviations of Acoustic Reflex Amplitude for Normal and Sensorineural Hearing-Impaired Subjects

Mean SD

Normal 7.46 mm 3.0 mm

Sensorineural 7.29 mm 2.9 mm

Reflex Measurements

Table 2 presents the mean values and standard devia­ tions of the reflex onset, duration and offset for the normal and sensorineural hearing-impaired subjects.

These data show only a small difference in the onset measures between the groups, however, the duration and offset values from the sensorineural group were larger than those from the normal group for each noise band.

In order to evaluate these reflex measurements independently, t-tests (Ferguson, 1971) were used to compare the mean values of the reflex onset, duration and offset between the groups for all stimuli. This analyses revealed significant differences in these Table 2

Mean Values (msec) and Standard Deviations of Acoustic Reflex Onset Duration and Offset Responses for Normal and Sensorineural Hearing-Impaired Subjects Using Various Noise Bands

Center Frequency

Wideband • 5KHz lKHz 2KHz 4KHz Grand Mean SD Mean SD Mean SD Mean SD Mean SD Mean

Onset 227.5 32.7 209.5 39.3 212.3 29.9 218.9 34.9 226.0 46.9 218.8

Normal Dura­ tion 1050.7 84.3 1070.8 60.4 1077.6 55.5 1078.5 79.9 1073.5 101.9 1070.2

Offset 283.5 70.8 272.7 39.4 281.7 51.4 285.0 61.3 287.6 69.7 282.1

Onset 228.0 58.5 197.1 60.4 216.5 63.5 238.8 57.4 242.3 52.2 224.7

Sensori­ Dura­ neural tion 1505.3 209.8 1546.9 172.4 1583.4 261.1 1569.5 216.3 1531.6 259.1 1547.3

Offset 714.7 229.9 751.7 190.2 798.4 299.9 815.8 234.7 771.9 298.3 770.5

I- 18 response characteristics at each noise band between the normal and sensorineural group at the .001 level of significance, An F-max test indicated that there was marked heterogeneity in the variance present between the groups; therefore, it was necessary to set a higher significance level before accepting this statistical analysis (Lindquist, 1953). Figure 2 presents the frequency distribution for reflex dura­ tion at all noise bands of the normal and sensori­ neural subjects. The two groups appear to be normally distributed, however, it can be seen that there is a marked difference in the variance between these groups.

Data points for Figure 2 were obtained from the last two columns of Appendix A which shows the frequency distribution of the subjects* durations for all noise bands.

Estimate of Probabilities of Sensorineural Deficit

Table 3 presents the probability that a person tested has a sensorineural loss based on their reflex duration. Column A shows the reflex durations in milliseconds. Column B gives the likelihood ratio of a subject from this study having a marked-to-severe sensorineural hearing-impairment. Column C presents Humber of Subject Durations for ail Noise Bands F i g u r e

2.

N D Impaired o i r s m t a r l i

b u (

t • ) i S u o b and n j e of c t

S s A e . c n o s u o s r t i i n e c u

R r e a f l

l e ( x ■ )

D H u e r a a r t i i n o g n -

f o r

20

Table 3

Estimates of Probabilities of Sensorineural Deficit

A B i C Reflex Duration Likelihood Ratio-1- Posterior (msec) Probability

949 .000 .000

950-1049 .000 .000

1050-1149 .014 .0122

1150-1249 .182 .134

1250-1300 1.000 .665

1301-1349 5.993 .886

1350-1400 5.993 .886

1401-1500 00 1.000

>1500 00 1.000

^Probability of reflex duration given sensorineural divided by probability of reflex duration given normal.

2 Alternative Prior P(Pos/alt) Joint Posterior Normal .667 x 72/150 » .320 - .988 Sensorineural .333 x 1/75 « .004 « .012 .324 1.000 21

the posterior probability (Schmitt, 1969) of any in­

dividual tested at our clinic with a reflex duration

listed in Column A, as having a marked-to-severe

sensorineural hearing loss. These probabilities are

based on a prior assumption that one out of three children

suspected of having a marked-to-severe sensorineural

loss of hearing sensitivity would indeed have such a

loss. The two to one odds were arrived at by observing

the fact that children who are emotionally disturbed or have a neurological disorder may exhibit behaviors that

cause them to be suspected of having a hearing impair­ ment. DISCUSSION

Since this investigation was undertaken with the

development of a diagnostic tool in mind, it was felt

that the results should be analyzed so as to be applic­

able to clinical investigations. For this reason, a

statistical probability approach to the data was em­

ployed, By using Bayes* Theorem any hearing-impaired

individual’s reflex duration or mean reflex duration

can be compared to Table 3 and specific weighted con­

clusions about his hearing can be drawn. However, be­

fore this can be performed, certain qualifications must

be stipulated. It must be kept in mind that the imped­

ance technique is only an indirectnmethod of measuring

the stapedial muscle reflex. Therefore the mean

values reported here represent absolute measurements

only when an otoadmittance meter is used. The prior

odds of two to one were selected because of our

specific clinical backgrounds, and therefore if an

audiologist feels that the prior odds for his clinical

setting would be different, it is suggested that he

calculate his own posterior odds as outlined by

Schmitt (1969).

It is suggested that an audiologist apply the data

from this study to his clinical findings in the following manner. For example, this investigation found that the 23 two populations do not overlap when a subject’s reflex durations using the five noise bands are averaged and then applied to Table 3. None of the sensori­ neural hearing-impaired subjects fell into the normal reflex duration area when their mean reflex duration was calculated. In addition, none of the normal hearing subjects had reflex durations that would cause them to be classified in the sensorineural group.

Although not reported in the results section, two individuals with mild sensorineural hearing losses (45 dB pure tone average) were also tested. One of these individuals had a reflex duration of 1252 msec and the other 1097 msec. Although these were the only two people tested with this degree of hearing loss, their results seem to indicate that a much greater overlap between people with this extent of loss and the normal group would occur. This would imply that further research with differing degrees of sensorineural hearing loss is needed before weighted predictions as outlined by this study can be made. For example, if an audiologist obtains a reflex duration of 1097 msec, he can be reasonably sure that the subject does not have a raarked-to-severe sensorineural hearing loss, but cannot rule out the possibility of a mild sensorineural 24

hearing loss.

The high correlation between the measurements of

the outside observer and those of the author indicated

that the reflex was easy to read and could be measured

accurately. In addition, it appears that only a brief

training period is necessary.

In this investigation, there were no differences

in reflex amplitudes between the groups. The present data can be considered relatively uncontaminated by any instrumentation variable.

The most important findings were the longer re­

flex duration and offset of the sensorineural group.

They took approximately 450 msec longer to reach base­

line than did the normal group. At this point, one might speculate as to the reason for the longer reflex duration and offset times in the sensorineural group.

It has been found that the resistance and reactance components at the tympanic membrane were abnormally high at low frequencies in VIII nerve lesion subjects

(Macrae, 1973). Macrae (1973) suggested that the abnormality is attributable to an increase in the input acoustic impedance of the produced by the in­ crease in protein content of the cochlear fluids and dilation of the cochlear duct known to occur in 25

acoustic neuromas. His explanation is supported by

theoretical calculations carried out on an electric

analog of the conductive system and he suggested that

similar abnormalities in the acoustic impedance at the

tympanic membrane might occur in other pathologies

which produce abnormal mechanical conditions in the

cochlea. The abnormal mechanical condition in the

sensorineural hearing-impaired cochlea would possibly

be the effect of striai atrophy and the debris of

degenerating cells that are found in the fluid spaces

of the in noise-induced losses (Engstrom

et al., 1970).

When the hair cells are demaged the nerve fibers

in the organ of Corti also degenerate (Engstrom et al.,

1970). This could have several effects on the analysis

of the stimuli at the cochlea. First, it could be

suggested that even though a hair cell was only par­

tially damaged the effects on the hair cell’s re­

ceptive and presynaptic regions could possibly account

for longer latencies in the signal processing time.

Since the auditory nerve fiber is known to degenerate with hair cell damage, one might speculate that the nerve fiber’s membrane permeability is affected causing an extended period for depolarization, which in turn 26

could cause longer reflex response times in sensori­

neural hearing-impaired individuals. It could also be

suggested that a combination of these factors would

attribute to differences found in this investigation.

However, these are pure speculation and no physiological

experimental evidence is available at this time to sup­

port these hypotheses.

Although the sensorineural group had greater mean differences across stimuli, because of the large dif­

ferences in variance the differences in means were not

large enough to suggest that one noise band is more sensitive than another.

Future Research

As stated earlier, the reflex duration data as analyzed in Table 3 can only be used to identify in­ dividuals with marked-to-severe sensorineural hearing- impairments, Therefore, future research should be concerned with expanding the use of reflex response time testing to identify differing degrees of sensori­ neural impairments.

Since it was necessary to study a sensorineural group from whom behavioral data, as well as reflex data, could be obtained, it is suggested that young 27

children suspected of having a hearing loss be tested

using this method. These results could then be com­

pared to existing and future audiometric thresholds to determine the usefulness of the reflex procedure as a

clinical tool'. REFERENCES

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APPENDIX A Frequency Distribution of Subject’s Durations for all Noise Bands

Totals Sensori- Acoustic WB . 5KHz 1KHZ 2KHz 4KHz Normal neurals Reflex Duration N S N S N S N S N S N s

^949 2 2 2 6 950-1049 13 8 10 11 12 54 1050-1149 12 17 17 14 12 1 72 1 1150-1249 1 3 3 1 4 2 1 13 2 1250-1300 1 2 1 2 2 1 3 6 1301-1349 1 2 2 2 1 1 7 1350-1400 1 4 1 1 2 1 7 1401-1500 1 4 4 2 5 16 >1500 7 8 5 9 7 36