ACOUSTIC-REFLEX DYNAMICS AND TEMPORARY THRESHOLD SHIFT IN OCTAVE-BAND NOISE EXPOSURES BY MICHAEL JAY MOUL A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1985 ACKNOWLEDGEMENTS I would like to thank those people whose help and cooperation facilitated this research. Major Ernest Hepler provided the conceptual background. His constant encouragement and friendship made the project feasible. Dr. Kenneth Gerhardt, chairman of my supervisory committee, had the patience and expertise needed to keep the project going over many rough spots. His teaching, guidance and friendship were such that I will always look to him as my mentor. My supervisory committee, Drs . Alice Holmes, Joseph Kemker, Patricia Kricos and Alan Agresti not only helped with this research effort, but also made my years on campus much more rewarding. Their support as well as that of my fellow students is very much appreciated. I am very grateful to the United States Army Medical Service Corps for my selection and funding for this educational experience. My military academic preceptor. Lieutenant Colonel Roy Sedge, supported my application for training and continued to support me throughout my schooling. n I owe my greatest thanks to my wife, JoAnne, and to my daughters, Michele and Marcia. Their love makes every project worthwhile and made this one possible. in TABLE OF CONTENTS Page ACKNOWLEDGEMENTS ii ABSTRACT vi CHAPTER I BACKGROUND AND PURPOSE 1 Introduction 1 Review of Literature 5 Temporary Threshold Shift 5 Measuring Temporary Threshold Shift ... 10 The Acoustic Reflex 16 Measuring the Acoustic Reflex 19 Acoustic Reflex Threshold 23 Acoustic Reflex Magnitude 27 Acoustic Reflex Latency 29 Acoustic Reflex Adaptation 31 The Acoustic Reflex and Temporary .... Threshold Shift 35 Susceptibility to Noise 40 Statement of Purpose 46 II METHODS 4 8 Subjects 48 Instrumentation and Procedures 49 Exposure 49 Behavioral Hearing Threshold Measurements 52 Acoustic Reflex Measurements 57 Calibration 6 4 Data Analysis 68 III RESULTS 69 Behavioral Threshold Shift 69 Acoustic Reflex Properties and TTS 80 Acoustic Reflex Threshold 80 Acoustic Reflex Magnitude 84 Acoustic Reflex Latency 92 IV CHAPTER Page III RESULTS (Continued) Acoustic Reflex Adaptation 96 Demographic Data and TTS 99 Pre-Exposure Audiometric Threshold and TTS 102 Multiple Regression Modeling 103 IV DISCUSSION 110 Interpretation of Results 110 Behavioral Threshold Shift 110 Acoustic Reflex Threshold 113 Acoustic Reflex Magnitude 114 Acoustic Reflex Latency 117 Acoustic Reflex Adaptation 119 Ear Canal Volume/Resonance 121 Demographic Differences 123 Predictive Modeling 124 Summary 127 REFERENCES 129 BIOGRAPHICAL SKETCH 146 v Abstract of Dissertation Presented to the Graduate Council of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ACOUSTIC-REFLEX DYNAMICS AND TEMPORARY THRESHOLD SHIFT IN OCTAVE-BAND NOISE EXPOSURES By Michael Jay Moul May, 1985 Chairman: Kenneth J. Gerhardt, Ph.D. Major Department: Speech Clinical and experimental observations indicate that there are individuals uniquely susceptible to noise-induced shifts in auditory sensitivity. Experimentally induced temporary threshold shift (TTS) varies widely in subjects exposed to the same noise source. The acoustic reflex (AR) provides a protective function that also varies widely in individuals. The AR therefore may be one of the complex factors that relate to individual susceptibility to TTS. This study examined the relation of dynamic properties of the AR to the TTS that developed in thirty human subjects. The following experimental question was asked. Can pre-exposure measurements of AR activity (threshold, magnitude, latency and adaptation) be used to predict the vi . TTS that develops in normal hearing subjects following two, separate, two-hour octave-band noise exposures (centered at 0.5 kHz and 3.0 kHz). All subjects were exposed to both noises. Significant TTS occurred that was more pronounced for the high- frequency exposure. The audiometric pattern of TTS differed for the two exposures. Pre-exposure behavioral hearing threshold and several AR parameters were found to be significantly correlated to the TTS that resulted from both noise exposures. Multiple regression modeling showed AR adaptation, onset latency and magnitude (as elicited with a broad-band noise stimulus) to be only slightly predictive of the TTS that resulted from the low-frequency noise exposure. Other elicitor/variable combinations were less predictive particularly for TTS resultant from the high-frequency noise These results suggest that the AR is a factor in the development of TTS. Only a slight predictive relationship exists however. Previously reported data on variation in both the AR and in TTS according to the spectral content of noise were confirmed. vix . CHAPTER I BACKGROUND AND PURPOSE Introduction to Loss of hearing sensitivity caused by exposure problem. noise is a significant economic and social States Expenditures by both private industry and the United year government total hundreds-of-millions of dollars per costs in disability payments alone. The additional care involved in funding for diagnostic and rehabilitative performance, are equally staggering. The reduction in job increase in absenteeism and employee retraining/replacement are costs associated with reduced communication ability incalculable. No dollar figure can be placed on the personal and social impact of this problem. Hearing loss it not only effects the impaired individual's life style, also changes that of family, co-workers and friends. Annoyance, fear, embarrassment, loss of self-esteem and gradual withdrawal from society often accompany hearing loss. All these costs, both monetary and human, make noise-induced hearing loss one of our nation's most serious occupational issues (Myklebust, 1964; Kryter, 1970; Hepler, Moul and Gerhardt, 1984) 1 . 2 Hearing conservation measures have been mandated in several federal government documents. Program success has been constantly undermined by a lack of worker education and by industry's reaction to implementation costs and perceptions of excessive governmental control. A United States Court of Appeals decision on November 8, 1984 (in response to industry's complaints) virtually invalidated the Occupational Safety and Health Administration's hearing conservation standard and will adversely effect efforts to deal with this problem nationwide (Department of Defense, 1978; Code of Federal Regulations, 1981; Occupational Safety and Health Administration, 1983; Cherow, 1985) However, even if such programs could be effectively implemented there is still an unresolved issue of individual susceptibility to the damaging effects of noise. Clinical experience and data from military and industrial noise-exposed populations indicate that there are individuals who are uniquely susceptible to noise- induced hearing loss (Burns, 1973; Hepler, Moul and Gerhardt, 1984) . Such individuals develop a loss of hearing sensitivity after relatively limited exposure to noise. The identification of "at risk" individuals prior to their placement in jobs involving noise exposure (or prior to induction into military service) could significantly lower the incidence of noise-induced hearing loss. Unfortunately, no single predictive index of this 3 susceptibility has been identified (Jerger and Carhart, 1956; Simmons , 1963; Ward, 1965; Michael and Bienvenue, 1977; Humes, 1977). Research efforts in the area of individual susceptibility have therefore been focused upon the identification of patterns or relations between various auditory and non-auditory indices that might improve prediction. This "test battery" approach must include measures of all factors that potentially protect an individual's auditory system from damage, or conversely, that might predispose it to damage. The acoustic reflex (AR) is a contraction of middle- ear muscles in response to intense acoustic stimulation. The AR reduces the intensity of sound transmitted through the middle-ear system and may therefore provide some degree of protection to the inner-ear structures known to be damaged by noise. The extent of this protection is varied and appears to be dependent upon the nature of the noise and upon certain dynamic properties of the individual's AR. The implication is that individuals with, for example, an AR response that adapts quickly will be more susceptible to damage than those with AR responses that are sustained for longer time periods. The AR/susceptibility literature is replete with inconsistent results due, in part, to the wide range of equipment and noise exposure paradigms that have been used (Coles, 1969; Johansson, Kylin and Langfy, 1967; Popelka, 1984). Many studies reporting no demonstrated 4 AR/susceptibility relation were accomplished using short- term exposure to noise (minutes) . Other research involving long-term exposure to noise (hours) suggests that the AR may be a factor in individual susceptibility (Borg and Odman, 1979; Gerhardt, Melnick and Ferraro, 1979; Gerhardt and Hepler, 1983) . Documenting individual variation in susceptibility to noise as related to AR parameters required that a series of coordinated studies be completed. Hepler (1984) provided data on AR properties for a noise exposure (broad-band noise at 90 db SPL for two hours) that was chosen to closely approx- imate the actual