EFFECTS OF INTENSE ACOUSTIC NOISE ON COCHLEAR FUNCTION IN INFANT ANT ADULT CUTNEA PIGS By HARVEY BRUCE ABRAMS 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 1980 ACKNOWLODGIiMl-NTS The author wishes to express his deepest appreciation to and respect for Dr. Teas, Chairman of his supervisory committee and academic advisor, whose guidance and patience have been a continual source of encouragement and inspiration throughout this endeavor. The author also wishes to express his sincere gratitude to Drs. William E. Brownell and William H. Cutler, for their valuable assistance. The writer is indebted to the Veterans Administration which provided a traineeship during his doctoral studies at the University. TABLE OF CONTENTS Page ACKNOWLEDGEMENTS i;L ABSTRACT ... IV CHAPTER I. INTRODUCTION . . 1 II. METHOD . . . 12 Animals Surgical Procedure ■-••-. 12 Signal Generation '.'.'.'''" 12 Noise Stimulus Generation . . . . ]t Recording . . • - 19 b 1 Q Procedure . Histology . 2i' S} 25 III. RESULTS . . . 27 Preexposure r 27 Response Waveform Threshold Effects of Frequency and Intensity on Nj and P4 Response Amplitudes ,, Effects of Frequency and Intensity on Nj and P4 Response Latencies 41 Postexposure . . r AQ Effects of Noise on Threshold ° Effects of Noise Exposure on Response Amplitude and Latency Histology . . 70 IV. DISCUSSION . . . 80 Development of the Auditory System Effects of Noise on the BSR .....'.'.'.' .' .' — Susceptibility . LIST OF REFERENCES 88 BIOGRAPHICAL SKETCH 94 Abstract of Dissertation Presented to the Graduate Council oT the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy EFFECTS OF INTENSE ACOUSTIC NOISE ON COCHLEAR FUNCTION IN INFANT AND ADULT GUINEA PIGS BY HARVEY BRUCE ABRAMS March 1980 Chairman: Donald C. Teas Major Department: Speech Brain stem responses (BSR) to filtered acoustic clicks were ob- tained from four groups of guinea pigs, two newborn and two adult, with chronically implanted recording electrodes. One adult group and one newborn group were exposed to two hours of a 108 dB SPL narrow band of noise centered at 4 kHz. The two other groups served as controls. After one month following the exposure, BSRs were again obtained, the animals sacrificed and the cochleae prepared to deter- mine the number of missing hair cells. Threshold, response amplitude and response latency values were analyzed for the N^ and P4 waves of the BSR for all animals. Preexposure results indicated that while the Nj response functions in the adult and newborn groups were gener- ally similar, the P4 functions differed between the two groups. Postexposure results indicated a greater Nj and P4 threshold shift for the exposed newborn group in response to the 6 kHz filtered click than for the adult group. In addition, the postexposure N: and P4 response amplitude functions of the newborns showed a greater relative decrease in response to the 6 kHz filtered click than did the adult functions. Postexposure latency measures did not indicate differences between the groups although the latencies of Hi and P4 in response to the 1 kHz filtered click were longer at higher click intensity levels than those obtained prior to the noise exposure. Histological analysis failed to show any differ- ences in the number of missing hair cells among the exposed adults and newborns. The results of this study are discussed in relation to maturation of the auditory system, effects of noise on the BSR and susceptibility to noise-induced hearing loss. CHAPTER I INTRODUCTION Modern man pays a price for technological progress in the form of industrial and environmental related health disorders. Tie effects of noise on the auditory system are of considerable interest today and have been studied since the early 19th centry. Fosbroke (1831) identi- fied individuals suffering noise-induced hearing loss on the basis of their exposure to weapon fire and forge operations. Yoshii f 1909) was able to identify the hair cell region of the cochlea as the principle site of noise-induced damage. Investigations through the 1950 's were directed toward the quantification of cochlear damage as a function of stimulus intensity and frequency (Crowe, Guild and Polvogt, 1934; Davis, Derbyshire, Kemp, Lurie and Upton, 1935; Lurie, Davis and Hawkins, 1944; Wever and Smith, 1944; Davis and Associates, 1953; Wever and Lawrence, 1955). Throughout the last several decades the effects of noise upon cochlear electrophysiology have received atten- tion (Eldredge, Bilger, Davis and Covell, 1961; Price, 1968, 1972, 1974; Simmons and Beatty, 1962; Benitez, Eldredge and Tempi er, 1972; Mitchell, Brummett and Vernon, 1973; Eldredge, Mills and Bonne, 1973) as has the normal anatomical and physiological development of the cochlea (Nakai and llilding, 1968; Pujol and Ililding, 1975). Tie efforts of these and many other investigators have added much to our knowledge of the effects of intense noise on the auditory sys- tem. We know, for instance, that the cochlear structures principally i affected by high noise levels arc the outer hair cells (Lurie ct al. 1944; Davis et al. 1935). As the intensity or duration of the noise increases the extent of injury increases and may include other struc- tures such as the inner hair cells and afferent nerve fibers. It is generally accepted that the location of damage within the cochlea is related to the frequency of the stimulus. High frequency, high in- tensity noise causes damage to the basal end of the cochlea whereas lower frequency noise affects more apical portions of the cochlea (Wever and Smith, 1944). It has also been shown that damage to the structures of the cochlea results in electrophysiological changes such as a decrease in the magnitude of the cochlear microphonic (Price, 1968, 1972, 1974) and in the whole-nerve action potential (Mitchell, Brummett and Vernon, 1973, 1977). Although the effects of noise on cochlear structure and function have been well studied and documented, the specific mechanisms of cochlear damage remain unresolved. Spoendlin (1976) suggested that the specific mechanisms of noise- induced cochlear damage are a function of the intensity of the stimu- lus and its duration. Spoendlin described two critical intensities with respect to their effect upon the cochlea. Below 90 dB (critical intensity I) there appears to be little structural damage regardless of the duration of the exposure. Above 130 dB (critical intensity II) the structures of the cochlea undergo severe irreversible damage even with very short durations. As the duration of the exposure increases at 130 dB and above, the damage to the cochlea docs not increase in proportion to the duration. The mechanism of damage at these levels is apparently mechanical as evidenced by separation of the organ of Corti from the basilar membrane. The hair cells in the detached por- tion often have a normal looking appearance. The motion of the basilar membrane at high intensities is apparently so violent that the organ of Corti separates from the membrane. Between these two critical intensi- ties (90- 130 dB) the duration of exposure is an important consideration. As duration increases for moderately intense stimuli, cochlear damage increases. The specific mechanisms of damage between the two critical intensities have been the subject of considerable investigation (Spoendlin, 1970, 1971; Ward and Duvall, 1971; Beagley, 1965; Lipscomb and Roettger, 1973; Bohne, 1972, 1976). One theory proposes that cochlear damage to moderately intense noise is the result of what Spoendlin (1976) calls metabolic decompen- sation. Hair cells examined after noise exposure show damage that is consistent with the effects of changes in the metabolic activity of the cells. These changes include distortion or swelling of the cell bodies (Spoendlin, 1970, 1971) fusion of sterocilia (Spoendlin, 1971; Ward and Duvall, 1971) and an increase in the number of lysosomes in the outer hair cells (Beagley, 1965; Ward and Duvall, 1971). A second theory suggests that vascular changes occur as a result of noise exposure re- sulting in an interruption of blood supply, oxygen and nutrients to the cells. Lipscomb and Roettger (1973) found evidence of constriction and decrease in red blood cells in the vessels below the basilar mem- brane following noise exposure. Spoendlin (1971) discovered swelling of the afferent nerve fibers following noise exposure. This finding is similar to that seen after an interruption of the blood supply to the cochlea. A third theory, the Ionic Theory, as proposed by Bohne (1972, 19 76) suggests that damage to the structure of the organ of Corti is caused by the communication of peri lymph- like fluid of the organ of Corti (low potassium, high sodium ion content) with endolymph of the seal a media (high potassium, low sodium ion content) through the nor- mally impermeable reticular lamina. Bohne theorized that noise expo- sure somehow increases the permeability of the reticular lamina re- sulting in communication of the two fluids. To test her hypothesis, Bohne exposed chinchillas to a one hour exposure of 108 dB SPL octave band noise centered at 4 kHz and sacrificed the animals at different postexposure intervals to determine the course of injury over time. She discovered that while there was little evidence of damage shortly after the exposure « one hour) at one month postexposure there was total degeneration of a 1 mm segment of the organ of Corti approxi- mately 4 mm from the base. Bohne discovered small holes, the size of missing hair cells, in the reticular lamina two hours postexposure which were large enough to allow leakage of the endolymph into the organ of Corti fluid spaces. These holes later healed resulting in scars formed by the enlarged phalangeal processes. Some indirect support for Bohne 's theory is provided by Goldstein and Mizukoshi (1967) who suspended outer hair cells in artificial endolymph and perilymph and found swelling in those cells placed in the endolymph whereas those cells placed in the perilymph retained their shape. Stronger support is provided by Duvall, Sutherland and Rhodes (1969) who nicked the endolymphatic surface of the organ of Corti and dis- covered increasing damage to the ulstrastructure of the organ of Corti as the time increased following the procedure. A second unresolved issue in noise- induced hearing loss is the possibility that susceptibility to noise-induced trauma varies among organisms within the same species. There is, as yet, no way to de- termine susceptible Individuals within a population. As a result, those susceptibility studies which have been particularly valuable have dealt not so much with individual as with group susceptibility. The target for several of these investigations has been the very young. Falk, Cook, Haseman and Sanders (1974) exposed two-day, eight- day and eight-month old guinea pigs to 50 hours of white noise at 119- 20 dB SPL and found that the younger animals suffered significantly greater pathology as measured by hair cell loss. Price (1976) found a greater loss of cochlear microphonic (CM) as measured at the round window among kittens than among adult cats after an exposure to an intense 5 kHz pure tone for 50 minutes. The correlation between histological results and the CM measurements were somewhat inconsistent but the overall results suggested a positive relation between the two measures. Bock and Saunders (1977) proposed a critical period theory to explain the results of a study in which hamsters, 27-55 days of age, appeared to exhibit an increased susceptibility to noise trauma as mea- sured by CM sensitivity. The authors suggested that there may be de- velopmental changes in the young cochlea which increase the suscepti- bility to noise trauma. Danta and Caiazzo (1977) exposed newborn and adult guinea pigs to a 115 dB SPL narrow band of noise centered at 4 kHz for one hour and discovered an increased susceptibility to tem- porary threshold shift (TTS) among the newborns as measured behaviorally , Dodson, Bannister and Douek (1978) in attempting to simulate incubator conditions in a newborn nursery, exposed one week old guinea pigs to white noise at 76 dB SPL for seven days. They found appreciable loss of outer hair cells in these animals as compared with a control group. These results suggest that young organisms may be more sus- ceptible to noise-induced hearing loss than adult specimens. Of particular significance is the case of human infants remaining for weeks in incubators. If infants show similar susceptibility, these individuals may be undergoing noise-induced damage to cochlear hair cells . The basis for noise-induced permanent hearing loss is structural damage to the fine detail of the organ of Corti. Only in recent years, with the use of electron microscopy, has the fineness of de- tail required to effectively study noise-induced damage become suffi- ciently documented. It is hair cell damage that must be assessed in noise trauma and the central question is the relation between hair cell damage and the parameters of the traumatic stimulus. However, hair cell damage can only be assessed post-mortem. If one could de- termine the relation between an ante-mortem assay of hair cell loss, an estimate of the state of hair cell integrity might then be possible For this reason, there has been much interest in studying response activity in preparations subjected to stimuli which were chosen to in- duce acoustic trauma. One might expect, for example, that behavioral threshold would be a good correlate for hair cell damage if we assume that behavioral thresholds accurately reflect the integrity of coch- lear structures. However, Eldredge, Mills and Bohne (1973) showed that in the chinchilla there was a poor correlation between threshold sensitivity and hair cell loss. 7 The CM discovered by Wever and Bray (1930) and defined by Saul and Davis (1932) has been used frequently as an index for noise-in- duced cochlear damage. The CM is a stimulus- related cochlear poten- tial with waveform which duplicates the displacement-time pattern of the cochlear partition (Hallos, 1973). The input-output function of the CM is characterized by three segments. The first segment is a linear one in which output voltage is directly proportional to the stimulus intensity. CM. output has been measured as low as .005 mv (Wever, 1966) . The second segment is characterized by a departure from linearity toward the maximum output of the CM. 'Hie third seg- ment consists of a roll-over effect in which increases in stimulus intensity result in decreases in CM output. The CM has traditionally been measured by two techniques: a single electrode placed on or near the round window (RW) , and the differential recording technique (Tasaki and Fernandez, 1952) which utilizes two active electrodes placed in the same cochlear turn. The RW technique has the advantage of relative ease of recording but it has a serious drawback in that, in the case of low frequency stimulation, it cannot be determined whether the electrode is picking up proximally or distally generated CM. Another problem with the RW technique is that the electrode picks up the whole-nerve action potential (AP) with the CM. Under many stimulus conditions it is difficult to visually separate the two re- sponses which originate from different anatomical sources. The differential electrode technique, on the other hand, permits the study of locally generated CM and the separation of whole-nerve AP and CM components but it requires a more surgically invasive technique than RW recording. Unfortunately, RW recordings tend to yield little difference in the reduction of CM sensitivity as a function of test frequency (Simmons and Beatty, 1962; Price, 1968). In noise exposure experiments (Smith and Wever, 1949; Price, 1968) the frequency at which maximum depression of the CM occurred tended to be below the exposure frequency. Durrant (1976) concludes that, "... the loss of CM sen- sitivity and maximum output appear to be rather limited indicators of the detailed mechanisms involved with acoustic trauma, based on cur- rently available data" (p. 192). In contrast to the CM, the neural responses may be a more sensi- tive index of the degree of and perhaps the location of noise trauma (Davis and Associates, 1953; Benitez, Eldredge and Templer, 1972; Mitchell, Brummett and Vernon, 1977; Pugh, Horowitz and Anderson, 1974). The use of the eighth nerve action potential to gain frequency specific information was earlier felt to be limited since the prevail- ing thought was that the response reflected only synchronous activity in the basal portion of the cochlea. Analysis of the whole-nerve re- sponse with analytic procedures shows that some resolution of frequency representation can be obtained. The use of tone-pips (Davis, Fernandez and McAuliffe, 1950), filtered clicks (Aran, 1971; Zerlin and Naunton, 1975), click-pips (Coats, 1976) and selected masking of broadband click evoked AP (Teas, Eldredge and Davis, 1962; Eldredge, Mills and Bohne, 1973) have enabled investigators to obtain substantial frequency speci- fic information from the whole-nerve response. The whole-nerve AP appears to correlate well with anatomical data. Eldredge and his associates (1973) concluded that, "The close rank oreder correlation between loss of whole-nerve AP . . . and the loss of hair cells is very gratifying in terns of a quest for reliable physiological indices of injury. The Nj peak voltages as a function of input sound pressure may be a more sensitive index for loss of hair cells than any measure of threshold simply because this function examines responses over a wider dynamic range" (p. 791. The whole-ricrvc response in man is recorded most clearly with a t ranstympan i c electrode resting on the promontory. The response can also be recorded with a wick electrode in the external auditory mea- tus, but with some loss of clarity. The whole-nerve response is also included in the Brain Stem Response (BSR) as wave I. Jewett and his associates (Jewett, Romano and Williston, 1970; Jewett, 1970; Jewett and Williston, 1971], recording from the vertex of the scalp, described a series of waves consisting of seven peaks which appeared in the first nine msec after the onset of a click stimulus. Their evidence strongly suggested that wave I was generated by the eighth nerve while waves II through VII represented auditory evoked brain stem activity. The waves have since been referred to as the Brain Stem Response. In an attempt to correlate the individual waves with specific brain stem structures. Lev and Sohmer (1972) recorded intracranially in cats and concluded that waves I through V represented activity of the cochlear nerve, cochlear nucleus, superior olivary complex, and the inferior colliculus (waves IV and V) respectively. Buchwald and Huang (1975) dissected various auditory brain stem structures in the cat and came to similar conclusions regarding the generator sites of the BSR with the exception that wave IV appeared to represent activity in the ventral nucleus of the lateral lemniscus. In a similar inves- tigation, Henry (1979) concluded that the sources of the BSR in the 10 mouse closely resemble those in the cat. Wave V is of particular interest as it is the most prominent peak when recorded from the human scalp. Recent investigations have attempted to demonstrate relationships between wave 1 and wave V (Elberling, 1976; Klein and Teas, 1978). Hlbcrljng has shown that, regardless of the intensity, the latency of wave V varies as a con- stant (4.2 - 4.4 msec) when compared to the latency of wave I for a 2 kHz acoustic transient. Elberling suggests that this relationship is indicative of the close approximation of frequency specificity between the two waves. The significance of this relationship is two- fold. First, the clinician's task for measuring responses close to threshold is made easier with the larger wave V and secondly, infor- mation may be obtained which can add to our knowledge concerning the relationship between peripheral and central auditory processing. The use of BSR is rapidly becoming a valuable objective method for differentially diagnosing auditory pathway disorders (Sohmer, Feinmesser and Szabo, 1973; Sohmer, Feinmesser, Bauberger-Te 1 1 , Lex and David, 1972; Berry, 1976; Davis, 1976; Davis and Hirsh, 1977). At present, however, few studies have investigated the effects of intense noise on these brain stem neural potentials. In one study, Sohmer and Pratt (1975) studied the effects on the BSR in human subjects produced by noise exposure. The noise produced a temporary threshold shift (TTS) . The BSR to a click showed a greater latency and amplitude change for the earliest negative deflection (Nj) than the later waves which showed little change in the TTS condition. The authors suggested that this finding indicated that TTS is a peri- pheral, electrophysiologic event. 11 As humans cannot be used in permanent threshold shift (PTS] studies, the guinea pig has long been considered one of the animals of choice for several reasons: low cost, ease of procurement, surgi- cal accessibility of the auditory structures, and as Davis and Associates wrote in 1953, "In any extrapolation of . . . data to the problem of acoustic trauma in man we can probably assume with some confidence that the organ of Corti in man has about the same mechanical strength that it has in the guinea pig. The size and structure are very similar, particularly if we compare the corres- ponding regions that are most sensitive to the same frequency. For given amplitudes of movement of the footplate of the stapes we should expect rather similar injurious effects on the hair cells and similar probabilities of mechanical failure of supporting structures" (p. 1188) The use of the guinea pig also has produced considerable data of in- trinsic interest quite apart from their direct applicability to man. That is, the principles of function are also of interest. The purpose of this study was to contrast the effects of noise which produces PTS on the BSR's of newborn and adult guinea pigs. Specific data are presented on (1) the effect of age on the suscep- tibility to noise-induced hearing loss as measured by the BSR, (2) on the effects of PTS on the amplitude and latency of the early and late waves of the BSR and the relations among these waves, (3) on the correlation between noise-induced changes in the early and late BSR and hair cell loss, and (4) on the suitability of filtered clicks as a frequency-specific stimulus. chapter it methods and procedures An i ma 1 s Four groups of guinea pigs, 16 animals, were used. Five animals were in the adult experimental group (305-957 gms) and five animals were in the newborn experimental group (96-112 gms) . Three animals were in the adult control group (407-508 gms) and three animals were in the newborn control group (107-132 gms). In this study an animal under 72 hours of age was considered newborn. The adult animals were procured, fed, and managed by the Animal Resources Department of the University of Florida and caged in a laboratory. The newborns were born in this room and stayed with their mothers. The ambient noise level in the room was approximately 48 dBA and consisted mostly of animal vocalizations and movement within the cages. No frequency analysis of the ambient noise was done. Normal auditory acuity was determined by the presence of Pryor's Reflex (this reflex was often weak and/or missing in the newborns), normal otoscopic examination and acceptable BSR (first negative wave) threshold to broad band clicks. Pilot studies had determined this threshold to be approximately 30 dB SPL + 5 dB. Surgical Procedure The animal was anesthetized with Nembutal (0.5 cc/kg of body weight). A supplemental dose (1/3 of initial dose) was given after two hours if the animal showed signs of discomfort or excessive 12 13 movement. Temperature and heart rate were monitored continuously. Temperature was maintained between 36-38° C. In order to eliminate the contribution of the contralateral ear to the BSR the test ear (Taniguchi, Murata and Minami, 1976; Teas and Nielsen, 1975) the left cochlea was destroyed by rupturing the round and oval windows transtympanical ly and draining the cochlear fluids. Under clean, non-sterile conditions, a 2-3 cm anterior to pos- terior incision was made along the midline of the scalp. After the tissue was retracted to expose the skull, one hole was drilled at the parietal crest and another was drilled approximately 3 mm superior to the external auditory meatus (EAM) . In the adults, mounting screws (0.80" x 1/16") were screwed into these holes and platinum wires were wrapped about each screw (.008" diameter + .003" diameter teflon coated except for 2 mm of its tip). In the newborn, the tips of the electrodes were inserted directly into the drilled holes since the newborn skull was too thin to accept the screws. With the wires attached to a small socket, the implant was fixed onto the skull with cranioplast ic cement. Figure 1 illustrates the location of the electrodes in the skull. Soon after implantation was completed the animals were returned to their cages (the newborns to their mothers) and allowed to recover approximately 48 hours be- fore BSR measurements were made. One postsurgical complication found during pilot studies was that vestibular disturbances due to the contralateral labyrinthectomy could be so severe that feeding is inhibited. This occurred in one newborn animal which eventually died despite attempts at supplemental o (1) tn H 5" U (/) tr, rt O H *j Ph fc: 3 O 60 c « c CD !/) (H 1 — 1 rt CD > cd +J a; X M (H o ■'/ 3 '-; 00 CD CD U 15 16 feeding. It was found that if damage to the non-test cochlea was kept to a minimum, the animals would quickly recover from vestibular prob- lems (nystagmus and loss of balance). Another complication was the loosening and eventual loss of the implant plug in the newborn animals as a result of a rapid skull growth during the one month postexposure period. This problem was solved by periodically adding a small amount of cranioplastic to the perimeter of the implant during the one month period . Signal Generation A simplified block diagram illustrating the equipment used in this study is shown in Fig. 2. A Grass stimulator produced the square wave pulse which had a repetition rate of 10 pulses/sec and a duration of 120 msec. After being delivered to a buffer, attenuator and mixer, the signal was amplified (Mcintosh, MC 40) attenuated and fed into an IAC single wall, sound-treated room enclosed by a concrete block wall. 'Hie filtered clicks were generated by activating a band-pass filter (Krohn-IIite, 3100) with the pulse. The filter cutoffs were adjusted to produce the desired waveform of approximately four cycles in length with its plateau being reached within the second cycle. The filter imposed a frequency dependent delay between the synchronizing pulse and the signal at the transducer which ranged from .18 msec for the filtered click (FC) centered at 8 kHz to .89 msec for the 500 Hz centered FC. All latency measures reported in this study are corrected for this filter delay. The filtered clicks drove a Bruel and Kjaer (BSK) 1/2 in condensor microphone (type 4133) coupled to the opening of the guinea pig's CAM by a 1 mm diameter speculum. The spectra of 19 the filtered clicks (Fig. 3) were found by measuring the acoustic output from a speculum with a 1/8" condensor microphone (B§K 4138) connected to a sound-level meter (Bf,K 2209) with 1/3 octave filter (Bf,K 1616). The output of the meter was fed to a graphic level re- corder (BT,K 2306) which plotted the data semi -automat ically . The spectra were corrected for ear canal effects as measured previously in this laboratory (Teas and Nielsen, 1975) and for the difference in repetition rates used for calibration (90/sec) vs. the rate used during the experiment (10/sec) . The spectrum SPL at the ear drum at 0 dB attenuation was determined by subtracting 10 X Log of the band- width (bandwidth being determined at 6 dB down from the peak) from the averaged SPL values of those data points within 6 dB of the peak. Noise Stimulus Generation The stimulus for the narrow band noise exposure was generated by a Grason-Stadler 455C Noise Generator. After being filtered (Krohn- Hite, 3100) and amplified (Mcintosh, 4C40) the filtered noise was de- livered to an Altec, 802 D acoustic transducer whose cone was placed 2 cm from the EAM of the animal. The signal was calibrated before and after each exposure by a Ballantine, 320A True RMS meter and by a Bf,K Precision Sound Level Meter (model 2209) with a 1/2 in conden- sor microphone (type 4135) placed at oo incidence to the center of the cone's opening. The spectrum of this noise is shown in Fig. 4. Recording The two leads from the skull implant were connected to a Grass, model 11IP511B high impedance cathode follower connected to a Grass PS Series AC Preamplifier (gain set at 10k, band passed from 30 llz to rt h p, • T3 > H E , £ O O I/) ^ O r-t u: o +-> cd us- C03 BIT] DO C ,C ■C ft ifl " 03 u) cd C C i/> cd u. o u x 3 !h cr o ^r h E -a e c rt .-J ,q a. ) > f O OJ • •H -M (H ■M O) O 0) d 1/] O 'U n CD •— < 0) r-t X 0) 10 O M H 10 U O O (U .H c rt £ ,c M T> +-> O t/1 (D ,£3 10 •Hi ftBl ^ H 0 ^ 0 v) O • -H >H nj 0) tfl ifl T3 X fn CO ■P •** tu 3 +-> Mh u; h •H O UJ -H [x, 4n cq 4-1 23 (IdS 9P) A1ISN31NI 24 3 kHz) and Krohn-Hite model 3100 filter (also hand passed from 30 Hz to 3 kHz). The head holder served as ground. The filtered response was delivered to the A/D converter (40 psec bin width) of a Computer Automation mini-computer which performed the averaging of the BSR activity. The single responses were monitored on the Tektronix 565 oscilloscope to note changes in the waveform which would indicate equipment malfunction or excessive movement. Procedure The animal was placed in a head holder immediately after the anesthetic had taken effect and the implant was connected to the cathode follower. After the speculum had been placed into the open- ing of the EAM, intensity functions were performed with broad-band, 0.5, 1, 2, 4 and 8 kHz filtered clicks in that order. As the signal to noise ratio was poorest at the lowest frequencies, the averages tended to be satisfactory at these low frequencies while the animal was most heavily anesthetized. Intensity of the clicks was varied from 0 dB of attenuation to threshold, in 6 dB steps, for each stimulus. At 0 dB attenuation the calculated SPL was 89 dB at 8 kHz, 86 dB at 6 kHz, 75 dB at 4 kHz, 70 dB at 2 kHz, 77 dB at 1 kHz and 82 dB at 500 Hz. The attenuators were linear over the range utilized 'Hie number of responses averaged for each stimulus varied depending upon the intensity and CF of the stimulus. This number ranged from 32 responses for the higher frequency FCs at 0 dB of attenuation to 2038 responses at threshold. Threshold was determined as the point between the level at which the response was just visually detectable in the average and the level at which it was no longer visually 25 detectable for 2048 repetitions. The responses were recorded on data cassettes for later processing. After completion of the baseline measurements, the experimental groups were exposed to a narrow band of noise centered at 4 kHz at 108 dli SPI, for two hours. Following the four-week interval, input- output functions were again performed, the procedure for which was identical to that described for the baseline measurements. A four- week interval was selected as there is no indication of additional anatomical damage beyond this time in the chinchilla (Bohne, 1976) and pilot work did not indicate additional damage as measured elec- trophysiological ly in the guinea pig. Histology Immediately following the one month postexposure measures, the cochleae were processed for analysis by surface preparation (Engstrom, Ades and Anderson, 1966; Smith and Vernon, 1976). The animals were administered an overdose of Nembutal IP. As soon as respiration ceased, the animals were decapitated and the temporal bones removed and placed in vials of formalin fixative. Under a dissecting micro- scope at 12X magnification, the bulla was opened to expose the cochlea. A small hole was made in the apex and the cochlear windows were care- fully ruptured to allow gentle perfursion of fixative throughout the cochlea by means of a Pasteur pipette. The perfused cochlea remained in fixative for at least 48 hours. After two 10 minute rinses in phosphate-buffered solution, the cochlea were gently perfused several times with 1 percent 0S04 for 2 hours. After removal of the 0S04, the cochlea were rinsed twice with 35 percent alcohol, twice with 50 26 percent alcohol (at 5 minutes per rinse) and left to stand, and re- frigerated in 70 percent alcohol until dissected. Dissection was performed under a stereomicroscope . Beginning at the apex and continuing through the upper three turns, the hone portion of the cochlea was removed, followed by the stria vascularis and tectorial membrane. The organ of Corti was separated from the osseous spiral lamina with a capsule knife in 1/2 to full turns and placed in a vial of glycerol. The section was then placed In a drop of glycerol centered on a glass slide, covered with a glass cover slip, and sealed with permount. Due to the tenacity with which the organ of Corti adheres to the spiral ligament in the basal turn, the capsule knife was used to separate the organ of Corti from both the ligament and lamina. The tissue was carefully lifted out from the base in 1/4 turn sections for mounting. The specimens were viewed under phase-contrast microscopy at 1000X magnification. Missing hair cells were counted for each turn. CIIAPTI-R I [ I RESULTS The results of this study are presented in preexposure and post- exposure sections. Within each section the threshold measures and the effects of frequency and intensity upon response amplitude and latency will be shown. Histological results will be presented in a final section. Except for the threshold measures, where data are presented for all filtered clicks, the results will focus upon the responses to the 1 kHz and 6 kHz PC's. Response amplitude and latency contrasted sharply as a function of click intensity and in the effects of the noise exposure for these two signals. Responses to the 4 and 8 kHz FC"s were often similar to those of the 6 kHz signal. The 500 Hz FC often elicited a small dynamic range and the 2 kHz FC contained strong resonant energy at 4 kHz (Fig- 3) • Preexposure Response Waveform The BSR waveform is shown in Figure 5 and consists of four posi- tive and four negative peaks. The parietal lead was positive with reference to the EAM. For the purpose of this study the first nega- tive peak (Ni) and the fourth positive peak (P4) were analyzed as they were the earliest and latest waves that varied systematically and consistently in amplitude and latency with changes in stimulus intensity. A cursor was used to measure peak amplitude and latency Figure 5. Representative BSR waveforms produced by FC's of different center frequencies. A, 500 Hz; B, 1000 Hz; C, 2000 Hz; D, 4000 Hz; E, 6000 Hz; F, 8000 Hz; G, broad-band. All waveforms are shown at 0 dB of attenuation. The number of average responses varied from 32 for waveforms C thru G to 128 for waveform A. 29 30 values. Amplitude of Nj was measured from baseline to its most negative value and the amplitude of P4 was measured from the most negative value of N3 to the most positive value of Pa. Latency was measured from the onset of the signal at the transducer to the N] and P4 peaks at their- most negative and positive amplitude values respect i vely . In one pilot study electrode locations were varied. As the more caudal electrode was moved away from the external canal to more medial positions, the amplitudes of the later BSR waves increased. For this study the electrode configuration was chosen (Fig. 1) in order to emphasize the amplitudes of the earliest BSR waves. The waveforms in Figure 5 show that the amplitudes of the N] , No complex are larger than the later deflections. However, the latencies could be read easily with the cursor at all locations in the BSR waveform. Threshold Figure 6 illustrates the mean Nj and P4 preexposure threshold values and + 1 Standard Deviation (SD) for each FC . The solid line in each pair of curves represents the averaged responses of 5 experi- mental plus three control animals. 'Hie broken line represents the averaged responses of the three control animals measured 1 month following exposure of experimental animals. Tims, the threshold measurements over time are stable and the control group measures appear to be valid estimates of preexposure sensitivity. In all cases thres- holds measured 1 month following exposure were within 1 SD of the pre- exposure threshold values. However, the P4 thresholds of the newborn control animals measured 1 month after birth showed a decrease in M O H ^4 C x: o o o fn ,ld O PB. C C !s oj < s Ph <4H - — i o m o • H T3 o rH X! 4-> C 03 i/l o c! l/l +J o C -J C H 3 i/> 1— 1 cj XI m rH o ;■-■ 4-1 03 X ■H • 03 S w u O C ■H 4) o cd W 3 rH Ph c OJ o3 nj xj X o e 4-> CJ &H 10 /— > t/1 a> -O rH a/ a a X rH O XJ xj n 4J O rH H +j X +-> M B in c 4-1 3 O CL> O O ■rH Fh rH U l/l ^ 4h r; o CJ o 4-> try e rH rH a ■ H 3 1 — 1 en TTJ l/l o C~ C O 4-1 c3 -3 a, 13 ■H X o i: C — I rH D X! O 03 03 o +-> X! 4-> ai M V) HC e o >^ 0) I/) rH l/> Oh +-> o 3 O X c 4h rH CL, (1) 2 3 o t— ( rH h n3 03 Sh -3 • <; 0 r— I r— I «. rC o3 03 <0 C 4-> 6 CJ C rH tO rH O 'H C •« •H O U C -H H ix, x: *-> oJ rH is; Q 00 q5- T3 (\i I .a: — >- O 2 in Ld 3 O UJ CC u_ q: en Ld h- vT> z *r UJ o (M ^ o o (IdS 8P) anOHS3dHl 33 threshold for frequencies of 1 kHz and above. The average decrease in thresholds to the 5 filtered clicks (from 1 kHz to broad band) was 3.1 dB. There was no change in threshold for the 0.5 kHz FC. No differences occurred in threshold for P, in the adult control group and only a slight difference occurred in the Ni responses in the new- born control group. All four sets of curves showed maximum sensi- tivity to the 4 kHz FC, and there was a tendency for the thresholds to higher PC's to be slightly higher. 'Hie most extreme difference in the visual detection level among the frequencies of the PC's occurred in the adult group for P4 which showed a difference of 6 dB between 4 and 6 kHz. With the exception of the P4 response from the newborn group, the absolute threshold levels for N, and P4 were very similar among the newborn and adult animals. The maximum threshold values were approximately 67-69 dB SPL at 500 Hz and de- creased to a minimum of 28-29 dB SPL at 4 kHz. The mean P4 threshold values of the 8 newborn animals for PC's of 1, 2 and 4 kHz were 3 to 4 dB higher than the adults. The thresholds of the 3 newborn control animals measured one month later, however, were consistent with all other threshold measurements. Effects of Frequency and Intensity on N] and P4 Response Amplitudes Figure 7 illustrates the preexposure response amplitude vs. signal intensity for the Nj (Pig. 7A) and P4 (Fig. 7B) peaks produced by the 1 kHz and 6 kHz PC's averaged over the 8 adult animals. Figure 7A shows that Nj rises very steeply for the first 6 dB above threshold and then more gradually with increases in intensity of the 6 kHz FC. The Nj response to the 1 kHz FC requires greater intensity for Figure 7. Average preexposure response amplitude in the adult group (N=8) as a function of click intensity. Panel A illustrates the N\ amp- litude function; Panel B, the P4 amplitude function. The solid circles represent response to the 6 kHz FC; the cross marks, responses to the 1 kHz FC. See Figure 9 for variance data. 35 02- PREEXPOSURE 9-r- 30 50 70 90 30 50 CLICK INTENSITY (dB SPL) 70 90 36 detection and is lower in amplitude at all intensity levels above threshold. At suprathreshold intensities response magnitude in- creases at a slower rate for the 1 kHz FC than for 6 kHz. The in- tensity functions for P_, contrast with those for N, . The threshold for the P4 responses are the same as for Nj . Except near threshold, the amplitudes of the l\| responses are lower than the Ni responses at respective intensities, probably because of the electrode con- figuration used. The slope of the function of P4 , 6 kHz is less than that for the 1 kHz FC, but its maximum response is greater. The intensity functions for the newborn group are shown in Figure 8. The functions for Nj compare well with those for the adults. However, those for P4 differ in some respects. The re- sponse magnitudes and the slopes of the intensity functions are lower. Unlike the adult responses there is little difference in slope between the 1 and 6 kHz PC's for the P4 responses. Figure 9 shows the coefficient of variation (CV) for the pre- exposure response amplitude as a function of click intensity for the 1 kHz and 6 kHz FC's. The CV's for the adult animals (Fig. 9A) in response to 6 kHz are nearly 1.0 near threshold and decreases to about 0.5 by 18 dB above threshold and remains at about that level throughout the intensity range. For 1 kHz, the threshold is higher and the CV varies around the 0.5 value throughout the intensity function. The CV's for Nj and P4 responses are similar in the adult. The CV's for newborns are shown in Figure 9B. The wide variation in CV for the responses from newborns contrasts with the consistency shown by the adults responses. In a broad sense the CV's for the two responses, Nj and P4 , to each stimulus resemble each other. Figure 8. Average preexposure response amplitude in the newborn group (N=SJ as a function of click intensity. The legend is the same as that for Figure 7. 38 NTENSITY (dB SPL) c +-> 3 o bo 3 • H 13 ■10 cr o m Id CD h- _J D < CL >- h- z UJ o o — - o z UJ < A Nl P4 24 5 4 22 5.2- 2.0 5.0 1.8 4.8i 1.6 4.6 1.4 4.4 L2 4.2" U0 4.0- 0 N, P4 2.4 5.4 2.2 5.21 20 5i0- I.8 4.8- I.6 4.6 I. 4 4.4 I.2 4.2 I.O 4.0 kHz •ft— i r kH; B ADULT 6 kHz D NEWBORN 6 kHz PREEXPOSURE 7^-r- 30 50 70 90 30 50 CLICK INTENSITY (dB SPL) 90 •H 00 r-t -a c O O -H to to n tu j id (1 n) o u o M — • C C • a> i— i a> o u u (j a 'h m Hi CD a) a) 4-i U-i G x; .c -h -h nJ P ♦-> X) t) rt to •> (D CD &,^< xx x h h p p +-> -H 2 CD »1 (J O 4J 10 to U X 0) fl O Hi U) h u a, +-> 10 O 10 Ui IS) u (Ll 3 a. nj CD S 0) to J3 Z .O XI C 4J +J O ••> CD •h a< <* c 5 to a, a io 13 to C h r-H « C C .£ CD +-> Cd rH rt c c — i O ?h -H rt 3 10 -D -a a, -a +-1 S CD to < .-1 CD (-( cd a c 3 . .., -a cti .-I c_) r-i cti -a cd U* Z c > CD Cd Jh n « x 3 XX C +J to 4-1 •H C O e c-i 11. «)rt XI •H 3 +J i: -a 'H (It 1) H) to a O —1 .C Z CD ■M • H CD (0 X) .« O ,C •H 00 4-> 2 CD u rt +-> c £ ^ CD •> cd -H CD CD 10 +-> t— t +-> > C .-( to cd cd o 3 CD CD -< CL.T3 to to cd to < c c U Ph a. C to to O 4-> C 10 CD CD Ph C -H c U U to CD O a> to to -h ih rr Ci CD CD +-> Z o. »h 3 u a,r-i p; 0) CD • cd cd 3 X x M U > 4-i +-> +-> .— I to X X c c cd a) o o CD CD h — i C C CD CD 3 O 0) 0) S ^ M h tJ |J t-> 4-» ■H >H id (J CD CD LL, O r-H ,— ( _Q _Q NEWBORN LATENCY- ADULT LATENCY (MSEC) — to oj CD "* O O O ■ * „ M - (03SWJ ADN31V1 46 92 dB are slightly shorter in the adults than in newborns. On the right side of Figure 11, in panel E, the differences between the N, responses in adults and newborns for each signal are shown directly. All the infant latencies are longer than the adult but the difference; for the 1 kHz I'C below 66 dB are the smallest, i.e., the infant re- sponses are most similar to the adult. Above 66 dl?, the differences become larger abruptly, rising to 1.7 msec at 72 dB. For the 6 kHz FC, the infant responses gradually approach the adult values, i.e., the differences become less with increases in intensity. For P/j the latency functions for the two signals cross at about 64 dB. For signals greater than 64 dB, the responses to 1 kHz FC showed a shorter latency than those to 6 kHz, while at intensities less than 64 dB, P4 for 1 kHz has a longer latency than for 6 kHz. The latency functions also cross for the infant responses, but the functions appear to be truncated, i.e., while the latencies for weak signals are similar to the adult latencies, the infant P* re- sponses do not become as short as in the adults. The right-hand panel F shows the differences in P^ responses between adult and newborn groups. As for Nj , infant responses lag the adult. The sharp increase in P4 delay for infants seen for Ni is paralleled in the data on P4 . However, the difference for 6 kHz is minimal at weak intensities and reaches a fairly constant value at 0.2 msec as intensity increases. Table 1 shows the average SD for the N and P^ latency measure- ment for the adult and newborn groups. The overall variance was similar between the two populations (.227 msec for adults vs. .22 4 7 tabu: i Averaged S.I), for N, and !',-, Latency Measurements (msec) ADULT Nl P4 1kHz .259 .238 6kHz .143 . 266 NEWBORN Nl P4 210 .294 120 .256 '18 msec for newborns). Within populations, Nj latency displayed less variance than P4 in both groups, the difference being larger among the newborns. Response variance to the 6 kHz FC was less than to the 1 kHz FC, for both groups. The preexposure threshold, amplitude and latency data consis- tently illustrated that the Nj response functions in the adult and newborn were generally similar whereas the P4 functions differed between the two groups. Postexposure Effects of Noise on Threshold Figure 12 illustrates the mean Nj (Pig. 12A) and P4 (fig- 12B) threshold shift as a function of center frequency of the PC's for the two groups of guinea pigs. The solid lines represent the responses from experimental animals and the dashed lines represent conti'ol re- sponses measured one month following exposure of the experimental ani- mals. For both control groups, the threshold shifts for Nj and P,j were no more than + 6 dB from their preexposure values. Figure 12A shows that the average threshold shift of N] for the adult experimental group increased with increasing center frequency for the FC to a maxi- mum of 15 dB for the 8 kHz FC . The N threshold for the newborn group also decreased with increasing frequency and exceeded the threshold shift in adults. The threshold shift for the newborns showed a maximum shift of 31 dB at 6 kHz. The shift in threshold for P4 , shown in Figure 12B, shows the same trend of increasing shift with increasing center frequency of the PC. The P4 threshold for newborns reached a maximum shift of 30 dB for the 6 kHz PC and the P4 threshold for adults o ••> a a. a, o x d u tO -M cfl CD COX x: p i --h o Id o> XI C C 1 OJ 0 ci> (/) p rH o *J J3 u, c crt X o H X P rt CD U -H ■ H U <+_, c o P c o cd ♦J cd 3 XI ■H C cr-H P o 4-> u-i c/) O rt p ■H p M • > Tf+J a,x) c a. i—i 3 cd "I o -a o c TJ u u •H CO bo rt u X! (-U +J •-i C 4H o O rt <4h .H P fn +-> O X *J +J 10 CO c C c o O CD T O +-> CD Pi U (!) X CU O 3 ca +-> X) 4h u •H U-i p P( CO U •H H O 4-> u. rt 111 U Ih 3 rt a. a> ii i-h m ■M "4-1 ■* o rt XI ch vO ■ H ii! C C) -S P-H rt to X CO c D +-» ♦J ■H o tO <+H ■a p PVH P +J M 3 x: 0 CJ '+J O oo 4h CO •H X e U box) X) o 1/1 o C p X l-H O rt X o s rt x ♦J X CD ■M CO XI 4-> c C CD CD p '^. o 10 x> E X p to o a H +-> o P rt O CD CD CD P ft, ft£ U > ■p' x x +-> a. IN -H P +J CO < e u rt CD OCT) P r-H CO <+H E C 3 CD CO Cd t0 C O LO CD •H Ol p II X •> U-, cx o z +j rr 50 - CD - CO 00 CD - * >- o -z. UJ z> o UJ cr u. cr UJ h- -z. UJ o u u_ ddsap) ijihs cnoHsaum 51 TABLE 2 Standard deviation for averaged postexposure threshold shift (dB SPL) l-'C 0.5 kH N]^ exper CN=5) Adult +15 Newborn 13.8 Adult 6 Newborn 7 .3 Adult 6 Newborn 8 7 Adult 5 8 Newborn 6. 1 Adult 7. 4 Newborn 10. 3 Adult 11.2 Newborn 15.6 (N=3) +6.9 0 0 3 .4 3 .4 0 0 6 9 3 4 6 6 3. 4 ,■] exper. CN=5) Pa cont (N=3J +8 +3.4 14 6.9 4.9 0 6.8 6 9.5 3.4 8.8 3.4 8.6 9.1 8.8 6 5.5 6 8.1 6.9 11.8 3.4 15.5 3.4 reached a maximum shift of 19 dB at 4 kHz. Except for the 6 kHz FC, the difference between newborn and adult threshold shift was less for P4 than for N. . Fffcets of Noise 1-xposurc on Response Amplitude and Latency Response ampl it tide. Figures 13 and M illustrate the relations between postexposure response amplitude and signal intensity for the Nj and P4 peaks produced by the 1 kHz and 6 kHz PC's averaged over the five adult (Fig. 13) and five newborn (Pig. 14) experimental animals. The preexposure equivalent measures are shown in Figures 7 and 8. As in the preexposure amplitude functions, response ampli- tude was a continuously increasing function of stimulus intensity. The postexposure curves of Nj for 6 kHz show a greater decrease in response amplitude from preexposure values than the curves for 1 kHz. Both intensity functions show a shift to the right, i.e., requiring greater intensities to reach a given amplitude, but the greatest change occurs for the 6 kHz curve. The response threshold to the 6 kHz FC is elevated and a larger response was required for detection. There is less postexposure change in the Nj response amplitudes for the 1 kHz FC. The curves for P4 (adults) show relations similar to those described for Nj_ . For the newborns, however (Fig. 14), the postexposure curve for Nj to the 6 kHz FC is shifted so far to the right that it coincides with the intensity function for 1 kHz, corresponding to the maximum threshold shift seen in Figure 12 (about 30 dB) . Only very strong signals evoked P4 responses in the post- exposed newborn. Figure 13. Average postexposure response amplitude in the adult group (N=5) as a function of click intensity. See Figure 7 for comparison with preexposure functions and legend and Table 3 for variance data. 54 2- 6- UJ Q D 4 h- _l Q. 2 < .2 LU 00 z: 1 o o_ .08 oo UJ 06 or .04 .02 A ADULT 7^ N, 6 KHz B P4 * * i khz POSTEXPOSURE #- 30 50 70 90 30 50 CLICK INTENSITY (dB SPL) 70 90 Figure 14. Average postexposure response amplitude in the newborn group 0=5) function of click intensity. See Figure 8 for comparison with pre- exposure functions and legend and Table 3 for variance data. 56 NEWBORN B > 6- LU C/3 Z O I" 0- .08- LU 06- or PA POSTEXPOSURE 6 KHz I kHz .02- 50 CLICK ¥r- 70 90 30 INTENSITY 50 (dB SPL) 90 TABLE 3 Standard deviation for postexposure response amplitude (VV) dB SIM, 1 kHz (N=5) dB SPL 6 kHz (N=5) T~ __p_ 38 Nl ''4 41 Adult + .07 + .07 Adult + .18 + .06 Newborn - - Newborn - - Adult .07 .07 Adult .34 .23 47 44 Newborn - - Newborn - - Adult .12 .1 Adult .32 .45 53 50 Newborn .23 - Newborn - - Adult .17 .27 Adult .5 .63 59 56 Newborn .23 - Newborn .11 - Adult .23 .09 Adult .54 .5 65 62 Newborn .41 .44 Newborn .2 .12 Adult .25 .2 Adult .5 .68 71 68 Newborn .34 .35 Newborn .52 .05 Adult .34 .16 Adult .39 .69 77 74 Newborn .74 .68 Newborn .37 .1 Adult .84 .43 Adult .19 .66 83 80 Newborn 1.31 1.15 86 92 Newborn Adult Newborn Adult Newborn .58 .37 .82 .57 .71 .25 .58 .16 .4 .27 58 The differences in response amplitude between preexposure and postexposure conditions arc shown in absolute values in Figures 15 A-D and in relative values in 15 F. and F. Relative shift was computed by dividing postexposure response amplitude by preexposure response amplitude, subtracting the result from 1 and multiplying by 100 to obtain a percentage shift. The results shown in Figure 15 in- dicate that the noise exposure produced little change in the responses to the 1 kHz FC but substantial change in response to the 6 kHz FC. 'Hie absolute postexposure shift in the N] response to the 6 kHz FC was greater than that for the 1 kHz FC in both the adult and newborn populations. Similarly, response amplitude of F4 showed a greater shift in response to the 6 kHz FC than to the 1 kHz in both groups but to a lesser amount than Nx . Figures 15 E and F show the post- exposure response amplitudes in relation to the preexposure control responses. 'Hie Nj and P4 response amplitudes to the 6 kHz FC in the newborn group decreased more than did the responses of the adults. Except at low intensities, the Ni and P4 responses decreased about the same amount in the newborn group whereas the N-, response in the adults shifted more, relative to the preexposure control responses than did the P4 response. Response latency. Figure 16 illustrates the postexposure latencies vs. signal intensity for the Nj and P4 peaks produced by the 1 kHz and 6 kHz FC's averaged over the five experimental, adults (Figs. 16 A and B) and five newborns (Figs. 16 C and D) . The ordi- nate for P4 is labeled so that there is a 3.0 msec constant between Ni and P4 . The corresponding preexposure measures are shown in • 3 4-> < ■ H 4-> ■- 'A C rH C O rt ,14 3 O ft-H UTJ'H H ■H < 4-> HI ♦ -> O •• C 1/1 3 4-1 C 4h O O •H N C 4-> I o o ^ •H C U 4-1 c 3 -M 0) •H CO m -a • +-> ca i/i to a, 3 « 3 (J Pli hH • H O C C X! 1/1 o rt c c in J3 • H 0 0 d 4-1 UJ ft 'J (D u 0 -a to C i/i 0 3 C 3 1 — 1 X fj ■r-l 4-1 0 4-> ■H j: c >. i— 1 O 1 1 Ti h ft r-( im ft 0 0 e i — 1 x > 4- a o ■ H 4-1 4-J tO o ft O II fH Cj •V 0 O 2 3 X! 1/1 0 ft VI 4-> Cu u l/l T3 O 1—1 CO O C ft •» o U Kj X X 'm |-H o •H O 1/1 *-> o u U t ft C/1 > 4-J ft 3 o • H •3 c O ft 4-1 • H 0 -a U u r— 1 CJ c CO CJ CD O C3 > ftW ■» r— ( •H 1/1 CJ rt 4J CJ C 4-1 rt u • ■H c -T - — ( 'm 0 01 •s a. O E u cu c 4-1 ■ H fj *» 0 H ~) Rj e ^ r 1 CJ c U 0 X ft ri 4J o Fh ^ X l/l rQ £1 0 V 3 j: ^D 4-1 H o 0 3 f-H z ft *J X! ■ H cd 4J O 0 1/1 a !h 4J H ,0 x. C0<-H O < 2 •H 4-1 X X c Cfl l/l i/1 +J (H 4-1 II in o c 0) 5S i— i < 43 -^t- I- 24 5.4 < -1 22 52- 20 5.0- 1.8 4.8- 1.6 46 14 44-| 1.2 42 10 4.0- c -#- ADULT B D NEWBORN kHz 6 kHz 6 kHz POSTEXPOSURE ~30 50 ' 70 ' 90 30 ' sT) CLICK INTENSITY (dB SPL) 70 90 63 TABLE 4 Standard deviation for averaged postexposure response latency (msec) dB SPI 4J 47 53 59 65 71 77 8 5 Adult Newborn Adult Newborn Adult Newborn Adult Newborn Adult Newborn Adult Newborn Adult Newborn Adult Newborn J Jdlz fN=5) +.136 +.222 .114 .171 .236 .32 . 252 .328 .256 .337 .42 .182 .56 .171 . 222 .32 .494 .253 .291 .256 .141 .433 .199 .4 82 .129 dB SPI, ,437 .47 44 50 56 6 2 74 SO 8d 92 6 kHz N] (N=5) Adult + .27 - Newb o rn - - Adult .121 .357 Newborn .333 - Adult .088 .284 Newborn .382 - Adult .131 .355 Newborn .389 - Adult .07 .298 Newborn .455 .166 Adult .09 .261 Newborn .327 .186 Adult .088 .191 Newborn .285 .25 3 Adult .06 .24 Newborn .27 .439 Adult .067 .288 Newborn .195 .169 Adult .121 . 336 Newborn .192 .499 64 Figure 10. As in the preexposure functions, response latency for all stimulus conditions was a continuously decreasing function of stimulus intensity. There are relatively fewer data points for the newborn population due to the restricted dynamic range caused by the increase in postexposure threshold. The slopes of the postexposure response latencies from adult 1 kHz functions are not as steep as the corresponding preexposure curves (.016 msec/dB postexposure vs. .05 msec/dB preexposure). By contrast, the slopes of adult 6 kHz functions (Fig. 16B) for pre and postexposure are similar, but the postexposure latency values are shorter than the preexposure measures. It is difficult to ob- serve definite trends in the postexposure response latencies for the newborn groups because of the relatively few data points. The post- exposure latency function for NT in response to the 6 kHz FC (Fig. 16D) is very similar but earlier (shifted left) than the preexposure measure. The P4 response latency function shows a large decrease in postexposure latency values throughout the intensity range and re- quires signals greater than 56 dB for detection. The postexposure Nj and 1'4 shift in latency as a function of FC intensity is shown in Figure 17. Figures 17 A and B illustrate latency shifts in the adult and Figures 17 C and D show the shifts in the newborn group. The latencies for the control groups (dashed lines) decreased for both the 1 kHz and 6 kHz FC ' s . The Nj and P. postexposure response latencies to the 6 kHz FC for adults and the N2 responses in the newborns also decreased in latency by a similar amount. The latency of P4 from newborns increased in the responses to 6 kHz. However, the postexposure latencies to the 1 kHz }:C shows C 2 H n o w o -h z: c -a a e 66 o Q ~ ■ t < c a c o x c X X X X — — - f- co Z O LjJ 'OQ |_ .o (33SW) ADN31V1 dX33dd — dX3±S0d 67 an increase in latency with increasing signal intensity for N, and throughout the intensity range for P^ for both adult and newborns. Even though the Nj response to the 1 kHz FC changed little in magni- tude (Tigs. ISA and 15C) , the latency increase for the postexposure measures was consistent. The response latency to the 6 kHz FC also decreased but less than for the 1 kHz FC, even though the amplitude change was large. The P4 functions in the newborn group (Fig. 1 70) differ from the others in that there is a large decrease in Latency for the experimental group in response to the 6 kHz FC as compared with the control group. Also, the response latency to 1 kHz de- creased at all signal levels except at the highest intensity, al- though the function becomes less negative with increasing signal strength . Figure 18 illustrates the P4 - Nj latency differences as a func- tion of click intensity. For the adult responses to 1 kHz (Fig. 18A) the postexposure P4 - N^ differences are larger than the preexposure differences, but for the responses to 6 kHz (Fig. 1SB) there is no change in the P4 - Nj latency differences. For the newborns (Figs. 18C and 18D) postexposure P4 - Nj differences are shorter than pre- exposure values for both stimuli. Although N] latencies for the adult responses to the postexposure 1 kHz FC were slightly longer than the preexposure latencies, a greater increase in latency occurred for P4. No systematic latency effect occurred for responses to the 6 kHz FC. The latencies from newborns showed shorter P, - N. latency differences following exposure to the 4 kHz narrow band noise. The differences were consistent across the full range of intensities that cO XI c o CD *£! t/V D +J Cl 1/) c - o o rH h ft N in o Hh ™ o X o ^ X +-> c O h o *j o ■H to 4h l/l tJ •M o C (J 00 n 11 3 --3 K +-» cw < o u -, o u ft <4H u: (/I •H - cO c (J O n (h o !/> •H c CO fH o 4-1 (1) *-) u O ft I— 1 C c o CO o <-H CO fn e x M-i C 3 -H +-> ■ rl -H C/l fn X X H o ui 1 n o o 3 0 ^ > co ■ H CL. ■H -O ■ ; 4-> ■M -H ft U T> O i-H X 0) ■M W C f{ I/) Ti 1—1 <1> ■-I M Q) cd O y h T! • H c cd 3 X C !h cd O CI] -, 'M ■H Rl rt TJ ,G 74 §yw 7f, Figure 21 illustrates the averaged number of missing hair cells as a function of distance along the cochlear partition for the four categories of preparations. In general, both groups exhibited only minimal hair cell damage (in terms of missing cells). The adults (Fig. 21A) showed damage at the base in all four rows of hair cells with the amount of missing cells decreasing toward the apex. Outer hair cell row 3 exhibited the greatest amount of damage throughout most of the cochlea although the first outer hair cell row was the most damaged at the base. The newborn experimental group (Fig. 21B) exhibited maximum damage at the apex in the third outer hair cell row and minimum hair cell loss in the other turns and toward the base. The control animals showed very little hair cell loss throughout the cochlea. There appears to be little correlation between histological and electrophysiological data. The newborn group showed greater postex- posure threshold shift (Fig. 12) and relative amplitude shift (Fig. 15) than the adult group in response to the 6 kHz FC . The histological data, however, do not demonstrate greater hair cell loss for the new- borns. On the contrary there are fewer missing hair cells toward the basal end of the cochlea for the newborns than than for the adults. Much of the histopathology in the adult group was statistically influ- enced by one animal, EA6, which exhibited extensive hair cell loss in the lower turns. It is thought that this damage existed prior to ex- posure as FA6 was the oldest and largest (957 gms) animal used in this study. Furthermore, FA6 demonstrated minimal postexposure electro- physiologic changes. If the influence of FA6 is eliminated from the O C I- 1 'f> 0) 4-> 4-> O 0> -1 T3 w e, ■H '£ c •H O O -; h 0 ^i o o t/J c rt H, i—i 72 X • H (!) m t/1 O t~ o u nl rn rH > (1) ^H 4-» t/i (J t-i t/1 r_ n O H T3 3 n •^ <4H u 03 u i r, 4h bO,c o '.J 4-J 0) M H c C u o £3 o •H - H -; C - [ — i fj o ■H w f ) (LI (!) c r-t ■s ^ O 3 o ir, !/) U) H 4-1 O cC (L> Ul :- r: 13 w ifl +j CJ C p< a; > O u •H £3 4-> Jh ■> O H 0) C x: '-J 4-> o 3 •H rt ). "Central auditory fatigue," Audio logy 14, 72-83. Beagley, H.A. (1965). "Acoustic trauma in the guinea pig. Part I," Acta Otolaryngol . (Stockh.) 60, 437-451. 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(1975). "Physical and auditory specifi- cations of third-octave clicks," Audiology 14, 155-143. BIOGRAPHICAL SKETCH llai'vey Bruce Abrams was born on November 2.r), 1948, in Brooklyn, N.Y. He was graduated from Sheepshead Bay Higb School in June, 1966. In June, 1970, aided by a U.S. Office of Education Fellow- ship, he received the degree of Bachelor of Arts with a major in speech pathology and audiology from the George Washington University Mr. Abrams received the degree of Master of Arts with a major of audiology in August, 1971, from the University of Florida. Aid afforded by a Social Rehabilitative Services traineeship enabled him to complete this aspect of his education. After serving four years as an audiologist in the U.S. Army, Mr. Abrams returned to the University of Florida in January, 1976, to pursue work toward the degree of Doctor of Philosophy with a major in speech, aided by a Veterans Administration Traineeship. Mr. Abrams is an audiologist with the Veterans Administration Medical Center at Bay Pines, Florida. He is a member of the Ameri- can Speech -Language and Hearing Association, the Acoustical Society of America and the Military Audiology and Speech Pathology Society. He is married to the former Catherine L. Breder of Washington, U.C., and is the father of three children -- Lydia, Jesse and Emily. 94 I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Donald C. Teas' v Professor of Speech I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. ■K * William E. Browne lY ~ Asst. Professor Neuroscience I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. it /J sL William II. Cutler Adjunct Asst. Professor, Dept, of Speech This dissertation was submitted to the Graduate Faculty of the Department of Speech in the College of Liberal Arts and Sciences and to the Graduate Council, and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. Dean, Graduate School UNIVERSITY OF FLORIDA 3 1262 08553 5119