Contralateral suppression of linear and nonlinear transient evoked otoacoustic emissions in neonates at risk for hearing loss

Journal of Communication Disorders 41 (2008) 70–83

Contralateral suppression of linear and nonlinear transient evoked otoacoustic emissions in neonates at risk for hearing loss Alessandra Spada Durante a,*, Renata Mota Mamede Carvallo b,1 a

Medicine School of Santa Casa of Sa˜o Paulo, Rua Dr. Cesareo Mota Junior, 61, Zip 01221-020, Sa˜o Paulo - SP, Brazil b Medicine School of Sa˜o Paulo University, Rua Cipotanea, 51 Cidade Universitaria, Sa˜o Paulo - SP, Brazil

Received 19 April 2006; received in revised form 28 March 2007; accepted 8 May 2007

Abstract To investigate the transient evoked otoacoustic emissions (TEOAE) contralateral suppression in neonates at risk for hearing loss, 55 neonates at risk for hearing loss (risk group) and 72 fullterm neonates not at such risk (control group) were bilaterally tested. In all neonates, the TEOAE were recorded in two stimulation modes (linear and nonlinear clicks), with and without contralateral acoustic stimulation. Findings revealed significant contralateral suppression of otoacoustic emissions in both groups, but the amount of TEOAE contralateral suppression was reduced for at risk group ( p = 0.001), supporting the hypothesis that medial olivocochlear bundle function is reduced in neonates at risk for hearing loss. The combination of contralateral acoustic stimulation and TEOAE enables easy and noninvasive study of auditory efferent function. However it should be emphasized that the reduction in TEOAE contralateral suppression in the risk group, statistically identified as a group effect, might not be detectable in individual cases. Further studies are needed in order to determine whether the lower amount of TEOAE contralateral suppression in neonates at risk for hearing loss represents a risk for developing auditory processing disorders. Learning outcomes: The reader will be introduced to the study using auditory efferent pathway activation by contralateral acoustic stimulation (CAS), resulting in the TEOAE suppression effect. The characteristics of TEOAE suppression in the neonatal population, in which it provides evidence * Corresponding author. Tel.: +55 11 33677785. E-mail addresses: [email protected] (A.S. Durante), [email protected] (R.M.M. Carvallo). 1 Tel.: +55 11 30917452. 0021-9924/$ – see front matter # 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcomdis.2007.05.001

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of the reduced medial olivocochlear system function in those at risk for hearing loss, will also be addressed. # 2007 Elsevier Inc. All rights reserved.

1. Introduction Evoked otoacoustic emissions (EOAEs) are sounds in the external ear canal in response to acoustic stimulation and can first be recorded at 28–29 weeks postconception. The EOAE amplitude increases up to postconception week 38 and decreases to adult-like values by the age of 3 years (Chabert et al., 2006). Generated by normal nonlinear mechanisms within the cochlea, EOAEs and are thought to be the result of motile activity by the outer hair cells, which are innervated by the efferent nerve fibers of the medial olivocochlear bundle (Brownell et al., 1985; Davis, 1983; Dallos, 1992; Rasmussen, 1946). These myelinated fibers project from the medial zones of the superior olivary complex and terminate in the outer hair cells, predominantly those on the opposite side. Only 24–26% of the medial efferent fibers are uncrossed (Warr & Guinan, 1979). This network implies that auditory input can be modified peripherally at the cochlear hair cell level. Early experiments established that the medial olivocochlear system (MOCS) has a predominantly inhibitory function (Buno, 1978; Desmedt, 1962; Galambos, 1956; Gifford & Guinan, 1987). It has therefore been hypothesized that this system serves to protect the cochlea against acoustic injury (Cody & Johnstone, 1982; Liberman & Kujawa, 1999). This system also seems to be involved in the detection of sound in noise (Micheyl & Collet, 1996; Micheyl, Morlet, & Giraud, 1995). Evidence of this modulation comes mainly from numerous studies of otoacoustic emissions (OAEs). The combination of contralateral acoustic stimulation (CAS) and transient evoked otoacoustic emissions (TEOAEs) allows the study of sound-evoked olivocochlear feedback and therefore of the efferent cochlear innervation. It has been shown that MOCS activation can reduce TEOAE levels (Collet, Veuillet, Bene, & Morgon, 1992; De Ceulaer et al., 2001; Hood, Berlin, & Hurley, 1996; Veuillet, Collet, & Duclaux, 1991). This effect, known as TEOAE contralateral suppression, cannot be explained by technical artifacts, crosstalk or middle-ear effect, providing evidence of MOCS involvement in TEOAE contralateral suppression in normal-hearing human subjects (Collet et al., 1990; Veuillet et al., 1991). The suppression of OAEs has been reported to be absent or reduced in patients with auditory neuropathy (Abdala, Sininger, & Starr, 2000; Berlin, Hood, Hurley, & Wen, 1994; Berlin, Morlet, & Hood, 2003; Hood, Berlin, Bordelon, & Rose, 2003), learning disabilities (Veuillet, Collet, & Bazin, 1999) or auditory processing disorders (Muchnik et al., 2004). Therefore, the objective and non-invasive CAS-TEOAE technique could facilitate differential diagnosis, particularly in neonates and infants. Healthy neonates have TEAOAE contralateral suppression already present at term birth (Durante & Carvallo, 2002; Morlet, Collet, Salle, & Morgon, 1993; Morlet et al., 1999; Ryan & Piron, 1994). Chabert et al. (2006) investigated the early maturation of the efferent regulation of cochlear function in a population of preterm neonates and showed that the first statistically

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significant decrease in TEOAE amplitude in the presence of CAS occurred at postconception weeks 32–33 in preterm neonates, reaching adult-like values by postconception week 37. However, there is no data in the literature regarding MOCS function in neonates at risk for hearing loss (HL). The question of whether the TEOAE contralateral suppression observed in healthy newborns is the same as that in neonates at risk for HL remains unanswered. According to the Joint Committee on Infant Hearing (2000), all infants who pass newborn hearing screening but have risk indicators for other auditory disorders or speech/ language delay must be monitored for communication development. The functions of the medial efferent system include the modulation of signal detection in noise, are related to peripheral hearing sensitivity (Kawase & Takasaka, 1995). Infants with risk factors for HL, such as low Apgar score (Kaga, Ichimura, Kitazumi, Kodama, & Tamai, 1996), prematurity/low birth weight (Davis et al., 2001), and bacterial meningitis (Huggoson et al., 1997), are more prone to auditory processing disorder (Bamiou, Musiek, & Luxon, 2001). Auditory processing disorders result from the activity of various neurobiological mechanisms, which may be influenced by either the afferent or efferent auditory systems, and the MOCS plays a small and specific role in such disorders (Muchnik et al., 2004). If the amount of TEOAE contralateral suppression is reduced or absent in neonates at risk for HL, determination of the suppression effect could be employed as an initial procedure for monitoring auditory development. It may have clinical relevance as a nonbehavioral predictor of auditory processing dysfunction. Current screening methods (OAEs, Brainstem Evoked Responses Audiometry, Joint Committee on Infant Hearing high-risk register) focus on identifying infants with sensitivity loss. Including TEOAE suppression testing would extend the capabilities of hearing screening to include infants at risk for more central auditory deficits. However, the optimal TEOAE contralateral suppression protocol for neonates has yet to be defined. The choice of stimulus is crucial to OAE procedures. The appropriate stimulus can provide better results for a specific measurement. Linear mode stimulation has proven more sensitive for detecting shifts in the TEOAE recorded in the presence of competitive noise than in those recorded without such extraneous noise (Hood et al., 1996). However, despite the fact the linear stimulus is more sensitive to OAE levels, this mode uses a combination of four acoustic impulses (identical in size and polarity) and has the limitation of recording middle-ear and cochlear response components together. Therefore, some studies (De Ceulaer et al., 2001; Durante & Carvallo, 2002; Morlet et al., 2004) have proposed using the nonlinear mode to investigate contralateral suppression of TEOAE in a clinical setting. In this recording mode, the stimulus consists of three stimuli of a certain size and a fourth that is three times larger and of the opposite polarity. This ensures the near total elimination of meatal- and probe-related acoustic artifacts and is designed to achieve optimum results in neonate screening (Kemp & Ryan, 1991; Newmark, Merlob, Bresloff, Olsha, & Attias, 1997). Therefore, the purpose of this study was to determine whether the amount of TEOAE contralateral suppression, which is a measure of efferent system function, is different in neonates who are at risk for HL than in healthy neonates without risk factors.

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2. Methods This research was performed in accordance with the principles of the Declaration of Helsinki. Approval for this study was granted by the local ethics committee (CAPPesq-HC 647/01). Written informed consent was obtained from the parent or guardian of each neonate tested in this study. 2.1. Subjects Data were obtained from 128 infants born at the University Hospital of the University of Sao Paulo in Sao Paulo, Brazil. All 128 infants passed the regular hearing screening test. They presented bilateral TEOAE to nonlinear click stimulation at 80  3 dB peSPL, with an overall reproducibility level of at least 50%, and the signalto-noise ratio at 2.4, 3.2 and 4.0 kHz exceeded 6 dB with at least 75% of reproducibility. The groups met the following inclusion criteria:  Control group: 72 full-term neonates (36 females and 36 males) with no risk factors for HL (Joint Committee on Infant Hearing, 1994). The average birth weight was 3240  320 g, the average Apgar score was 9.0  0.6, and the average gestational age at birth was 38.6  1.0 weeks.The test was performed within 72 h (prior to discharge).  Risk group: 56 neonates (28 females and 28 males) with at least one risk factor for HL (Joint Committee on Infant Hearing, 1994). The average birth weight was 2720  860 g, the average Apgar score was 8.0  2.0, and the average gestational age at birth was 37.3  3.3 weeks. The risk factors for HL found in this group are described in Table 1.  15 pre-term neonates with an average gestational age of 39.5  1.5 weeks at the time of the test.  41 full-term neonates tested during the neonatal period (28 days of life) but prior to discharge.

Table 1 Distribution of risk factors for hearing loss (N = 56) Risk factors for hearing loss

N

(%)

Exchange for hyperbilirubinemia Congenital infection Family history of childhood hearing impairment Birth weight less than 1500 g Mechanical ventilation > 5 days Ototoxic medications > 7 days Low Apgar scores Craniofacial anomalies Bacterial meningitis Stigmata associated with a syndrome

12 9 9 9 8 7 5 5 3 0

21.43 16.07 16.07 16.07 14.28 12.50 8.93 8.93 5.36 0

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2.2. Procedure During the neonatal period, TEOAEs were recorded using the ILO 292 OAE analyzer, version 5.61 (Otodynamics Ltd., Hatfield, UK), using a ILO newborn TE SNS8 probe (insert phone). In all neonates, the TEOAE were recorded in two modes (linear and nonlinear) and with and without CAS. Under all conditions, the mean intensity of the clicks was 60–65 dB peSPL, and total of 100 sweeps were recorded for each ear. This particular intensity was chosen based on previous studies conducted in humans (Collet et al., 1992; Hood et al., 1996; Ryan & Kemp, 1996; Veuillet, DuverdyBertholon, & Collet, 1996) in order to rule out the involvement of middle ear mechanisms (Giraud, Collet, Che´ry-Croze, Magnan, & Chays, 1995). The response level was determined by measuring the signal-to-noise ratio, adopting an analysis time window of 4–20 ms. The TEOAEs were registered on channel A, the (linear and nonlinear) click channel. Linear clicks, a combination of four identical acoustic impulses of 80-ms duration, were presented by the ILO 292 system protocol (Lyon Mode-Otodynamics, Ltd.), which presents alternating blocks of linear clicks (with and without CAS). The data set from the test with CAS was designated memory store 1, and that from the test without CAS was designated memory store 2. Nonlinear clicks consisted of four standard 80-ms rectangular electric pulses presented at 50 pulses/s, the fourth pulse having inverse polarity and increased amplitude. The CAS consisted of continuous broadband white noise at 60 dB SPL, delivered through channel B of the ILO and presented by an ILO General Purpose TE SGS-8 probe (insert phone). The neonates were tested in their cradles, in a quiet although not soundproof room, next to the nursery. All subjects were tested bilaterally in a randomized fashion. After the two probes were in place, TEOAE were recorded in alternating blocks (with and without CAS) for the nonlinear and linear modes, always in the same order. TEOAE contralateral suppression was calculated by subtracting the with-CAS TEOAE level from the withoutCAS TEOAE level. The TEOAE were recorded in a single session, or some time later in the same day, when conditions were suitable (when noise and subject movement were sufficiently reduced). If TEOAE recordings were not completed, the subject was excluded from the study. The statistical analysis using ANOVA showed that the intensity of the stimulus and stability measures were equivalent ( p  0.5) under the with-CAS and without-CAS conditions for all groups, ears and genders. 2.3. Statistical analysis The without-CAS TEOAE, with-CAS TEOAE, and TEOAE suppression levels were categorized by group (control or risk), by ear (right or left), by gender (male or female) and by mode (linear or nonlinear). The analysis of variance was performed for the TEOAE levels (ANOVA 1, TEOAE level without-CAS; ANOVA 2, TEOAE level with-CAS) and suppression levels (ANOVA 3, TEOAE suppression level) including these factors (Appendix A).

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The chi-square test was used to perform an additional analysis in which the two groups were compared in terms of the proportion of subjects presenting the suppression effect. In all cases, the criterion of significance was p < 0.05. For the ANOVA of TEOAE suppression levels (ANOVA 3) the assumptions were satisfied after the exclusion of six values located out of the normal residual curve. A cutoff point of 0.6 dB was adopted as the criterion for identifying the suppression effect (Prasher, Ryan, & Luxon, 1994).

3. Results As seen in Fig. 1, the mean TEOAE levels without-CAS showed greater values than the with-CAS in all conditions. TEOAE levels without-CAS in the left ear were lower than in the right ear by 1.03 dB (0.42 dB) ( p = 0.005, ANOVA 1). There were no significant interactions only main effects of gender and ear in the TEOAE with-CAS condition levels ( p  0.02, ANOVA 2). The differences in emission levels between the control group and the risk group under the without-CAS and with-CAS conditions were not significant ( p  0.07, ANOVA 1 and 2). A cutoff point of 0.6 dB was adopted as the criterion for identifying the suppression effect (Prasher et al., 1994). Occurrence of the TEOAE suppression effect by stimulus

Fig. 1. Mean of TEOAE levels in right and left ears by stimulation mode (linear and nonlinear), with and without CAS (contralateral acoustic stimulation) and group (control (CG) and risk (RG)) for females and males.

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Table 2 TEOAE contralateral suppression by stimulus mode (linear and nonlinear) and by group Condition

Group Control, N = 72

Risk, N = 56

Linear and nonlinear suppression Only linear suppression Only nonlinear suppression No suppression

60 (83%) 6 (8%) 4 (6%) 2 (3%)

43 (77%) 4 (7%) 7 (12.5%) 2 (3.5%)

Chi-square test

p = 0.68

mode can be seen in Table 2. A high proportion of infants presented suppression, regardless of group according to the chi-square test ( p = 0.68). The TEOAE contralateral suppression levels by gender, group, ear and stimulation mode presented considerable intersubject variability (Fig. 2) but the confidence intervals (95%) indicate lower values in the risk group. A significant effect was found for the stimulation mode and ear ( p < 0.001), with greater suppression values in the non-linear mode and right ears ( p = 0.027). Investigating the group effect of the TEOAE contralateral suppression (ANOVA), regardless of the gender, ear and mode of stimulation, a significant reduction was identified through the statistical analysis. In the control group, the TEOAE contralateral suppression was 1.04 dB (0.08 dB) compared with 0.73 dB (0.09 dB) in the risk group ( p = 0.001).

Fig. 2. Mean of TEOAE contralateral suppression levels in right and left ears by stimulus mode (linear and non linear) and group (control (CG) and risk (RG)) for females and males.

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4. Discussion The main finding in the present study was that the amount of TEOAE contralateral suppression was significantly reduced in neonates at risk for HL. Furthermore, despite the fact that there were no differences between TEOAE level, at the without-CAS condition, per group ( p = 0.070), the ANOVA analyses indicated a group effect in the TEOAE contralateral suppression ( p = 0.001). Even though the differences in emission levels between the CG and RG in the without-CAS and with-CAS conditions were not significant, the trend for lower emission levels in all conditions for the RG might suggest a cochlear problem. Considering the TEOAE responses, the suppression effect was greater ( p < 0.01) in neonates at no risk for HL (1.04 dB  0.08 dB). In the group of neonates at risk for HL, the suppression effect was 0.73 dB  0.09 dB. The importance of the amount of TEOAE contralateral suppression, which has primarily been described in normal-hearing adults (Berlin et al., 1993; Berlin et al., 1994; Hood et al., 1996; Veuillet et al., 1991), is found in its association with mechanisms of central MOCS feedback in the peripheral auditory system. When OAE suppression is absent or reduced, it is frequently associated with difficulties in sound localization and lateralization, auditory discrimination and recognition, difficulty in understanding speech in the presence of background noise, and auditory neuropathy/dyssynchrony (Abdala et al., 2000; Berlin et al., 1994; Berlin et al., 2003; Hood et al., 2003; Kumar & Vanaja, 2004; Muchnik et al., 2004; Sanches & Carvallo, 2006; Veuillet et al., 1999). The risk factors associated with the high risk group in this study also indicate the potential for developing auditory processing disorders. Although preliminary, these results are similar to those previously obtained in a study of children with auditory processing disorders, indicating that the reduced TEOAE suppression in the high risk group is a potentially useful indicator of future auditory processing problems. There is strong evidence that contralateral suppression of OAEs reflects efferentmediated effects (Puel & Rebillard, 1990; Ryan, Kemp, & Hinchcliffe, 1991). In the present study, TEOAE levels were suppressed by CAS, whether recorded using linear or nonlinear stimuli, thereby indicating functional maturity of the MOCS in neonates (Durante & Carvallo, 2002; Morlet et al., 1993; Morlet et al., 1999; Ryan & Piron, 1994). Despite the large intersubject variability in the amount of TEOAE contralateral suppression, and larger effects in the nonlinear mode, both stimulation modes produced a significant amount of TEOAE contralateral suppression. Considering these results, it would be possible, in a clinical setting, to use either the linear or nonlinear mode with confidence, thereby shortening the procedure time. A recent study (Morlet et al., 2004) confirmed previous findings (Durante & Carvallo, 2002), using the Quickscreen mode (time window of 12 ms) to demonstrate an effective amount of TEOAE contralateral suppression in neonates. The data also revealed that the MOCS appears to be more efficient in the right ear than in the left ear when the nonlinear mode is used. This laterality of the suppression effect has been reported by several authors (Khalfa & Collet, 1996; Khalfa, Morlet, Mycheyl, Morgon, & Collet, 1997; Khalfa, Mycheyl, Veuillet, & Collet, 1998; Morlet et al., 1999). Further studies are needed in order to investigate whether the reduced amount of TEOAE contralateral suppression in neonates at risk for HL constitutes a risk for

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developing auditory processing disorders. Auditory processing dysfunction is not usually associated with loss of sensitivity, and targeting the population of infants at risk for HL would miss those infants in the general population who present normal hearing but have auditory processing deficits. Ongoing attempts to understand the functions of the efferent system are important because the cochlear structures, key physiological elements in peripheral auditory sensitivity and frequency selectivity, are innervated by fibers from both the lateral and medial zone of this system. These preliminary results suggest that reduced efferent system function, as measured by the amount of TEOAE contralateral suppression, seems to be common in neonates at risk for HL and underscore the importance of periodic audiological evaluation. The combination of CAS and linear or nonlinear TEOAE enables easy and noninvasive study of auditory efferent function. Concerning possible practical applications, it should be clarified that the reduction in TEOAE contralateral suppression was only statistically significant, and that this technique might therefore have limited clinical utility.

Acknowledgments Financial support for this study was provided by the Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior (CAPES, Coordination of the Advancement of Upper-Level Personnel) and by the Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP, Foundation for the Support of Research in the state of Sa˜o Paulo; Grant 01/09605-5). We are also grateful to the staff of the Estatistica Aplicada do Instituto de Matematica e Estatistica da Universidade de Sao Paulo (CEA-IME USP, University of Sa˜o Paulo Institute of Mathematics and Statistics, Center for Applied Statistics).

Appendix A Factors

Sum of squares

ANOVA 1: TEOAE level without-CAS Model Gender Group Ear Stimulus mode Gender  group Gender  ear Group  ear Gender  group  ear Gender  stimulus mode Group  stimulus mode Gender  group  stimulus mode Ear  stimulus mode

153858.65 185.00 79.90 192.80 5.15 6.03 34.02 5.16 0.05 9.55 0.57 0.01 5.12

d.f.

16 1 1 1 1 1 1 1 1 1 1 1 1

Mean square

F

p

9616.17 185.00 79.90 192.80 5.15 6.03 34.02 5.16 0.05 9.55 0.57 0.01 5.12

398.02 7.66 3.31 7.98 0.21 0.25 1.41 0.21 0.00 0.40 0.02 0.00 0.21

0.000 0.006 0.070 0.005 0.644 0.617 0.236 0.644 0.964 0.530 0.878 0.984 0.645

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Appendix A (Continued ) Factors

Sum of squares

d.f.

Gender  ear  stimulus mode Group  ear  stimulus mode Gender  group  ear  mode Error

0.41 0.08 1.30 11983.35

1 1 1 496

Total

165842.00

512

ANOVA 2: TEOAE level with-CAS Model Gender Group Ear Stimulus mode Gender  group Gender  ear Group  ear Gender  group  ear Gender  stimulus mode Group  stimulus mode Gender  group  stimulus mode Ear  stimulus mode Gender  ear  stimulus mode Group  ear  stimulus mode Gender  group  ear  mode Error

137211.27 152.12 32.90 137.96 92.60 6.41 20.49 3.55 0.01 3.49 0.41 0.06 0.74 0.08 0.75 1.04 13038.94

16 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 496

Total

150250.21

512

ANOVA 3: TEOAE suppression level Model Gender Group Ear Stimulus mode Gender  group Gender  ear Group  ear Gender  group  ear Gender  stimulus mode Group  stimulus mode Gender  group  stimulus mode Ear  stimulus mode Gender  ear  stimulus mode Group  ear  stimulus mode Gender  group  ear  mode Error

483.21 0.07 11.90 3.56 41.47 0.34 2.11 1.62 0.66 0.54 1.29 0.04 5.49 1.65 0.91 0.01 546.44

16 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 491

1029.65

507

Total

Mean square

F

p

0.41 0.08 1.30 24.16

0.02 0.00 0.05

0.897 0.955 0.817

8575.70 152.12 32.90 137.96 92.60 6.41 20.49 3.55 0.01 3.49 0.41 0.06 0.74 0.08 0.75 1.04 26.29

326.22 5.79 1.25 5.25 3.52 0.24 0.78 0.14 0.00 0.13 0.02 0.00 0.03 0.00 0.03 0.04

0.000 0.017 0.264 0.022 0.061 0.622 0.378 0.713 0.985 0.716 0.900 0.962 0.867 0.956 0.866 0.842

30.20 0.07 11.90 3.56 41.47 0.34 2.11 1.62 0.66 0.54 1.29 0.04 5.49 1.65 0.91 0.01 1.11

27.14 0.06 10.69 3.20 37.27 0.30 1.89 1.45 0.59 0.48 1.16 0.04 4.93 1.48 0.82 0.01

0.000 0.806 0.001 0.074 0.000 0.582 0.170 0.229 0.443 0.487 0.282 0.848 0.027 0.225 0.367 0.930

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Appendix B. Continuing education 1. The medial olivocochlear pathway consists of: a. Myelinated fibers, 74–76% of which cross at the opposite side and terminate in the outer hair cells. b. Unmyelinated fibers, 74–76% of which cross at the opposite side and terminate in the outer hair cells. c. Myelinated fibers, 74–76% of which are uncrossed and terminate in the outer hair cells. d. Myelinated fibers, 74–76% of which cross at the opposite side and terminate in the inner hair cells. e. Myelinated fibers, 24–26% of which cross at the opposite side and terminate in the outer hair cells. 2. Functional hypotheses regarding the medial olivocochlear system include: a. auditory discrimination and recognition b. predominantly excitatory function c. sound localization and lateralization d. modulation of signal detection in noise 3. Indicate the sequence that correctly fills in the spaces above (‘T’ = ‘true’; ‘F’ = ‘false’). a. F,T,F,T b. T,F,T,F c. T,F,T,T d. F,F,T,T e. T,T,F,T 4. Otoacoustic emission suppression is . . . a. a reduction in OAE stability b. explained by technical artifacts, crosstalk and middle-ear effect c. a reduction in OAE reproducibility d. a reduction in OAE level e. a reduction in OAE latency 5. This study shows that TEOAE contralateral suppression occurs in both linear and nonlinear stimulus mode in the following percentage for the control and risk group, respectively a. 8%; 7% b. 83%; 77% c. 6%; 12.5% d. 3%; 3.5% e. 15%; 75% 6. These findings regarding TEOAE suppression in neonates allow which of the following assumptions to be made? a. TEOAE contralateral suppression was significantly reduced in neonates at risk for hearing loss (0.73 dB  0.09 dB)

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b. The suppression effect was greater ( p < 0.01) in neonates at no risk for hearing loss (1.04 dB  0.08 dB) c. TEOAE levels were suppressed by contralateral acoustic stimulation whether recorded using linear or nonlinear stimuli d. The reduced TEOAE suppression effect observed in neonates at risk for hearing loss underscores the importance of periodic audiological evaluation e. All of the above

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