Adaptation of distortion product otoacoustic emission in humans.

Previous studies of animals observed a phenomenon of adaptation of distortion product otoacoustic emission (DPOAE) and found that the phenomenon was mediated to a large extent by the medial olivocochlear (MOC) reflex. The present study investigated DPOAE adaptation in humans. The following stimuli were used: f2/f1 = 1.2; f2 = 2, 4, or 5.65 kHz; L2 = 50-65 dB SPL re 20 microPa rms, L1 - L2 = 0-15 dB, where L1 and L2 represent levels of the f1 and f2 tones, respectively; duration of two-tone burst = 5.5 s; interburst gap = 20 or 30 s; number of repetitions = 40 or 64. We analyzed the 2f1 - f2 DPOAE as a function of time using a method of heterodyne envelope detection. The subjects were 20 humans aged from 15 to 54 years (median = 21 years) with normal hearing. We observed that (1) humans exhibited DPOAE adaptation phenomenon; (2) the time course of DPOAE level was characterized by a 2-exponential function; (3) distributions of the fast and slow time constants were well separated with their median values being 69 ms and 1.51 s, respectively; (4) distributions of the magnitudes of the fast and slow adaptation components were largely overlapped with their median values being 0.65 and 0.40 dB, respectively; and (5) the combined magnitude of the adaptation ranged from 0.4 to 3.0 dB with a median of 1.10 dB. To our knowledge, the present study is the first published article to describe adaptation of DPOAE in humans. These results should help advance the basic knowledge of human cochlear mechanics operating under the control of the MOC feedback system and contribute to the development of practical applications such as identifying people at high risk of acoustical injury and a clinical test of the functional status of the MOC system.

Single-unit responses to cholinergic agents in the rat inferior colliculus.

Single units were recorded from the inferior colliculi of adult male rats anesthetized with urethane. Units were driven with tonal stimuli, and changes in unit responses to the tones were monitored during iontophoretic application of cholinergic drugs. The cholinergic agonists acetylcholine and carbamylcholine potentiated responses in about 50% and suppressed responses in about 35% of units tested. Cholinergic antagonists typically produced effects when delivered alone. Both d-tubocurarine and atropine methyl nitrate excited over 80% of units tested, while mecamylamine and scopolamine inhibited the majority of tested units. Dihydro-beta-erythroidine was generally ineffective. Alpha-bungarotoxin was generally ineffective when delivered alone, but blocked agonist effects. Post-stimulus time histogram (PSTH) patterns, response-intensity functions and response areas were examined for changes during drug delivery. Cholinergic agents did not differentially affect either time periods within the PSTH or frequency bands of response areas, but were especially effective for those intensities producing larger response rates. Taken together with evidence from biochemical studies, our results suggest the presence of a functional cholinergic input into the inferior colliculus which acts to modulate acoustic processing.

Auditory brainstem response results as predictors of behavioral auditory thresholds in severe and profound hearing impairment.

Pediatric cochlear implantation is restricted to patients with stable, bilateral profound sensorineural hearing losses who derive no benefit from conventional amplification. Obtaining reliable audiologic thresholds in a young child with sudden or early-onset hearing loss can be challenging. This study examines the accuracy with which auditory brainstem response evaluation can predict unaided and aided behavioral thresholds in a child with severe-to-profound hearing loss. Reliable behavioral thresholds were obtained on 119 children who had no measurable click-evoked auditory brainstem responses at instrumentation limits of 100 dB HLn. These data show that an absent auditory brainstem response at 100 dB HLn does not necessarily indicate the absence of measurable unaided hearing for test frequencies ranging from 250 Hz to 4000 Hz. Average aided thresholds of better than 60 dB were present in 43% of the children for 500, 1000, and 2000 Hz and in 53% for 500 and 1000 Hz. Therefore, the absence of a click-evoked auditory brainstem response at 100 dB HLn in a young child is not prima facie evidence of the child's cochlear implant candidacy.

Cerebrospinal fluid parameters and auditory brainstem responses following meningitis.

Auditory brainstem responses were measured in 94 children under 24 months of age immediately following treatment for bacterial meningitis. Evidence of peripheral hearing loss (thresholds of 30 dB HLn or greater) was found in 47% of the patients. In addition, 9% had prolonged interwave latencies, indicating the possible presence of retrocochlear pathology. Other clinical data were examined as well. CSF glucose concentration correlated with both the presence and magnitude of hearing loss (as measured by auditory brainstem responses). Magnitude of hearing loss also was associated with the presence of seizures. Although all children recovering from meningitis should be assessed for hearing loss, those who have had low CSF glucose concentrations and seizures appear to be at high risk.

Auditory brainstem responses elicited by 1000-Hz tone bursts in patients with sensorineural hearing loss.

Auditory brainstem responses were measured in response to 1000-Hz tone bursts from 115 patients with sensorineural hearing loss, presumably of cochlear origin. Mean wave V latencies and variability were comparable to those observed in normal hearing subjects for similar stimuli. The range of interaural differences in wave V latencies for 1000-Hz tone bursts were slightly greater than those observed for clicks, which may not be surprising, given the greater variability in wave V latencies for tonal stimulation, even in normal-hearing subjects. These differences, however, were not affected either by the magnitude or symmetry of hearing loss for frequencies at and above 1000 Hz. These data suggest that tone burst ABRs might be useful in otoneurologic evaluations, especially for patients with asymmetric hearing loss.

Effects of stimulus phase on the latency of the auditory brainstem response.

Auditory brainstem responses were measured in five normal-hearing subjects, using single-cycle sinusoids at octave frequencies ranging from 250 to 2000 Hz. These sinusoids, gated with Blackman functions, were presented either at 0 or 180 degree phase and were varied in level from 90 dB SPL to threshold in 10-dB steps. Stimulus phase affected wave V latencies for low-frequency stimuli, with the effect decreasing as frequency increased. These data are thought to represent an evoked potential manifestation of known phase-locking abilities within the auditory system.

Some issues relevant to establishing a universal newborn hearing screening program.

This article describes some of the factors relevant to the establishment of a universal newborn hearing screening (UNHS) program. First, the difficulty in providing precise estimates of test sensitivity and specificity are reviewed. This section is followed by hypothetical estimates of overall programmatic costs, first for a fixed number of babies to be screened and then as a function of the number of babies to be screened in a year. Included in these estimates are the costs for equipment, disposables, personnel, and follow-up testing. These estimates are provided for three different screening protocols: auditory brainstem response (ABR) alone, otoacoustic emission (OAE) alone, and OAE followed by ABR only for those babies who failed the OAE screening. If follow-up costs are not included, it is less expensive to screen newborns with OAEs compared with the other two protocols. However, once follow-up testing is included as part of the program costs and there are at least 400 births per year, procedures in which OAEs are performed first, followed by an ABR on those infants who do not pass the OAE test, result in the lowest costs. Hospitals with as few as 400 births per year should expect per-baby costs not exceeding $30, regardless of which protocol is used. For all three protocols, the unit costs decrease as the number of babies screened increases. The final section describes data from a local UNHS program in which all infants are screened first with an OAE test, followed by an ABR test on infants not passing the OAE screening. Idiosyncratic features to this program are described, including the fact that all screening tests are performed by audiologists, who are paid on a part-time basis, adding cost to the program. Even under these circumstances, the unit cost is under $30. These data lead us to conclude that all infants can be screened in a cost-effective manner.

Case of recurrent, reversible, sudden sensorineural hearing loss in a child.

This paper describes audiologic, electrophysiologic, and medical test results for a now 10-year-old girl who has had 45 episodes of reversible, sudden sensorineural hearing loss over the last 8 years. Episodes have lasted from 6 to 72 hours and often have been accompanied by a mild illness. Acoustic immittance measures have been consistent with normal middle-ear function with the exception of absent ipsilateral and contralateral acoustic reflexes. Mechanically evoked perioral reflex activity was markedly asymmetric following lower lip stimulation. The asymmetry of R1 activation between right and left side lower lip inputs raises questions about the integrity of central connections within the brain stem, including internuncial pathways coursing between trigeminal sensory relay nuclei and the facial motor nucleus. An electrocochleographic evaluation revealed cochlear microphonic but absent or markedly abnormal whole nerve action potentials. Auditory brainstem responses (ABR) have been either absent or poorly formed and significantly delayed, regardless of hearing sensitivity. Middle and late auditory evoked potentials were essentially normal. Both transient-evoked and distortion-product otoacoustic emissions were present regardless of peripheral auditory sensitivity. All medical tests have been essentially normal. Although no definitive diagnosis has been reached, beta blockers have been used with some success. Taken together, these data document a very unusual case of fluctuating hearing loss. The electrocochleographic and otoacoustic emission data suggest that the outer hair cells are functioning normally and that the loss is not cochlear in origin.(ABSTRACT TRUNCATED AT 250 WORDS)

Preliminary descriptions of transient-evoked and distortion-product otoacoustic emissions from graduates of an intensive care nursery.

Transient-evoked (TEOAE) and distortion-product otoacoustic emissions (DPOAE) were measured in 51 graduates of an intensive care nursery and compared to data obtained from 80 normal-hearing children and adults. All infants had click-evoked auditory brainstem responses (ABR) at 30 dB nHL or less while the older subjects had pure-tone thresholds of 20 dB HL or less for octave frequencies from 250 to 8000 Hz. OAE data were collected using commercially available devices. All data were analyzed in terms of emission amplitude, emission-to-noise ratio, and response reproducibility as a function of frequency. DPOAEs were measured at three points per octave between f2 frequencies of approximately 500 and 8000 Hz. TEOAEs were elicited by clicks and were analyzed in both octave and 1/3-octave bands centered at frequencies from 500 to 4000 Hz, as well as in the broadband condition. In addition, stimulus amplitudes for the clicks used to elicit TEOAEs were analyzed within octave and 1/3-octave bands to determine whether any age-related differences in responses can be accounted for on the basis of stimulus differences. Both emission amplitude and noise amplitude were greater in neonates than adults, although there was variability across frequency. Emission-to-noise ratio and response reproducibility were more similar between groups. For TEOAEs, high-frequency emission-to-noise ratios were larger in neonates compared to older subjects, while the reverse was true in the lower frequencies. Less obvious frequency effects were observed for DPOAEs. These findings are discussed in relation to the potential use of OAEs as screening measures for neonatal hearing loss.

AP measurements of short-term adaptation in normal and in acoustically traumatized ears.

A forward-masking paradigm was used to measure short-term adaptation in normal and in acoustically traumatized ears. AP amplitude was measured to a tone burst of fixed frequency and level while masker intensity (Lm) and duration (Tm), as well as the internal between masker offset and probe onset (delta t) were varied. Masker and probe frequency were held constant at 4 kHz. In one experiment, short-term perstimulatory adaptation was examined through the measurement of probe-elicited N1 amplitude at a fixed delta t while both Lm and Tm were varied. These probe response-versus-T functions were modeled exponentially, yielding time constants that did not differ between was examined by holding Tm constant and measuring probe-response amplitude as a function of delta t. When the decrement- (amount by which the probe-elicited N1 amplitude was reduced by the masker) versus-delta t functions were fit with an exponential model that accounted for long-term effects, no differences were observed between recovery time constants from normal and traumatized ears. Finally, growth of response to the masker was estimated by examining the decrement in normalized amplitude of response to the probe as a function of Lm. Acoustically traumatized ears revealed steeper growth functions, suggesting steeper single-fiber rate-versus-level functions.

Forward-masking AP tuning curves in normal and in acoustically traumatized ears.

Compound action potential tuning curves (APTC's) were developed in a series of normal and acoustically traumatized cats. Probe frequency was set to 4 kHz ad probe level was set to 15-20 dB SL re: AP threshold for both normal and impaired ears. In addition, normal ears received the probe tone at a level of 70-75 dB SPL. Probe-elicited AP amplitude was measured in a forward-masking paradigm as a function of both masker level and frequency. Results indicated that the sharpness of the APTC's (defined by Q 10) was unaffected by acoustic trauma. The relative difference between required masker level on the tail and at the tip was reduced for the noise-damaged ears. Thus sharpness around the tip was essentially normal yet low-frequency energy was more effective at masking high-frequency region in these impaired ear. When slope of the decrement in normalized, probe-elicited AP amplitude-versus-masker level functions are calculated for different frequency maskers, normal ears showed a frequency dependence such that low-frequency maskers resulted in the steepest slopes with progressively less steep slopes as masker frequency increased. Some of the noise-damaged ears did not show this same frequency dependence. Specifically, the slopes of these functions were not as different for low-and high-frequency maskers. These slope data may suggest that nonlinearities, present in the normal system, may be reduced as a consequence of this cochlear insult.

An empirical evaluation of the effects of high-pass noise on the whole-nerve action potential.

A number of methods are presented for evaluating the effect of high-pass noise on the whole-nerve action potential (AP). These methods include measurements of AP thresholds, amplitude-versus-level functions, decrement in AP amplitude-versus-masker level functions, and AP tuning curves. Examinations of threshold shifts as a function of tone-burst frequency and AP amplitude-versus-level with and without the presentation of high-pass noise indicate that basal portions of the cochlear partition can be masked effectively. Decrement in AP amplitude-versus-masker level functions and subsequently constructed AP tuning curves were used to verify that the presentation of high-pass noise did not alter the frequency response of that region of the basilar membrane responding to a 4000-Hz tone-burst probe. As a result, we conclude that high-pass noise may be used to mask the response from remote regions of the cochlea without altering response characteristics from lower frequency regions.

AP responses in forward-masking paradigms and their relationship to responses of auditory-nerve fibers.

The amplitude of N1 peak of whole-nerve AP was measured in cats using a tone-burst probe with tonal maskers in a forward-masking paradigm. Experiments examined the effects of masker level, frequency, and duration (Tm), as well as interstimulus interval (delta t). Results are consistent with the interpretation that amplitude of N1 reflects activity of a limited group of fibers with CF near the probe frequency. For single fibers, decrement in discharge rate (relative to an unmasked condition) is dependent upon rate to the masker [Smith, Neurophys. 40 (1977)]. For the AP, decrement in probe-elicited amplitude should reflect the rate to masker in those fibers excited by the probe. Thus measurements of N1 decrement versus masker frequency and level are similar to single unit rate versus level functions. Since decrement in N1 reflects amount of adaptation, increasing Tm decreases probe response. N1 amplitude as a function of Tm thus resembles PST histograms of nerve fibers. Finally, plots of N1 amplitude as a function of delta t are interpreted as a measure of recovery from adaptation.

Comparison between intensity and pressure as measures of sound level in the ear canal.

In-the-ear calibration of sound pressure level may be problematic at frequencies above 2 kHz, because the pressure can vary significantly along the length of the ear canal, due to reflection of sound waves at the eardrum. This issue has been investigated by measuring behavioral thresholds to tones in a group of human subjects (N = 61) for two different insertion depths of an insert earphone. The change in insertion depth was intended to alter the distribution of pressure in the ear canal, shifting the frequency at which spectral notches occur. The inset earphone or "probe" (Etymotic ER-10C) also contained a calibrated microphone, allowing the recording of sound pressure levels in the ear canal. Prior to the threshold measurements in each subject, the Thevenin acoustic source characteristics of the probe were determined by a special calibration procedure. This calibration allowed the expression of the sound level at threshold in terms of acoustic intensity (W/m2). The impact of changes in insertion depth was determined by measuring behavioral threshold at each depth. Because cochlear sensitivity remained constant, the level of sound entering the ear at threshold should have been the same (within measurement error) for both insertions. The difference in sound pressure level (SPL) at threshold between the two probe insertions was greatest at the notch frequency of the first insertion. At this notch frequency, the SPL at threshold increased by an average of 11.4 dB. The change in sound intensity level (SIL) at threshold was almost always less than the change in SPL. At the notch frequency, the SIL decreased, on average, by only 0.5 dB. These results suggest that SIL may be a better indicator than SPL of the sound level entering the ear, especially for frequencies in the 4-8 kHz range.

The identification and diagnosis of hearing loss in infants.

The goal of early identification of hearing loss is being reached gradually. The technological options for assessing hearing loss in neonates and infants are expanding to the point that we are now capable of identifying hearing loss, regardless of developmental level. The latest revision of the JCIH (1991) risk criteria, in conjunction with an increasing number of statewide programs, provides optimism for meeting the goal of finding and remediating hearing loss early in the first year of life.

Temporal characteristics of the acoustic reflex.

Temporal aspects of the acoustic reflex response were estimated for normally hearing subjects, subjects with noise-induced hearing loss and children with sensorineural hearing impairment. Onset latency and rise/fall times of admittance change were measured from the averaged responses (8 presentations) to tone bursts of 250 ms durations and 10 ms rise/fall time. Tone burst frequency was varied from 500 to 4 000 Hz. No differences between groups were observed for onset responses. Compared to the data from normally hearing subjects, the offset responses were slightly longer for subjects with noise-induced hearing loss and were considerably longer for hearing-impaired children. Due to the prolongation seen for noise-exposed subjects, it is difficult to interpret these data in terms of site of lesion. Still, the differences between hearing-impaired children and normal subjects suggest that acoustic reflex offset latency may be a useful screening device.

The choice of stimuli for ABR measurements.

This tutorial describes some of the stimulus choices available to clinicians performing ABR evaluations. The motivation for choosing particular stimuli is described in relation to the clinical question being asked. Thus, emphasis is directed toward the rationales for choosing different stimuli. Data are reviewed only briefly to illustrate these rationales. Finally, some effort has been made to relate clinical ABR results for certain stimulus conditions to underlying physiological processes in the hope that these discussions will lead to a more complete understanding of the stimulus-response relation.

Comparison of onset and steady-state responses of hearing aids: implications for use of the auditory brainstem response in the selection of hearing aids.

Input-output (I/O) functions of hearing aids were measured in response to a 2000-Hz tone burst, having 0.5 ms rise/fall time and 10 ms duration. I/O functions, measured with a hearing-aid analyzer, served as reference conditions. Hearing-aid outputs at onset and during the steady-state portion of the waveform differed; these differences often depended upon stimulus rate. The relation between onset and steady-state estimates of output were not always predictable from hearing-aid attack and release times. These findings indicate that the steady-state output limitation characteristics of hearing aids cannot be estimated from their onset responses. In turn, this suggests that ABR measurements may not provide accurate estimates of the compressive characteristics of hearing aids.

Auditory brain stem responses from graduates of an intensive care nursery using an insert earphone.

Auditory brain stem responses (ABR) were measured from graduates of an intensive care nursery using an insert earphone. Approximately 95% of all ears had click-evoked ABR thresholds of 30 dB nHL or less. Absolute latencies of waves I and V were within the range observed for a circumaural earphone, once the delay introduced by the insert earphone's sound delivery tube was taken into account. Finally, interpeak latency differences and interaural symmetry were comparable to values observed when a similar group of patients were tested with a circumaural earphone.

An alternate method for determining functional gain of hearing aids.

The functional gain of a hearing aid typically is determined by comparing aided and unaided behavioral thresholds. With this method, however, true gain may be underestimated in frequency regions of normal or near-normal hearing sensitivity (i.e., in cases of sloping, rising, or trough-shaped audiograms). Internal hearing-aid noise and/or amplified room noise imposes a lower limit on obtainable aided thresholds. In these cases, comparing aided and unaided acoustic-reflex thresholds may be a valuable clinical alternative to traditional means of determining real-ear gain. This study compared sound-field behavioral threshold and acoustic-reflex threshold estimates of functional gain for individuals with a variety of audiometric configurations. The sound-field behavioral threshold measurements were found to underestimate functional gain if unaided thresholds approached the normal hearing range. In regions of greater hearing loss, behavioral and acoustic-reflex estimates of functional gain were in good agreement.