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The threshold of the acoustic reflex (TAR) and the magnitude of middle-ear muscle response were measured for a homophasic (0 degrees) and an antiphasic (183 degrees) 550-Hz tone in a background of in-phase noise. Signal-to-noise ratio ranged from - 20 dB to 5 dB. Whereas previously reported data shows an effect of phase on the percept of loudness, no evidence of a phase effect was measured for acoustic-reflex responses. These results are interpreted as evidence against a relation between loudness and acoustic reflex for binaurally presented signals.

This paper describes preliminary data on the use of click-evoked ABRs in the hearing aid selection process. Four normal-hearing and 4 hearing-impaired subjects were tested with a hearing aid set at three different frequency response settings. Estimates of gain were calculated using shifts in Wave V thresholds, shifts in Wave V latency-level functions, acoustic-reflex measurements, coupler gain measurements, and measurements of functional gain. Results suggest that the click-evoked ABR does not distinguish between differing amounts of low-frequency gain, although reasonable estimates of high-frequency gain appear possible. Also discussed are technical factors that must be considered when using the ABR in the hearing aid evaluation process.

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.

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.

Psychoeducational data were collected from the files of 1,250 hearing-impaired children in Iowa public schools in an effort to identify educational and linguistic profiles related to different degrees of hearing loss. The files of most mildly to moderately hearing-impaired children do not reflect complete assessment of language, academic, or intellectual skills even when support services are being provided. The data reveal deficits that often are inconsistent with the reports and patterns of achievement on which the allocation of support services for hearing-impaired children have been based. The appropriateness of many of the assessment tools in use is questionable.

Survey questionnaires eliciting incidence information about degree and type of hearing loss, educational placement, use of amplification, and other demographic data were completed by audiologists in 13 of the 15 Area Education Agencies in Iowa in an attempt to describe the characteristics of hearing-impaired children in public-school settings. The information revealed patterns of hearing loss related to age and sex, use of hearing aids, and classroom placement that may be useful in planning support services for this population.

Auditory brainstem responses (ABR) were measured in 535 children from 3 months to 3 years of age. The latencies reported in this paper should be unaffected by peripheral hearing loss because each child had bilateral wave V responses at 20 dB HLn. Wave V latencies decreased as age increased, at least to 18 months of age, while little or no change was noted in wave I latencies over the same age range. Thus, interpeak latency differences followed the same developmental time course as wave V. The shapes of wave V latency-level functions were comparable across age groups. These results suggest that changes in wave V latency with age are due to central (neural) factors and that age-appropriate norms should be used in evaluations of ABR latencies in children. Interaural differences in absolute wave V latencies and interpeak latency differences were similar to those observed in infants and adults, indicating that response symmetry is independent of age. Statistical analyses suggested that the distributions of absolute and relative latency measurements are normal, making it possible to describe norms in terms of means and standard deviations. A simple model is described that accounts accurately for changes in mean wave V latencies as function of age from preterm through the first three years of life.

Click-evoked auditory brainstem responses were measured in a patient with high-frequency conductive hearing loss. As is typical in cases of conductive hearing loss, Wave I latency was prolonged beyond normal limits. Interpeak latency differences were just below the lower limits of the normal range. The Wave V latency-intensity function, however, was abnormally steep. This pattern is explained by the hypothesis that the slope of the latency-intensity function is determined principally by the configuration of the hearing loss. In cases of high-frequency hearing loss (regardless of the etiology), the response may be dominated by more apical regions of the cochlea at lower intensities and thus have a longer latency.

A technique is described to estimate the sound pressure level developed by a broad frequency response transducer at the tympanic membrane. Real-ear probe tube measurements near the tympanic membranes of 10 subjects were used to obtain frequency-dependent correction values for a custom-designed flat-plate coupler. These latter measures can be used for routine calibration of the transducer. Audiometric thresholds from 250 to 16000 Hz were obtained on 14 children (5-18 years). Threshold estimates were found to be comparable to previously reported values. Potential application and limitations of this technique are discussed.

Two-tone "synchrony suppression" was studied in responses of single auditory nerve fibers recorded from anesthetized cats. Suppression thresholds for suppressor tones set to a fiber's characteristic frequency (CF) were approximately equal to discharge rate thresholds for CF tones. Suppression thresholds above and below CF were usually lower than the corresponding discharge rate thresholds. However, at all frequencies studied (including CF), suppression thresholds were higher than the corresponding thresholds for discharge synchronization. Across fibers, rates of suppression growth for suppressors at CF were greatest in low-CF fibers and least in high-CF fibers, and there was a systematic decrease in suppression growth rate at CF as CF increased. Within fibers, rates of suppression growth above CF were typically less than at CF, and slopes were monotonically decreasing functions of frequency. Within-fiber rates of suppression growth below CF were variable, but they usually were greater than rates of growth at CF. Iso-suppression contours (frequencies and intensities producing criterion amounts of suppression) indicated that tones near CF are the most potent suppressors at near-threshold intensities, and that the frequency producing the most suppression usually shifts downward as the amount of suppression increases. These data support the notion that synchrony suppression arises primarily as a passive consequence of hair cell activation.

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.

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.

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.

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.

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.

The interactions between phase, signal level, and interstimulus interval (delta t) were examined in the transition region between simultaneous and nonsimultaneous masking. Phase effects as large as 22 dB were observed in forward masking. These trends can be explained by considering the effects of phase upon the stimulus envelope.

Auditory brainstem responses (ABR) were obtained from graduates of an intensive care nursery (ICN) when those babies were in stable physiological states and ready for hospital discharge. Intensity ranged from ABR threshold to 80 dB nHL, and all recordings were made in a sound-isolated chamber. The data reviewed here are from 585 babies having presumably normal hearing, based upon bilateral ABR thresholds of 30 dB nHL or less. To insure that estimates of population statistics were not biased by high correlations between ears, only the data from the left ears were used in most analyses. Larger correlations were observed between conceptional age (CA) and ABR latencies than between either gestational age (GA) or chronological age (CHA) and the same latencies. Data were grouped into six CA groups for further analyses. Distributions of all response-component latencies were similar in shape and depended upon CA, showing orderly decreases in latency with increasing age. None of these distributions differed significantly from normal, and they were well fitted by normal ogives. Thus, accurate estimates of percentiles can be obtained from the means and standard deviations. The results indicate that it is important to take CA into account when evaluating ABR latencies.

Auditory brainstem responses were recorded from 20 normally hearing subjects using tone-burst stimuli that were gated with cosine-squared functions. Clear responses were observed over a wide range of frequencies and levels. These responses were highly reproducible within individual subjects and were reliably measured by two independent examiners. ABR thresholds were higher than behavioral thresholds for all frequencies, especially for lower frequencies. Intersubject variability also was greater for lower frequencies. Wave-V latencies decreased with increases in both frequency and level for frequencies from 250 to 8000 Hz and for levels from 20 to 100 dB SPL. The standard deviations seldom exceeded 10% of the mean wave-V latency for any combination of level and frequency. These latencies can be viewed as the sum of both a peripheral and a central component. Assuming that the central component is relatively independent of both frequency and level, changes of wave V latency must be related to peripheral factors, such as travel time along the cochlear partition, and to stimulus characteristics, such as rise time.

Probe-elicited wave V amplitudes of the auditory brainstem response (ABR) were measured using a forward-masking paradigm. Subjects were anesthetized cats. For individual experiments, probe frequency and intensity were fixed and masker frequencies and intensities were varied. For each masker frequency, the extent to which the probe-elicited wave V amplitude was reduced by the preceding masker was plotted as a function of masker intensity. The rising segments of the masking functions were fitted with straight lines, using a least-squares procedure, to obtain estimates of their slopes. Masking grew most rapidly for masker frequencies below probe frequency, becoming progressively less steep as masker frequency increased. ABR tuning curves were constructed by using the linear fits to define the masker intensity that caused a 50% reduction in probe-elicited wave V amplitude. The shapes of these tuning curves were comparable to whole-nerve action potential (AP) tuning curves obtained under similar stimulus conditions. These results indicate that ABR amplitude measurements in a forward-masking paradigm can be used to estimate the growth of response to masking stimuli and frequency selectivity in a manner similar to AP amplitude measurements.

ABR and behavioral thresholds were estimated as a function of stimulus duration for three normal and two hearing-impaired subjects. Stimuli were 2000-Hz tone bursts with 0.5-ms rise/fall times and durations ranging from 1 to 256 or 512 ms. For both groups of subjects, ABR thresholds were independent of stimulus duration. Normal subjects showed greater improvement in behavioral thresholds as a function of duration than did subjects with hearing losses. Thus, it appeared that ABR and behavioral thresholds were affected differently by changes in stimulus duration and that the magnitude of these differences could depend upon the presence of hearing loss. These data indicate that temporal integration may be one factor which makes comparisons between ABR and behavioral thresholds complicated. In the present study, the magnitude of hearing loss, measured by the ABR, would have been underestimated if normal behavioral thresholds for short-duration stimuli were used as the reference.

Performance-intensity functions for monosyllabic words were obtained as a function of signal-to-noise ratio for broadband and low-pass filtered noise. Subjects were 11 normal-hearing listeners and 13 hearing-impaired listeners with flat, moderate sensorineural hearing losses and good speech-discrimination ability (at least 86%) in quiet. In the broadband-noise condition, only small differences in speech perception were noted between the two groups. In low-pass noise, however, large differences in performance were observed. These findings were correlated with various aspects of psychophysical tuning curves (PTCs) obtained from the same individuals. Results of a multivariate analysis suggest that performance in broadband noise is correlated with filter bandwidth (Q10), while performance in low-pass noise is correlated with changes on the low-frequency side of the PTC.

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.

The effect of activating-signal bandwidth upon the threshold of the acoustic reflex (TAR) was measured. Subsequently, loudness measurements were made for the same signals at the same intensity levels that were required to elicit an acoustic-reflex response. When loudness and TAR are compared at comparable levels, similar trends emerged. Results from this experiment provide evidence for both qualitative and quantitative similarities between acoustic reflex and the perception of loudness. This, in turn, suggest that signals at TAR may be equally loud for listeners with normal hearing.