Deafness Research Foundation Research Articles

Two‐microphone measurement of acoustic intensity in the ear canal as a calibration of high‐frequency hearing

As an alternative to measures of ear canal pressure in the vicinity of the tympanic membrane, we propose an acoustic intensity measurement slightly downstream from the point of sound delivery as a convenient and meaningful calibration of high‐frequency hearing. If sound absorption in the air medium and at the walls of the ear canal is neglected, the power flow through any cross section of the ear canal would be equal to the power flow into the middle ear. A sound delivery and intensity measurement system has been built using a two‐port earmold. The tube of an Etymotic Research ER‐2 sound delivery system passes through one port and a pair of probe tubes (outer diameters = 1 mm, tip separation ≈ 3 mm) pass through the other. The latter are connected to ER‐7 microphones and used for acoustic intensity measurements based on a cross‐spectrum algorithm. Data are given on the sensitivity of measurement results with respect to the relative positions of the sound delivery point and probe tube tips. [Work supported by a grant from the Deafness Research Foundation.]

Vowel discrimination in primates

Discrimination often steady-state synthetic English vowels was measured in Old World monkeys (Macaca, Cercopithecus) and humans using a repeating standard AX procedure and positive-reinforcement operant conditioning techniques. All monkeys had difficulty discriminating spectrally similar vowels such as |ɑ-Λ| and |ν-μ|, but macaque monkeys were superior to cercopithecus monkeys. Human reaction times reflected spectral similarity and correlated with monkey percent correct discriminations. Macaque DLs for formant frequency changes along vowel continuua |ε-æ| and |i-ɪ| were about 30 Hz, compared to about 10 Hz for humans. Formant filtering and sensation level variation had only minor effects on DLs. Results are related to other comparative psychoacoustic data and vocal communication in the various monkey species. [Work supported by NIH and the Deafness Research Foundation.]

Masking level difference in infants

The binaural masking level difference (MLD) for a 500-Hz pure tone was estimated for infants between 6 and 8 months of age. A masker with bandwidth of 600 Hz (No = 35 dB SPL), geometrically centered at the test frequency, was presented in phase to a pair of TDH-49 earphones. Each subject was tested in each condition (NoSo and NoSπ) in a single session, the sequence counterbalanced among the subjects. A computer-controlled visual reinforcement head-turn procedure was used. Masked threshold was estimated using a simple up-down staircase procedure with a 5-dB stepsize. Infant MLD estimates compare favorably with those of an adult control group. However, the infants' masked-threshold estimates, used in computing the MLD, were evaluated relative to the adult group for each condition. [Work supported by Deafness Research Foundation.]

Pulsation threshold and simultaneous masking: 2AFC results

The pulsation threshold is the signal level at which a series of brief signals appears to be (illusorily) continuous when the intervening intervals are filled with a “masker” (different signal source). It is generally measured by an adjustment or tracking procedure. As a function of the frequency relation of the signal and “masker,” pulsation threshold results are similar in many respects to simultaneous masking patterns. It has been suggested that the two techniques are generally measuring the same aspects of hearing. This was tested by using a 2AFC PEST procedure: pulsation threshold was defined as the signal level for 75% correct discrimination between illusory continuity (signal on only when masker off) and true continuity (signal on at all times). Results were essentially identical to those obtained with a 2AFC PEST simultaneous masking procedure, as well as with a standard tracked pulsation threshold procedure. Given the difficulty which naive subjects often have with finding a stable criterion for pulsation threshold judgments, the 2AFC procedure may be preferable to adjustment tasks. [Work supported by the Deafness Research Foundation.]

A computer-controlled psychoacoustic laboratory for under $10 000

Given a setup time of only a few months and a budget of only $10K, a researcher with very little background in electronics or computing can assemble an amazingly versatile system. Our configuration uses a popular microcomputer and commercially available A-D-A and parallel digital port and add-on boards for control functions. Analog voltage outputs control frequency and amplitude of two phase-lockable signal generators. After bandpass filtering, the signals are shaped and gated by electronic switches, and fine level adjustments are made by programmable attenuators (all under software control); signals are then mixed and delivered to headphones. Simple programs can be written in BASIC lot other languages) for a wide variety of psychophysical procedures and experiments, such as psychophysical tuning curves, automatic up-down tracked fixed-frequency audiograms, and 2AFC PEST masked thresholds; data and summary statistics are printed on paper. The only difficulty facing the would be researcher is finding (or finding funds for) an adequately sound-isolated test chamber. [Work supported by the Deafness Research Foundation.]

Crib-o-gram (COG) and ABR effect of variables on test results

This study compared seven categories of apparently relevant information concerning the Crib-o-gram (COG) as screening test and evaluated the following variables: birth weight, gestational age at birth, Apgar scores at 1 and 5 min, apnea, ototoxic drugs, number of days on assisted ventilation, and gentamicin blood levels. Low birth weight, early gestational age, and apnea were significantly correlated with COG results. Thirteen of the 55 newborns tested failed the COG at least twice before having the ABR test given. Twelve infants had normal ABR and one had same degree of hearing loss. Consequently the rate of false positive finding is very high. In spite of the high risk factors of the newborns, gentamicin, Apgar scores, or number of days on a ventilator were not implicated as a cause of hearing loss. [Work supported by Deafness Research Foundation.]

Asymmetrical frequency discrimination in primates

Auditory frequency difference limens at 0.5, 1.0, 2.0, and 4.0 kHz were measured monaurally in humans and in four Old World monkey species: Japanese and rhesus macaques (Macaca fuscata, Macaca mulatta) and vervet and deBrazza monkeys (Cercopithecus aethiops, Cercopithecus neglectus). A go, no‐go, repetitive‐standard discrimination procedure was employed with positive reinforcement. At certain frequencies and sensation levels, humans and macaques exhibit superior discrimination for frequency increments, while under these same conditions the Cercopithecus species exhibit superior discrimination for frequency decrements. Possible coding mechanisms to account for these results will be discussed, including monitoring rate increases versus rate decreases in asymmetrically tuned auditory nerve fibers. In one species, the Japanese macaque, there is some evidence that superior discrimination of frequency increments may relate to the design features of the animal's vocal communication system. [Supported by NSF, Sloan Foundation, and Deafness Research Foundation.]

Evidence for a resonant mechanism in auditory hair cell stereocilia

A mathematical analysis of stereocilia height and cochlear tonotopic organization along the length of the receptor epithelium in six species provided the basis for a resonant model of hair cell stereocilia. The analysis demonstrated an inverse power function between the distribution of frequency (f) and stereocilia height (h). The average empirical relation was: f∝h−2.67. These results are consistent with a resonant “clamped bar” model which has the relation: f∝h−2.00. The model provides conceptual insight into the mechanical properties of stereocilia by showing that Young's Modulus remains constant for all stereocilia in a given cochlea. The mechanical properties of stereocilia can be understood further in terms of their macromolecular structure and the bending of crossbridge molecules. The uniform bending of every crossbridge molecule, coupled with the constant number of crossbridges per unit volume within the stereocilia, leads to a constant value for Young's Modulus. Finally, the model raises the intriguing possibility that nature may have evolved two frequency selective mechanisms in the cochlea. [Work supported in part by The Deafness Research Foundation and NINCDS.]

Species differences in neural lateralization of species-specific calls

Petersen et al. [Science 202, 324–327 (1978)] reported that Japanese macaques exhibited a right ear advantage (REA} when listening to a semantically distinctive feature of their vocalizatioos. Several heterospecific monkeys tested in identical conditions with the Japanese monkeys' calls failed to show any reliable ear advantage. However, recent studies with humans have shown that the size and/or direction of ear advantages vary with the physical feature to which subjects attend. This raised the question of whether the species differences in lateralization obtained in our setting represented (1) a processing difference for the same signal feature or (2) a difference between species in the signal feature to which they attended. To choose between those alternatives we ascertained whether Japanese and heterospecific monkeys were in fact attending to the same acoustic feature when they showed their respective patterns of lateralization. Two Japanese and two heterospecific monkeys received extensive training on a discrimination task that required them to classify 15 vocalizations into two categories. During this time the Japanese monkeys exhibited REA's but the heterospecifics showed no ear advantage. The animals were then tested for generalization to 25 novel, natural vocalizations and six synthetic calls. These tests revealed that all four animals were attending to the same feature of the Japanese monkey calls. The animals were then returned to the original training stimuli and they exhibited their earlier patterns of lateralization. Thus this study shows that even when attending to the same physical feature, Japanese and heterospecific monkeys employ different neural processing strategies. This suggests that the communicative valence of the signals used in testing may account for the lateralization differences observed between species. [Work supported by NSF. Deafness Research Foundation and Sloan Foundation.]

Accuracy of Crib-o-gram as a screening test on high risk newborns

In the present study, Crib-o-gram (COG) and auditory brainstem responses (ABR) were recorded in fifteen high risk newborns in the intensive care unit selected by the following criteria: birth weight < 2200 g, asphyxia, artificial ventilation, respiratory failure, and potential ototoxic medication (aminoglycosides). The infants tested ranged in gestational age from 27 to 40 weeks. The COG testing was performed by placing a motion-sensing transducer beneath the infant's crib mattress. All 15 infants failed the COG at least two times before having the ABR test given. The ABR testing was performed in a soundproof room using a Nicolet 1170. Twelve infants had normal ABR, one had some sensorineural loss and two infants were deaf. The rate of false positive finding is very high. Consequently, the accuracy of the COG is questionable. In spite of the high risk factors of the newborns it is difficult to implicate aminoglycosides as a single cause of hearing loss. We feel very strongly that a routine ABR evaluation should be performed on all high risk newborns. [Work supported by Deafness Research Foundation.]

Cochlear effects of locally applied inhibitors

The round window has been used as a route for introducing ototoxic substances into the inner car in order to bypass barriers between the systemic circulation and the inner car. We have used this method to locally administer sodium, potassium‐ATPase inhibitors and chloride transport inhibitors to the inner ear of the chinchilla. Drugs dissolved in Ringer solution were applied on the round window membrane. Endocochlear potential (EP) was recorded from the basal turn using the round window approach. The EP was not altered following application of Ringer solution as a control. Following application of ouabain (10 mM), the EP steadily declined. After vanadate (10 mM), the EP initially increased, and subsequently declined. Sanguinarine did not alter the EP. The loop diuretics furosemide and piretanide caused a marked decline in the EP after local application. However, the stilbene derivative DIDS did not alter the EP after topical application. These findings raise questions about whether the loop diuretics have any effect on chloride transport in the cochlea and makes it appear unlikely that active chloride transport contributes to the normal EP. [Work supported by NIH and Deafness Research Foundation.]

Relation between simultaneous‐masking psychophysical tuning curves and cubic‐difference‐tone amplitudes for Fm < Fp

The detection of cubic difference tones (CDTs) generated by the masker and probe could influence the shapes of simultaneous‐masking psychophysical tuning curves (PTCs) if listeners increase the masker level until the CDT, as well as the probe, becomes inaudible. To test this possibility, simultaneous‐masking PTCs were measured for a 125‐ms, 2000‐Hz probe at 30, 45, and 60 dB SPL temporally centered in a 250‐ms masker in four normally hearing listeners using a method‐of‐adjustment procedure. The amplitudes of CDTs produced by the combination of masker and probe from the low‐frequency side of the PTC (Fm < Fp) were then determined using pulsation‐threshold and cancellation procedures. In all cases, a CDT was measurable, i.e., above masked threshold, indicating that CDT threshold did not establish the tuning‐curve masker level. Measurements of CDT amplitude as a function of L 1 for fixed L 2s (30,45, and 60 dB SPL) also suggest that for pulsed masker and probe stimuli, listeners did not rely upon the masking of the CDT in the simultaneous‐masking PTC task. [Work supported by the Deafness Research Foundation and NIH.]

The relations among critical bands, critical ratios, and intensity difference limens in man

Band‐narrowing estimates of the critical bandwidth (CB) are consistently larger than critical‐ratio (CR) estimates for the same signal frequency. Bilger [in Hearing and Davis: Essays Honoring Hallowell Davis, edited by S. K. Hirsh et al. (Washington U. P., St. Louis, 1976), p. 191] hypothesized that this difference can be accounted for by reference to intensity‐discrimination performance (CF[Hz]/CB[Hz] = ΔI/I). To test this hypothesis, band‐narrowing, critical‐ratio, and intensity‐discrimination data were collected from four subjects. Signal frequency was 2000 Hz and two noise spectrum levels were used: 20 and 50 dB. The psychophysical procedure was an adaptive, two‐interval, forced‐choice, converging on 70.7% correct. Based on critical‐bandwidth estimates obtained from the band‐narrowing experiments using common two‐line regression techniques, critical‐ratio estimates for pure‐tone detection in broadband noise, and intensity‐discrimination for critical bands of noise, the results across experimental conditions for individual subjects are not well described by Bilger's equation. [Work supported by NIH and the Deafness Research Foundation.]

A comparison of frequency and intensity discrimination in humans and monkeys

Although Stebbins [Am. J. Phys. Anthropol. 38, 357–364 (1973)] has reported that humans are “slightly” superior to monkeys in frequency and intensity discrimination, these comparisons were made across separate studies using different test procedures. The present experiment compared the discriminative capacities of humans and monkeys (Macaca fuscata) under identical test conditions. A repetitive standard technique was used in which subjects reported changes in the frequency or intensity of an ongoing train of 1-kHz tone pulses, presented monaurally at 50 dB SL. Human and monkey ΔI's for intensity increments were very similar (1.5–2.5 dB). However, human ΔF's for frequency increments (1–2 Hz) were clearly superior to those of monkeys (15–20 Hz). Species differences also occurred when subjects were required to detect decrements in frequency or intensity. Human − ΔF's (2–3 Hz) were slightly larger than those for frequency increments, but monkeys − ΔF's were about two to three times as large (30–50 Hz). Finally, human − ΔI's were similar to those for intensity increments, but we have as yet been unable to train monkeys to reliably report intensity decrements as large as −10 dB. Implications of these results for using monkeys as models of human speech sound perception and for the design of species-specific vocalizations will be discussed. [Work supported by NSF, NIH, and Deafness Research Foundation.]

Psychophysical tuning curves: Stimulus ensemble and level effects

Monaural masked thresholds were obtained for tone bursts in forward-masking, simultaneous masking, and pulsation-threshold paradigms as a function of masker frequency and probe level. Thresholds were obtained in both the presence and absence of a continuous, broadband background noise adjusted to a constant E/N0 = 15. Probe frequencies were 300, 1000, an 3000 Hz and probe levels were 10, 30 and 50 dB SL (re threshold in the absence of the background noise). Data were collected primarily with a method-of-adjustment psychophysical procedure. Some data were also collected for the limiting conditions with a two-interval forced-choice procedure for comparison. The data were plotted as psychophysical tuning curves. The extent to which physical parameters of the stimulus ensembles are sufficient to account for differences in tuning-curve parameters and the necessity to consider alternative explanations will be discussed. [Research supported by PHS Grant No. RR-07096, and grants from NINCDS and the Deafness Research Foundation.]

The search for a structural basis of temporary threshold shift: Some old and new ideas about hair cell fatigue

A number of observations have been made about changes in hair cell structure after noise exposure which causes a temporary hearing loss. These include a proliferation in the subsurface cistern of the endoplasmic reticulum, changes in membrane permeability on the reticular lamina, swelling of the intracellular compartment, decoupling of the stereocilia—tectorial membrane attachment in outer hair cells, the fusion of stereocilia, and the loss of stereocilia rigidity. Recent evidence suggests that this latter observation may be important. The actin filaments contained within the stereocilia are packed with paracrystalline order. Evidence will be presented to show that the macromolecular bridges between actin filaments appear to be damaged by noise. Loss of these crossbridges might reduce stereocilia rigidity and mechanically decouple the hair cell from its driving force. The possibility that stereocilia may regain their paracrystalline order, and thus their rigidity, during post‐exposure recovery will be considered. [Supported in part by the Deafness Research Foundation.]

The origins of the cochlear efferent bundle of Rasmussen in primates

The spatial distribution of neurons emitting fibers of the olivocochlear bundle was determined by injecting horseradish peroxidase into the round window in a series of new and old world monkeys. Animals were perfused 24–28 h later. Sections through the pons and medulla were reacted with tetramethyl benzidine and counterstained with thionin and/or neutral red. Perikarya containing reaction product numbered over 800 in one case. The greatest concentration of labeled cells was located in the dorsal periolivary area intermediate between the dorsal margins of the lateral and medial superior olivary nuclei. Approximately 400 cells were ipsilateral to the injection, opposed to 100 contralaterally. The dorsolateral periolivary area contained over 100 labeled cells ipsilaterally, and only about one-third as many contralaterally. While 95 labeled cells occupied the region of the contralateral medial preolivary nucleus, about two-thirds as many labeled cells were found in the comparable ipsilateral nucleus. Although a few labeled neurons were present bilaterally in the lateral preolive and the lateral superior olivary nucleus, no labeled cells were found in the region of the dorsomedial or ventromedial periolivary nuclei (medial to the medial superior olive). [Work supported by the Deafness Research Foundation.]

The effects of probe duration on simultaneous-masking tuning curves

Monaural masked thresholds were obtained for tone bursts in a simultaneous-masking paradigm as a function of masker frequency and probe duration. Probe frequency was 1000 Hz and probe durations ranged from 20 to 250 ms. Temporal-integration functions were determined for the probe and it was presented at both fixed SLs SPLs. Masker duration was fixed at 250 ms. Data were collected primarily with a method-of-adjustment procedure. The extreme probe durations were also measured using a two-interval forced-choice procedure. The effects of probe duration in simultaneous-masking paradigms on estimates of frequency selectivity based on tuning-curve parameters will be discussed. [Research supported by PHS Grant No. RR-07096, and grants from NINCDS and the Deafness Research Foundation.]

Critical periods of susceptibility to acoustic trauma in animals

In recent years the possibility that a maturing cochlea passes through a period of time in which it is unusually susceptible to acoustic trauma from noise exposure has been studied in a number of species. The conclusion from several sources suggests that the critical period for susceptibility to acoustic trauma is associated with developmental events within the cochlea. The data that support this conclusion will be reviewed. Evidence will also be presented to show the long‐term maturation of middle‐ear admittance in certain species. This phenomenon indicates that cochlear input may continue to develop over an extended period, and the implications of this for susceptibility to acoustic trauma in young ears will be considered. [Work supported by the Deafness Research Foundation.]

Effects of glutamate and aspartate on tone-evoked and spontaneous activity in posterior ventral and dorsal cochlear nucleus

As previously reported [Caspary and Havey, Neurosci. Absts. 5, 17 (1979)], iontophoretic application of the excitant amine acids, glutamate and aspartate, can affect thresholds and response patterns of cochlear nucleus (CN) neurons. This and recent data [Godfrey et al., J. Histochem. Cytochem. 25, 417 (1977); Wenthold, Brain Res. 143, 544 (1978); 162, 338 (1979)] support the hypothesis that glutamate and/or aspartate may be the neurotransmitter(s) at acoustic nerve endings. Control rate-intensity curves were obtained from neurons in the posterior ventral CN “on” and “chopper” responders) and dorsal CN (“pauser” and “build-up” responders). Rate-intensity curves shifted to the left with iontophoretic application of effective dose levels of glutamate and aspartate. Data plotted as percent increase (above control response) against intensity demonstrated that the largest changes occurred at near threshold intensities. “Chopper” neurons displayed increased non-tone-evoked activity at lower doses (2–150 hA) of excitant amino acids than “on,” “pauser,” and “build-up” neurons (100–250 nA). These data further demonstrate that glutamaro and aspartate can differentially affect CN neurons. [Supported by NIH and the Deafness Research Foundation.]