Hearing Res Research Articles

Developmental time course of auditory perception of modulation speech cues

Speech contains strong amplitude modulation (AM) and frequency modulation (FM) cues, which are commonly assumed to play an important role in speech identification for adults. We will review recent studies aiming to characterize the development of auditory perception of AM and FM speech cues for 6 and/or 10-month-old infants learning French or Mandarin [e.g., Cabrera, L., Tsao, F.-M., Gnansia, D., Bertoncini, J., & Lorenzi C. (2014), J. Acoust. Soc. Am. 136, 877–882; Cabrera, L., Bertoncini, J., & Lorenzi, C. (2013), J. Speech, Lang., Hearing Res. 56, 1733–1744]. These studies were based on vocoders, which are analysis and synthesis systems designed to manipulate the modulation components of speech sounds in a given number of frequency bands. Overall, the results suggest that: (i) the auditory processing of AM and FM speech cues is “functional” by 6 months, (ii) the auditory processing of the AM and FM cues is fine-tuned by language exposure between 6 and 10 months. These findings may help improving current models of modulation processing that do not take into account the plasticity of the auditory and speech-processing system.

Phonetics exercises using the Alvin experiment-control software

A collection of computer exercises was developed for use in teaching phonetic transcription to students taking introductory coursework in phonetics. The exercises were developed using Alvin, a software package for the design and online control of behavioral experiments [Hillenbrand and Gayvert, J. Speech, Lang., Hearing Res. 48, 45–60 (2005)]. The main goal was to allow students to work independently on the routine drill that is needed to learn sound-symbol associations. For example, for a vowel transcription exercise, students hear naturally spoken /hVd/ syllables and are asked to choose among twelve buttons labeled with phonetic symbols. Feedback is provided on each trial, and students have the option of repeating any trials with incorrect responses. Also included are word/phrase transcription exercises in which students hear an utterance and are asked to provide a phonetic transcription. Correct transcriptions are provided following each trial, and a summary of the student's performance is displayed at the end of the exercise. Reverse transcription exercises are also included in which a phonetic transcription is displayed and the student's job is to enter the word or phrase in ordinary orthography.

Incorporating cepstral peak prominence as an acoustic method for assessing variation in voice quality

Cepstral Peak Prominence (CPP) has been proposed as a means of assessing breathiness from recordings (Hillenbrand et al., J. Speech and Hearing Res. 37(1994):769-78). CPP involves computing the deviation of the cepstral peak in a smoothed cepstrum from a regression function for the smoothed cepstrum. It has not been tested extensively, though the few evaluative studies, involving comparisons against auditory judgments of voice quality, have yielded positive results. Previous evaluations have not assessed the use of CPP for comparisons of inter-group or intra-individual variation. A reliable way to gauge breathiness is useful not only for therapeutic purposes, but also for informing researchers about a speaker’s repertoire and identity construction. This study examines the efficacy of CPP for analyzing social variables at the individual and group levels, using African American and European American subjects producing read and spontaneous speech under controlled conditions. Results show intra-speaker consistency across speaking tasks, but no consistent inter-ethnic differences. These findings suggest that some aspects of phonation may be more important for individual identity than for group identity.

Protection by low-dose kanamycin against noise-induced hearing loss in mice: Dependence on dosing regimen and genetic background

We recently demonstrated that sub-chronic low-dose kanamycin (KM, 300 mg/kg sc, 2×/day, 10 days) dramatically reduces permanent noise-induced hearing loss (NIHL) and hair cell loss in 1 month old CBA/J mice (Fernandez et al., 2010, J. Assoc. Res. Otolaryngol. 11, 235–244). Protection by KM remained for at least 48 h after the last dose, and appeared to involve a cumulative effect of multiple doses as part of a preconditioning process. The first month of life lies within the early ‘sensitive period’ for both cochlear noise and ototoxic injury in mice, and CBA/J mice appear exquisitely vulnerable to noise during this period (Ohlemiller et al., 2011; Hearing Res. 272, 13–20). From our initial data, we could not rule out 1) that less rigorous treatment protocols than the intensive one we applied may be equally—or more—protective; 2) that protection by KM is tightly linked to processes unique to the sensitive period for noise or ototoxins; or 3) that protection by KM is exclusive to CBA/J mice. The present experiments address these questions by varying the number and timing of fixed doses (300 mg/kg sc) of KM, as well as the age at treatment in CBA/J mice. We also tested for protection in young C57BL/6J (B6) mice. We find that nearly complete protection against at least 2 h of intense (110 dB SPL) broadband noise can be observed in CBA/J mice at least for ages up to 1 year. Reducing dosing frequency to as little as once every other day (a four-fold decrease in dosing frequency) appeared as protective as twice per day. However, reducing the number of doses to just 1 or 2, followed by noise 24 or 48 h later greatly reduced protection. Notably, hearing thresholds and hair cells in young B6 mice appeared completely unprotected by the same regimen that dramatically protects CBA/J mice. We conclude that protective effects of KM against NIHL in CBA/J mice can be engaged by a wide range of dosing regimens, and are not exclusive to the sensitive period for noise or ototoxins. While we cannot presently judge the generality of protection across genetic backgrounds, it appears not to be universal, since B6 showed no benefit. Classical genetic approaches based on CBA/J × B6 crosses may reveal loci critical to protective cascades engaged by kanamycin and perhaps other preconditioners. Their human analogs may partly determine who is at elevated risk of acquired hearing loss.

Alveolar and velar stop releases during speech development.

It has been shown that children learning English tend to use tongue-to-palate contact patterns while producing velar and alveolar stop consonants that are not as well differentiated as for adults [Cheng et al., J. Speech, Language, Hearing Res. 50, 375–392 (2007)]. The electropalatographic experiments suggest that young children have not developed the fine motor control necessary for mature articulation of lingual consonants. It is possible that this may persist to become a speech disorder as they mature. We offer a complementary perspective that focuses on the mismatch between the acoustic and aerodynamic scaling from children’s vocal tracts to adult vocal tracts. In learning the motor control necessary to make a good distinction between alveolar and velar stops may be hindered by scaling mismatches. In this paper we will highlight acoustic measures of four children recorded every 6 months from the ages of 12–48 months playing with a care giver. We will consider both the formant frequencies at release and the shape of the burst spectrum to both help characterize the development acoustically and to infer what is causing the acoustics in the articulation. [Work supported by Grant No. NIDCD-RO1-001247.]

Determining underwater auditory thresholds in fish based on particle motion.

Acoustic particle motion is believed to play a significant role in the auditory scene of fishes even though most all hearing sensitivity data are measured with respect to sound pressure level. Auditory motion thresholds have been measured in air for goldfish (Carassius auratus), oscars (Astronotus ocellatus), and toadfish (Opsanus tau) by moving the fish’s body using a carefully calibrated shaker table [R. R. Fay, Science 225, 951–954 (1984); Z. Lu, A. N. Popper and R. R. Fay, J. Comp. Physiol. A 179, 227–233 (1996); R. R. Fay and P. L. Edds‐Walton, Hearing Res. 111, 1–21 (1997)]. These responses depend on both direction and amplitude of the motion. Thus, underwater particle motion thresholds must include both direction and amplitude, but most experimental setups are not designed for this purpose. Passive and active methods to control the direction and amplitude of acoustic particle motion underwater are presented. It is not enough just to measure the local particle motion in a small tank and report its amplitude as a threshold, especially when incidental sound pressure could be contributing to the auditory response and the orientation of the fish with respect to the direction of the particle motion vector is ill defined.

Statistical Analyses of Temporal Information in Auditory Brainstem Responses to Tones in Noise: Correlation Index and Spike-distance Metric

Gai and Carney (J Neurophysiol 96:2451-2464, 2006) previously explored the detection of tones in noise based on responses in the anteroventral cochlear nucleus; that study focused on temporal information in discharge reliability and analyses of neural responses related to the fine structure or envelope of the stimulus. Two additional temporal approaches, the correlation index (Joris et al., Hearing Res 216-217:19-30, 2006) and the spike-distance metric (Victor and Purpura, J Neurophysiol 76:1310-1326, 1996; Netw Comput Neural Syst 8:127-164, 1997), are tested in the present study. Trends in the correlation index as a function of stimulus level are similar to those of the synchronization coefficient (also called the vector strength) when the tone is presented alone. However, the present study found that trends in the correlation index did not agree with those of the synchronization coefficient for tones presented with relatively high-level background noise. Instead, trends in the correlation index generally agreed with those of the temporal reliability metric discussed in Gai and Carney (J Neurophysiol 96:2451-2464, 2006); that is, the correlation index decreased with increased tone level in the presence of relatively high-level background noise. The spike-distance metric, which was based on absolute spike times or on interspike intervals, was compared to the temporal measures described above, which were generally based on relative spike times. The results confirm that the spike-distance metric is not an optimal temporal metric. In addition, absolute spike times of primary-like responses generally contained much less temporal information than absolute spike times of chopper response types. The present study highlights the importance of relative spike-timing information as characterized by traditional and novel temporal measures.

A multicompartmental cochlear model with piezoelectric outer hair cells

A hydromechanical, multicompartment model of the cochlea, which employed a phenomenological outer hair cell (OHC) cell‐body electro‐motility, was able to mimic the physiologically measured response of the basilar membrane (BM) [A. E. Hubbard et al., ‘‘Time‐domain responses from a nonlinear sandwich model of the cochlea,’’ in Biophysics of the Cochlea: From Molecule to Model, edited by A. W. Gummer (World Scientific, Singapore)]. An improved model that included OHC electro‐anatomic parameters and scalar electrical parameters was able to mimic cochlear microphonic (CM) data at low frequencies, but not BM responses at high‐frequencies, because the OHC membrane capacitance severely reduces OHC transmembrane potential. To improve the high‐frequency performance, we used a piezoelectrical model of the OHC. Consequently, the mechanical loading of the OHC translates bidirectionally with the electrical impedance of the OHC. The new model compares well with physiological data from Gerbil at 40 dB SPL [T. Ren and A. Nuttall, ‘‘Basilar membrane vibration in the basal turn of the sensitive gerbil cochlea,’’ Hearing Res. 151, 48–60 (2001)]. [This work was supported by NIH.]

Exploring the role of spectral integration in the perception of vowel sounds

Building on the important work of Chistovich and Lublinskaya [Hearing Res. 1, 185–195 (1979)], Syrdal [Speech Commun. 4, 121–135 (1985)] proposed that vowel category boundaries tend to occur in quantal regions corresponding to a 3.5 Bark bandwidth of spectral integration. This proposal was shown to be consistent with acoustic distributions of certain American English vowels. Some initial perceptual results [K. A. Hoemeke and R. L. Diehl, J. Acoust. Soc. Am. 96, 661–674 (1994)] also tended to support the role of a 3.5 Bark integrator in determining major perceptual boundaries among vowel categories. However, other evidence, reviewed here, raises doubts about the generality of Syrdal’s proposal. The negative evidence includes both within- and cross-language variation in the location of vowel category boundaries assumed to be governed by the 3.5 Bark integrator and the absence of elevated discriminability in putative quantal regions corresponding to the spectral limit of integration. [Work supported by NIDCD.]

On generation of statoconia in gravireceptors of mollusks

Two models of development of statoconia in the statocyst of mollusks, based on the experimental data [Hearing Res. 49 (1990) 63; Hearing Res. 109 (1997) 125; Hearing Res. 109 (1997) 109;] are proposed. The purpose of the present work is to apply mathematical modeling to the analysis of mechanisms of statoconia formation and generation by supporting cells at the stage of their accumulation in the cyst lumen. In the case of Aplysia californica, it is not clear whether there is a temporal change of statoconia due to their growth in the cyst lumen similar to that in Biomphalaria, or whether the growth of statoconia occurs in supporting cells before they get into the cyst lumen. This question has to do with a more general and insufficiently investigated problem of the mechanisms of statoconia evolution during their stage of accumulation. This is related to A. californica as well as to the initial phase of development of Biomphalaria glabrata. This problem is of practical importance because the majority of experiments related to the study of the effects of altered gravity on the development of gravireceptors in the two mollusks A. californica and B. glabrata deals with the initial phase of statoconia development. It is assumed that two main processes determine the evolution of statoconia in developing mollusks: generation of new statoconia by growing supporting cells and growth of statoconia sizes in the cyst lumen. Analysis of experimental data related to the generation of statoconia in Aplysia and comparison of these data with the results of modeling of accumulation of statoconia suggest that the basic mechanism of evolution of size distribution of statoconia in Aplysia is growth of embryonic statoconia in supporting cells, that follows the growth of animal size. Thus, the large sizes of statoconia are determined by their development within supporting cells rather than by their growth in the cyst lumen. Analysis of the data concerning Biomphalaria allows us to assume that distribution of supporting cells which generate statoconia also varies. The results of modeling of evolution of statoconia specify necessary additional experiments, which are required to refine and test the model.

Improved temporal coding of sinusoids in electric stimulation of the auditory nerve using desynchronizing pulse trains.

Rubinstein et al. [Hearing Res. 127, 108-118 (1999)] suggested that the representation of electric stimulus waveforms in the temporal discharge patterns of auditory-nerve fiber (ANF) might be improved by introducing an ongoing, high-rate, desynchronizing pulse train (DPT). To test this hypothesis, activity of ANFs was studied in acutely deafened, anesthetized cats in response to 10-min-long, 5-kpps electric pulse trains that were sinusoidally modulated for 400 ms every second. Two classes of responses to sinusoidal modulations of the DPT were observed. Fibers that only responded transiently to the unmodulated DPT showed hyper synchronization and narrow dynamic ranges to sinusoidal modulators, much as responses to electric sinusoids presented without a DPT. In contrast, fibers that exhibited sustained responses to the DPT were sensitive to modulation depths as low as 0.25% for a modulation frequency of 417 Hz. Over a 20-dB range of modulation depths, responses of these fibers resembled responses to tones in a healthy ear in both discharge rate and synchronization index. This range is much wider than the dynamic range typically found with electrical stimulation without a DPT, and comparable to the dynamic range for acoustic stimulation. These results suggest that a stimulation strategy that uses small signals superimposed upon a large DPT to encode sounds may evoke temporal discharge patterns in some ANFs that resemble responses to sound in a healthy ear.

Frequency responses of two- and three-tone distortion product otoacoustic emissions in Mongolian gerbils.

The frequency responses of distortion product otoacoustic emission (DPOAEs) were investigated in adult Mongolian gerbils. The main goal was to investigate in this species the extent to which DPOAE measurements might be useful in estimating cochlear frequency-tuning characteristics. Specifically, this study investigated the parameter space for generation of DPOAEs to determine those regions, if any, where the emission responses gave "simple" frequency responses, i.e., responses similar in form to typical neural responses. At the same time, it was desired to determine in this species the existence, extent, and nature of the more complex three-tone emission frequency responses as observed in some other species [e.g., Martin et al., Hearing Res. 136, 105-123 (1999)]. In the present work, two-tone frequency response curves (f2/f1 ratio functions) were obtained by varying the lower frequency, f1, while holding the f2 frequency and both amplitudes (L1, L2) constant. Only for frequencies, f2, near 8 kHz did the response at the emission frequency, 2 f1-f2, form a simple, relatively broad peak. At all lower frequencies, the two-tone frequency response curve was typically complex and composed of multiple peaks. In comparison, three-tone frequency responses were constructed by fixing the primary stimulus pair (f1, f2) and varying a third tone widely in frequency (f3) and intensity (L3). Points in f3 and L3 which caused a criterion reduction in primary emission amplitude (at 2 f1-f2) were used to construct emission suppression tuning curves (STCs). Only for primary frequencies, f2, at 8 kHz and above were the emission STCs found to be simple, with shapes similar to neural frequency-tuning curves. At lower primary frequencies, particularly for relatively low primary frequency ratios (low f2/f1), three-tone responses were very complex. This complex response usually included a region of anomalous suppression in which very low suppression levels (L3) could result in significant decreases in the primary emission amplitude, often exceeding 12 dB. Regions of such anomalous suppression were typically observed under the following conditions: (1) for all f2 frequencies from 0.5 to 4 kHz; (2) for f3 frequencies between 1.4 and 8 kHz; (3) i.e., for f3 frequencies 1-3 octaves above the primary frequency, f2; (4) at L3 levels often 10 dB lower or more than the usual "best frequency" threshold, i.e., even lower than the relative minimum threshold found near the primary stimulus frequencies; (5) exhibiting sharp amplitude decreases often accompanied by emission phase shifts of about 180 deg; (6) present in both cubic emissions (2 f1-f2 and 2 f2-f1); (7) to be less extreme at larger primary stimulus frequency ratios (larger f2/f1); and (8) less extreme at larger intensity ratios (larger L1/L2). Because of the anomalous behavior at f2 frequencies below 8 kHz, "simple" emission STCs were typically only obtainable, if at all, near the extreme boundaries of the parameter space giving measurable emission amplitudes.

The phases of auditory-nerve fiber responses to tones: Dependence on stimulus frequency and intensity

As stimulus intensity increases, the phases of responses to tones of squirrel monkey auditory-nerve fibers [ANFs; Anderson et al., J. Acoust. Soc. Am. 49, 1131–1139 (1971)] and guinea pig inner hair cells [IHCs; Dallos, Hearing Res. 22, 185–198 (1986)] undergo lags and leads, respectively, at frequencies lower and higher than the characteristic frequency (CF) and remain relatively constant at frequencies near CF. A similar pattern holds for basal sites of the basilar membrane (BM) in several species. In contrast, tectorial-membrane (TM) vibrations at apical sites of the chinchilla cochlea exhibit phase shifts of opposite polarity [Rhode and Cooper, Aud. Neurosci. 3, 101–121 (1996)]. To explore this inconsistency between neural and mechanical behavior at apical cochlear regions, we investigated the phases of responses to tones of ANFs recorded in deeply anesthetized chinchillas. For stimulus intensities lower than 70 dB SPL, responses of most ANFs (regardless of CF) exhibited phase shifts similar to those observed in squirrel monkey ANFs, guinea pig IHCs, and basal BM sites in several species. Thus, the inconsistency between neural and TM mechanical data cannot be ascribed to species differences or a dependence of phase behavior on CF. [Work supported by NIH Grant DC-00419.]

The dynamic range of inner hair cell and organ of Corti responses.

Inner hair cell (IHC) and organ of Corti (OC) responses are measured from the apical three turns of the guinea pig cochlea, allowing access to regions with best, or most sensitive, frequencies at approximately 250, 1000, and 4000 Hz. In addition to measuring both ac and dc receptor potentials, the average value of the half-wave rectified response (AVEHR) is computed to better reflect the signal that induces transmitter release. This measure facilitates comparisons with single-unit responses in the auditory nerve. Although IHC ac responses exhibit compressive growth, response magnitudes at high levels depend on stimulus frequency. For example, IHCs with moderate and high best frequencies (BF) exhibit more linear responses below the BF of the cell, where higher sound-pressure levels are required to approach saturation. Because a similar frequency dependence is observed in extracellular OC responses, this phenomenon may originate in cochlear mechanics. At the most apical recording location, however, the pattern documented at the base of the cochlea is not seen in IHCs with low BFs around 250 Hz. In fact, more linear behavior is measured above the BF of the cell. These frequency-dependent features require modification of cochlear models that do not provide for longitudinal variations and generally depend on a single stage of saturation located at the synapse. Finally, behavior of dc and AVEHR responses suggests that a single IHC is capable of coding intensity over a large dynamic range [Patuzzi and Sellick, J. Acoust. Soc. Am. 74, 1734-1741 (1983); Smith et al., in Hearing--Physiological Bases and Psychophysics (Springer, Berlin, 1983); Smith, in Auditory Function (Wiley, New York, 1988)] and that information compiled over wide areas along the cochlear partition is not essential for loudness perception, consistent with psychophysical results [Viemeister, Hearing Res. 34, 267-274 (1988)].

Mice Lacking the Basolateral Na-K-2Cl Cotransporter Have Impaired Epithelial Chloride Secretion and Are Profoundly Deaf

In chloride-secretory epithelia, the basolateral Na-K-2Cl cotransporter (NKCC1) is thought to play a major role in transepithelial Cl(-) and fluid transport. Similarly, in marginal cells of the inner ear, NKCC1 has been proposed as a component of the entry pathway for K(+) that is secreted into the endolymph, thus playing a critical role in hearing. To test these hypotheses, we generated and analyzed an NKCC1-deficient mouse. Homozygous mutant (Nkcc1(-/-)) mice exhibited growth retardation, a 28% incidence of death around the time of weaning, and mild difficulties in maintaining their balance. Mean arterial blood pressure was significantly reduced in both heterozygous and homozygous mutants, indicating an important function for NKCC1 in the maintenance of blood pressure. cAMP-induced short circuit currents, which are dependent on the CFTR Cl(-) channel, were reduced in jejunum, cecum, and trachea of Nkcc1(-/-) mice, indicating that NKCC1 contributes to cAMP-induced Cl(-) secretion. In contrast, secretion of gastric acid in adult Nkcc1(-/-) stomachs and enterotoxin-stimulated fluid secretion in the intestine of suckling Nkcc1(-/-) mice were normal. Finally, homozygous mutants were deaf, and histological analysis of the inner ear revealed a collapse of the membranous labyrinth, consistent with a critical role for NKCC1 in transepithelial K(+) movements involved in generation of the K(+)-rich endolymph and the endocochlear potential.

Effect of removing physiologically relevant sound-localization parameters on tuning properties of barn owl tectal neurons

Barn owls use the interaural time difference (ITD) for locating sounds in azimuth and the interaural level difference (ILD) for locating sounds in elevation. Neurons in the optic tectum have spatially restricted receptive fields. Recently, Keller et al. [Hearing Res. 118, 13–34 (1998)] have demonstrated that the spatial patterns of responses were almost indistinguishable in response to virtual [stimuli filtered with head-related transfer functions (HRTFs)] and to free-field stimulation in neurons of the inferior colliculus. The inferior colliculus provides input to the optic tectum. The spatial restriction of tectal neurons is mainly due to their sensitivity to ITD and ILD. This paper deals with the effect of removing ITD and/or ILD from the virtual stimuli on the tuning of tectal neurons. HRTFs of two barn owls were recorded, and noise stimuli with ITDs, ILDs, filtered with HRTFs, filtered with HRTFs but containing no ITDs, and filtered with HRTFs but without ILDs were used in the experiments. Preliminary results suggest that removal of ITDs or ILDs caused a substantial loss of spatial tuning in tectal neurons. These results support earlier findings that suggested an essential role of the tectal neurons in sound-localization behavior. [Work supported by GIF.]

Estimating the phase characteristic of the auditory system from masking by harmonic complexes

As shown by several recent studies of positive and negative ‘‘Schroeder-phase’’ stimuli, masking by harmonic complexes is strongly affected by the temporal waveform of the masker [e.g., V. Summers and M. Leek, Hearing Res. 118, 139–150 (1998)]. Differences in masking by the Schroeder waves provide a starting point for estimating the phase response of the auditory system. In this study, the gradient of phase change across masker bandwidth was varied by altering the Schroeder-phase algorithm. Depending on signal frequency (ranging from 500 to 4000 Hz), differences in masking could be 5–10 dB for phase gradients changing by only 10%. The phase gradient that resulted in a minimum amount of masking varied systematically with signal frequency, with low frequencies masked least effectively by waveforms with rapidly changing component phases, and high frequencies by waveforms with more shallow phase gradients. This relationship between phase gradient and signal frequency is taken to reflect the phase response of the auditory filter at that frequency. The phase gradients associated with minimal masking at each frequency can be used to construct a piecewise-estimate of the phase characteristic of the auditory system for individual listeners. [Work supported by NIH DC00626.]

Are inner or outer hair cells the source of summating potentials recorded from the round window?

The relative contribution of inner hair cells (IHCs) and outer hair cells (OHCs) to the production of the summating potential (SP) is unresolved in the literature. Since OHCs in the base of the cochlea have been reported to produce little dc receptor potential except at very high sound pressure levels [I. J. Russell and P. M. Sellick, J. Physiol (London) 284, 261-290 (1983)], the IHCs appear to be the dominant source of the SP. However, results of intracochlear recordings are conflicting, although deriving from measurements in different turns of the cochlea [e.g., I. J. Russell and P. M. Sellick, J. Physiol. (London) 284, 261-290 (1983) versus P. Dallos and M. A. Cheatham, Sensory Transduction (1992)]. To determine which type of hair cells is the dominant source of the SP recorded at the round window, we used carboplatin to selectively destroy IHCs or a combination of IHCs and OHCs in the chinchilla. Related work, using measurements of distortion product otoacoustic emissions and cochlear potentials to assess the functional status of the OHCs served to validate this animal model [Trautwein et al., Hearing Res. 96(1-2), 71-82 (1996)]. The SP, cochlear microphonic (CM), and compound action potential (CAP) were recorded from the round window, and cochleograms were determined using well-established histological methods. The results were reasonably distinctive among three groups of ears--control (from untreated normal chinchillas), IHC-loss (extensive IHC loss with minor or no loss of OHCs), and IHC-OHC loss (total IHC loss plus extensive loss of OHCs over the basal half of the cochlea). Ears of chinchillas in the IHC loss group had a decrease of over 50% in SP output compared to control ears with the exact reduction depending somewhat upon the stimulus conditions. Ears in the IHC + OHC loss group, nevertheless, showed even further reduction in SP output which was clearly attributable to destruction of OHCs in the cochlear base. It was concluded that, although the IHCs are responsible for a greater contribution of dc-receptor potential to the SP recorded at the round window, a significant contribution is made by the OHCs, as well. The results suggest, specifically, that the round window "sees" SP output roughly in inverse proportion to the IHC:OHC. Lastly, the complexity of SP production, as recorded from the round window, precludes a completely straightforward interpretation of the SP:CAP in clinical ECochG.

Functional anatomy of the middle ear of insectivores

An electrical model has been constructed of the middle ear of land mammals [Hemilä et al., Hearing Res. 85, 31–44 (1995)]. This model can roughly predict the nature of air hearing in an animal, given the values of certain anatomical parameters for that animal. In the present study, light microscopy was used to investigate the anatomy of the middle ears of some representative members of the Insectivora. The nature of air hearing in these mammals was predicted, using an augmented version of the model. The model predicts that within the family Chrysochloridae (golden moles) there are genera (e.g., Chrysochloris and Chrysospalax) that have ears which appear to be very poorly adapted to the impedance-matching function required for efficient air hearing. The structure of the relatively enormous mallei of golden moles in these genera suggests that their ossicles are primarily adapted towards transmission of gound vibrations. A mechanism of ossicular inertia has been proposed. The electrical model predictions imply that subterranean mammals may be preadapted to evolve a more efficient mechanism for bone conduction of ground vibrations. Certain unusual features of the middle ear apparatus of other subterranean mammals may be interpreted in this light. [Work supported by B.B.S.R.C.]

Interactions of forward and simultaneous masking in intensity discrimination.

Intensity coding mechanisms are explored in a paradigm involving both forward and simultaneous masking. For intensity discrimination of 1000-Hz pure tone in quiet, a near-miss to Weber's law is observed. However, as more stimulus components are added to this relatively simple experiment, interactions among components produce a more complex pattern of results. An intense forward masker, while not causing any threshold shift for the test tone, produces a nonmonotonic intensity discrimination function ["the midlevel hump," Zeng et al., Hearing Res. 55, 223-230 (1991)]. The midlevel hump can be removed by the presence of additional notched noise [Plack and Viemeister, J. Acoust. Soc. Am. 92, 1902-1910 (1992)] or narrow-band noise whose level is increased along with the test tone's standard level. The same midlevel hump can also be enhanced by a fixed-low-level notched noise or a high-level, high-pass noise which causes minimal masking at the test frequency. Interactions of forward masking and simultaneous masking present a serious problem for a clear interpretation of these results. For example, the notched noise was originally intended to restrict off-frequency listening, but on-frequency masking compromised this original purpose and confounded the interpretation of the notched noise effects. By measuring systematically the growth-of-masking functions, the present study identified various interactions of forward and simultaneous masking and clarified the role of off-frequency listening in forward-masked intensity discrimination. Both peripheral and central mechanisms may have contributed to the occurrence, reduction and enhancement of the midlevel hump under these masking conditions.