Interaural Time Difference Thresholds Research Articles

Temporal weighting for interaural time differences in low-frequency pure tones

In contrast to envelope-based interaural time differences (ITD) at high frequencies, where sound onsets play a dominant role, the reliability and salience fine-structure ITD at low frequency (<1500 Hz) suggests uniform sensitivity to information across periods of an ongoing stimulus waveform. Several past studies, however, have demonstrated low-frequency ITD thresholds to improve sub-optimally with increasing sound duration [e.g. Houtgast & Plomp 1968, JASA 44:807-12], suggesting that the initial periods of a brief tone play a greater role in ITD processing than do later periods. Here, we measured the temporal profile of ITD sensitivity in pure tones ranging from 250-1000 Hz. Sounds were presented with ITD that either remained fixed over the sound duration (condition RR) or progressed linearly to eliminate the ITD cue from either the beginning (condition 0R) or end (R0) of the sound. Durations varied from 40-640 ms, including 20 ms ramps applied diotically to minimize envelope cues. ITD detection thresholds demonstrated (a) suboptimal improvement with duration and (b) greater sensitivity to ITD available early (R0) rather than late (0R) in the stimulus, a pattern nearly identical to that observed for high-frequency envelope ITD. [Supported by NIH R01 DC011548.]

Discrimination of auditory motion patterns: The mismatch negativity study

The aim of the present study is to test whether mismatch negativity (MMN) response can be elicited by changes in auditory motion dynamics. The discrimination of auditory motion patterns was investigated using psychophysical and electrophysiological methods in the same group of subjects. Auditory event-related potentials (ERP) were recorded for stationary midline noises and moving noises shifting to the left/right from the head midline. Two patterns of auditory motion were used with gradual (Motion) and stepwise (Step) movements which started and ended at the same loci. Auditory motion was produced by linear and abrupt changes of interaural time differences (ITD) in binaurally presented stimuli.In Experiment 1, ERPs were recorded for stationary midline standards and for Motion and Step deviants. It was found that Step deviants result in larger MMN amplitudes than Motion deviants with the same distance travelled, which implies that information contained in the stimulus midportion could be involved in the processing of the auditory motion. The threshold ITD values for the detection of Step and Motion stimuli displacement obtained during psychoacoustic tests were greater than the minimal ITD changes which elicited significant MMN. Experiment 2 demonstrated that Step deviants elicited significant MMNs in the context of Motion standards, although these stimuli could not be discriminated behaviourally. MMNs elicited by Step deviants in different acoustic contexts are discussed from the viewpoint of different brain processes underlying the discrimination of the abrupt ITD change.These results suggest that the early cortical mechanism of auditory motion processing reflected by MMN could not be considered as a spatial discriminator of the onset/offset stimulus positions, that is, a simple onset-offset detector. Combining psychoacoustic data with MMN results we may conclude that motion discrimination in auditory system might be better at the preattentive level.

Using low-frequency threshold interaural time differences to test models of binaural hearing

All models for detecting an interaural time difference (ITD) begin with model cross-correlator cells. Models differ in their inputs (excitatory/inhibitory) and in the distribution of cells with respect to interaural delay and frequency. For instance, the Jeffress model postulates a broad distribution on delay. Also, alternative binaural displays are based on different moments of the cross-correlation. For instance, the centroid is based on the first moment. The different models predict different frequency dependences of the threshold ITD in the limit of low frequency. Limiting behavior was computed for the various models using different assumptions about the frequency dependence of the synchrony of inputs to cross-correlator cells and about the sharpness of the rate-ITD function from the cells. The results were compared with the measured low-frequency functional behavior of ITD thresholds for four human listeners, which ranged from ƒ-0.8 to ƒ-1.4. These measured exponents disagree with predictions from some combinations of models. In particular, the popular centroid display within the Jeffress model tends to lead to slopes that are steeper than observed experimentally. [Work supported by the National Natural Science Foundation of China grant 61175043 and the Air Force Office of Scientific Research grant 11NL002.]

Interaural spectral asymmetry and sensitivity to interaural time differences

Listeners' ability to discriminate interaural time difference (ITD) changes in low-frequency noise was determined as a function of differences in the noise spectra delivered to each ear. An ITD was applied to Gaussian noise, which was bandpass filtered using identical high-pass, but different low-pass cutoff frequencies across ears. Thus, one frequency region was dichotic, and a higher-frequency region monotic. ITD thresholds increased as bandwidth to one ear (i.e., monotic bandwidth) increased, despite the fact that the region of interaural spectral overlap remained constant. Results suggest that listeners can process ITD differences when the spectra at two ears are moderately different.

Lateralization of acoustic signals by dichotically listening budgerigars (Melopsittacus undulatus)

Sound localization allows humans and animals to determine the direction of objects to seek or avoid and indicates the appropriate position to direct visual attention. Interaural time differences (ITDs) and interaural level differences (ILDs) are two primary cues that humans use to localize or lateralize sound sources. There is limited information about behavioral cue sensitivity in animals, especially animals with poor sound localization acuity and small heads, like budgerigars. ITD and ILD thresholds were measured behaviorally in dichotically listening budgerigars equipped with headphones in an identification task. Budgerigars were less sensitive than humans and cats, and more similar to rabbits, barn owls, and monkeys, in their abilities to lateralize dichotic signals. Threshold ITDs were relatively constant for pure tones below 4 kHz, and were immeasurable at higher frequencies. Threshold ILDs were relatively constant over a wide range of frequencies, similar to humans. Thresholds in both experiments were best for broadband noise stimuli. These lateralization results are generally consistent with the free field localization abilities of these birds, and add support to the idea that budgerigars may be able to enhance their cues to directional hearing (e.g., via connected interaural pathways) beyond what would be expected based on head size.

Frequency dependence of the interaural time difference thresholds in human listeners.

Interaural time difference (ITD) thresholds for sine tones were measured as a function of frequency with unprecedented resolution along the frequency axis. The tone level was 70 dB SPL, and the method was a two‐interval forced‐choice, three‐down one‐up staircase. Overall, the lowest thresholds occurred near 1000 Hz. At lower frequencies, thresholds varied more rapidly than the expected 1/f law, suggesting a growing deficit in elemental ITD processors as characteristic frequency decreases. At higher frequencies, thresholds increased dramatically with increasing frequency. Measurements at 50‐Hz increments were able to obtain a threshold for only one listener at 1500 Hz, but no threshold at 1550 Hz. In summary, performance varied from best to impossible over a range of about half an octave. In that sense, ITD thresholds appear to show the most dramatic frequency dependence of any auditory quantity. [Work supported by the Vice‐rectorate for Faculty and Academic Planning, Universidad Politecnica de Valencia an...

The effect of binaural coherence on envelope interaural time difference thresholds.

Room reflections alter the envelope of sounds differently at both ears, reducing binaural coherence. Experiment 1 measured interaural time difference (ITD) discrimination thresholds for broadband and transposed speech. A sentence was convolved with binaural room impulse responses for different source-receiver distances, and the envelopes extracted and multiplied with 4-kHz tones. Preliminary results indicate that envelope ITD thresholds increased from <100 μs without reverberation to >700 μs at four times the reverberation radius. ITD thresholds for unprocessed speech were more robust to the addition of reverberation, only rising above 350 μs at the same distance. To investigate whether reverberation is detrimental owing to reduction in binaural coherence, stimulus envelopes were created by temporally jittering raised cosine pulses around a 10-ms separation. Bilaterally independent jittering allowed variation in coherence while minimizing change in other envelope parameters. Preliminary results show ITD thresholds <100 μs for coherent envelopes, increasing to >700 μs for coherences of 0.6, a value consistent with the room simulations above. Envelope coherence strongly affects ITD discrimination, suggesting that ITDs extracted from high-frequency channels may not provide useful information in many realistic situations. This has implications for bilateral cochlear implant users, as current devices provide ITDs only in envelopes.

Interaural time difference thresholds as a function of the duration of the beginning of a sound in hearing-impaired adults.

The precedence effect indicates that the auditory system places most weight on spatial information at the onset of a sound. In this experiment, we measured how well hearing-impaired adults can ignore what comes immediately after the onset of a sound, as any limitations in that should interfere with localization in environments with lots of reflections. The stimulus was designed to be a simplified analog of a room impulse response: it consisted of two bursts of speech-shaped noise concatenated together: the first was short and given some ITD (representing an ideal onset that marked direction correctly), but the second was far longer and was interaurally uncorrelated (representing later reflections and reverberation from all possible directions). We measured psychometric functions (over the ITD and duration of the first burst) to determine the minimum duration of the first burst needed to report its direction. Presently, 19 listeners have completed the task. No significant correlation was found between minimum first-burst duration and hearing loss (better-ear loss = 3–50 dB), but the individual variation was considerable, especially so in those listeners with larger hearing losses. The results indicate that some listeners will have difficulty in ignoring irrelevant information after a sound’s onset.

Interaural Time and Level Difference Thresholds for Acoustically Presented Signals in Post-Lingually Deafened Adults Fitted with Bilateral Cochlear Implants Using CIS+ Processing

The main purpose of the study was to measure thresholds for interaural time differences (ITDs) and interaural level differences (ILDs) for acoustically presented noise signals in adults with bilateral cochlear implants (CIs). A secondary purpose was to assess the correlation between the ILD and ITD thresholds and error scores in a horizontal-plane localization task, to test the hypothesis that localization by individuals with bilateral implants is mediated by the processing of ILD cues. Eleven adults, all postlingually deafened and all bilaterally fitted with MED-EL COMBI 40+ CIs, were tested in ITD and ILD discrimination tasks in which signals were presented acoustically through headphones that fit over their two devices. The stimulus was a 200-msec burst of Gaussian noise bandpass filtered from 100 to 4000 Hz. A two-interval forced-choice adaptive procedure was used in which the subject had to respond on each trial whether the lateral positions of the two sound images (with the interaural difference favoring the left and right sides in the two intervals) moved from left-to-right or right-to-left. In agreement with previously reported data, ITD thresholds for the subjects with bilateral implants were poor. The best threshold was approximately 400 microsec, and only five of 11 subjects tested achieved thresholds <1000 microsec. In contrast, ILD thresholds were relatively good; mean threshold was 3.8 dB with the initial compression circuit on the implant devices activated and 1.9 dB with the compression deactivated. The ILD and ITD thresholds were higher than previously reported thresholds obtained with direct electrical stimulation (generally, <1.0 dB and 100 to 200 microsec, respectively). When the data from two outlying subjects were omitted, ILD thresholds were highly correlated with total error score in a horizontal-plane localization task, computed for sources near midline (r = 0.87, p < 0.01). The higher ILD and ITD thresholds obtained in this study with acoustically presented signals (when compared with prior data with direct electrical stimulation) can be attributed-at least partially-to the signal processing carried out by the CI in the former case. The processing strategy effectively leaves only envelope information as a basis for ITD discrimination, which, for the acoustically presented noise stimuli, is mainly coded in the onset information. The operation of the compression circuit reduces the ILDs in the signal, leading to elevated ILD thresholds for the acoustically presented signals in this condition. The large magnitude of the ITD thresholds indicates that ITDs could not have contributed to the performance in the horizontal-plane localization task. Overall, the results suggest that for subjects using bilateral implants, localization of noise signals is mediated entirely by ILD cues, with little or no contribution from ITD information.

Localizing noise in rooms via steady‐state interaural time differences

Noise lateralization experiments with headphones show that the steady‐state interaural time difference (ITD) is, or is not, a valuable cue for sound lateralization depending on the height of the peak of the interaural cross‐correlation function (cc). To discover whether these headphone results could be extended to the localization of sound in rooms, binaural recordings were made of one‐third‐octave noises in diverse environments: a dry classroom, a laboratory, and a reverberation room. Sound sources and a KEMAR recording manikin were moved about in each room to vary the cc from 0.2 to 0.8. The recordings, with ITDs digitally controlled, were presented to five listeners by headphones to measure listener sensitivity to ITD variations. It was found that sensitivity was almost entirely a function of cc with little weight attached to the actual room environment. ITD thresholds decreased monotonically with increasing cc. In agreement with earlier headphone studies using computed noises instead of room recordings, typical thresholds were higher by a factor of about 6 for a 2800‐Hz band where envelope ITD is used compared to a 700‐Hz band where waveform ITD is used. [Work supported by the NIDCD.]

Contributions of Intrinsic Neural and Stimulus Variance to Binaural Sensitivity

The discrimination of a change in a stimulus is determined both by the magnitude of that change and by the variability in the neural response to the stimulus. When the stimulus is itself noisy, then the relative contributions of the neural (intrinsic) and stimulus induced variability becomes a critical question. We measured the contribution of intrinsic neural noise and interstimulus variability to the discrimination of interaural time differences (ITDs) and interaural correlation (IAC). We measured discharge rate versus characteristic frequency (CF) tone ITD functions, and CF-centered narrowband noise ITD and IAC functions in interleaved blocks in the same units in the inferior colliculus of urethane-anesthetized guinea pigs. Ten "frozen" tokens of noise were synthesized and the responses to each token were separately analyzed to allow the relative contributions of intrinsic and stimulus variability to be assessed. ITD and IAC discrimination thresholds were determined for a simulated two-interval forced-choice experiment, based on the firing rate distributions, using receiver operating characteristic analysis. On average, between stimulus variability contributed 19% (range, 1.5-30%) of the variance in noise ITD discrimination and 27% (range, 3-50%) in IAC discrimination. Noise ITD thresholds were slightly higher than tone ITD thresholds. Taking the mean of the thresholds for individual noise tokens gave a similar result to pooling across all noise tokens. This implies that although the stimulus induced variability is measurable, it is insignificant in relation to the intrinsic noise in ITD and IAC discrimination.

Learning to discriminate interaural time differences: An exploratory study with amplitude-modulated stimuli

The advent of bilateral cochlear implants (CIs) has increased interest in learning on binaural tasks, and studies in normal-hearing listeners provide important background information. However, few studies have considered learning with discrimination of interaural time difference (ITD). Here, learning with ITD was explored using stimuli that are more relevant to bilateral CIs than used previously. Inexperienced listeners were trained with envelope-based ITD using high-frequency amplitude-modulated tones with or without an interaural carrier frequency difference (IFD), the former to simulate asymmetrical bilateral CI insertions. All were tested with and without IFD before and after training. In most listeners, ITD thresholds improved substantially with training, not necessarily reaching asymptote after 3000 trials. In these, the magnitude and time-course of learning was larger than anticipated from a previous study with low-frequency ITD. Learning generalized across IFD and the effect of IFD on ITD thresholds at post-test was smaller than reported previously. These results have implications for studies of bilateral CIs, such as the need to provide extensive training to avoid over-estimating any apparent ‘impairment’.SumarioEl advenimiento de los implantes cocleares bilaterales (CI) ha aumentado el interés en aprender sobre tareas binauriculares, y los estudios en sujetos oyentes normales han aportado importante información pertinente. Sin embargo, pocos estudios han considerado el estudio del aprendizaje con base en la discriminación de diferencias interauriculares de tiempo (ITD). Aquí, el aprendizaje con ITD fue explorado utilizando estímulos más relevantes para los CI bilaterales que los usados previamente. Se entrenó a oyentes sin experiencia con una ITD con base en envolventes que comprendían tonos de alta frecuencia de amplitud modulada, con o sin diferencia interauricular de la frecuencia portadora (IFD), esta última para simular inserciones bilaterales asimétricas de CI. Todos fueron evaluados con y sin IFD antes y después del entrenamiento, sin alcanzar necesariamente la asíntota después de 3000 intentos. En ellos, la magnitud y el tiempo del aprendizaje fue mayor de lo anticipado en un estudio con una ITD de baja frecuencia. El aprendizaje en general en todas las IFD y el efecto de la IFD sobre los umbrales de ITD después de la prueba fue similar a los reportes previos. Estos resultados tienen implicaciones en los estudios de CI bilaterales, tales como la necesidad de proveer amplio entrenamiento para evitar la sobre-estimación de cualquier ‘impedimento’ aparente.

Neural Sensitivity to Interaural Envelope Delays in the Inferior Colliculus of the Guinea Pig

Interaural time differences (ITDs) are important cues for mammalian sound localization. At high frequencies, sensitivity to ITDs, which are conveyed only by the envelope of the waveforms, has been shown to be poorer than sensitivity to ITDs at low frequencies, which are conveyed primarily by the fine structure of the waveforms. Recently, human psychophysical experiments have demonstrated that sensitivity to envelope-based ITDs in high-frequency transposed tones can be equivalent to low-frequency fine-structure-based ITD sensitivity. Transposed tones are designed to provide high-frequency auditory nerve fibers (ANFs) with similar temporal information to that provided by low-frequency tones. We investigated neural sensitivity to ITDs in high-frequency transposed and sinusoidally amplitude modulated (SAM) tones, in the inferior colliculus of the guinea pig. Neural sensitivity to ITDs in transposed tones was found to be greater than that to ITDs in SAM tones; in response to transposed tones, neural firing rates were more modulated as a function of ITD and discrimination thresholds were found to be lower than those in response to SAM tones. Similar to psychophysical findings, ITD discrimination of single neurons in response to transposed tones for rates of modulation <250 Hz was comparable to neural discrimination of ITDs in low-frequency tones. This suggests that the neural mechanisms that mediate sensitivity to ITDs at high and low frequencies are functionally equivalent, provided that the stimuli result in appropriate temporal patterns of action potentials in ANFs.

Threshold differences for interaural time delays carried by double vowels.

Experimental measurements were made of threshold interaural time differences (ITDs) for a "target" vowel presented simultaneously with a fixed-ITD "distracter" vowel. Three double-vowel pairs were used, comprising an "er" (/e/) together with either an "ai," "ar," or "oo" (respectively, /e/, /c/, and /u/). Threshold ITDs were found to be larger for the target vowel when it was part of a double-vowel pair than in control conditions in which it was presented alone. The effect size depended upon the choice of target vowel and distracter vowel, the level of the target relative to the distracter, and whether the two vowels had the same or different fundamental frequencies. The experiment was analyzed using a multichannel modification of Heller and Trahiotis' [J. Acoust. Soc. Am. 99, 3632-3637 (1996)] model, which used a weighted combination of the detectabilities of the ITD of the target and the distracter. It gave predictions consistent with the observed effects of level and with some of the effects of the choice of target vowel, but it could not describe the effect of the target-distracter differences in fundamental frequency. It was found that a single-channel version of the model, in which the chosen channel was allowed to depend upon fundamental frequency (which could be derived using a monaural autocorrelation model) did give a set of predictions in qualitative accord with the data.

Precedence-effect thresholds for a population of untrained listeners as a function of stimulus intensity and interclick interval

Data are reported from 127 untrained individuals under lag- and single-click conditions in a precedence-effect task. In experiment I, each subject completed ten runs in a two-interval forced-choice design under a lag-click condition and three runs under a single-click condition. The cue to be discriminated was an interaural time difference (ITD). Stimuli were 125-micros rectangular pulses and the interclick interval (ICI) was 2 ms. Subjects were randomly assigned to three groups of approximately 30. Each group was tested at one stimulus intensity (43, 58, or 73 dB). Mean threshold within each group was greater than 500 micros for lag-click ITD conditions, although substantial intersubject variability and a clear effect of stimulus intensity on lag-click ITD thresholds were observed, with lower thresholds for higher intensities. In experiment II, the ICI was varied from 0.3 to 10 ms, and thresholds were obtained from groups of approximately 20 untrained subjects. Data were also collected from three highly experienced observers as a function of ICI. The best naive subject produced mean thresholds near, but not as low as those obtained from experienced subjects. Analysis of adaptive-track patterns revealed abrupt irregularities in threshold tracking, consistent with either losing the cue or listening to the wrong cue in an ambiguous stimulus.

Binaural hearing in dolphins

Binaural hearing is an advantage of having two ears. Human benefits are evident in a 3-dB threshold difference, the ability to localize sound sources in space, and the ability to isolate primary sounds from corresponding echoes. The binaural capabilities of dolphins are relatively unexplored. Studies show that their localization of pure tones underwater is mediated by the same mechanisms observed in terrestrial mammals. Behavioral evidence from free-field localization studies supports reliance on time and intensity cues. Two studies have examined binaural hearing in dolphins using contact hydrophones to isolate the hearing mechanisms. They provided masking level differences (MLDs) comparable to humans and interaural time and intensity difference thresholds that were better than any recorded for terrestrial mammals. Neurophsyiological studies using evoked potentials investigated interaural sensitivity and intensity differences as a function of multiple frequency stimuli presented at various angles around the head of a dolphin. Recent behavioral studies have mapped the acoustic sensitivity around the head and lower jaw. Those results suggest greater sensitivity forward of the panbone, thought to be the site of best reception, and an asymmetry in sensitivity that may be analogous to that observed in other animals such as the barn owl.

Beyond presbyacusis: Non-hearing loss-related temporal processing deficits in the elderly

A group of 56 elderly individuals were tested in a battery of tests including pure tone audiometry, various word- and sentence-level speech understanding tasks, and measurement of spectral, temporal, and spatial auditory abilities. Presbyacusic hearing loss, when present, was symmetrical in the two ears and at most mild to moderate, with no significant deficit below 1.5 kHz. Frequency resolution was estimated by calculating auditory filter width from notched-noise masking at 1 kHz; temporal resolution was estimated from gap discrimination and from auditory segregation of temporal patterns of harmonic complexes with components below 1.2 kHz (in the low envelope frequency range); spatial resolution in the horizontal plane was estimated from interaural time difference (ITD) thresholds for 500-Hz clicks. The largest differences between results of the elderly and those of normal-hearing young subjects were seen for speech understanding in spatially distributed babble, word recognition in reverberant rooms, ITD thresholds, and segregation of temporal patterns. The results portray the decline of temporal processing as an important component of auditory and speech understanding dysfunctions in aging and strongly suggest that a non-negligible part of these dysfunctions may be of central origin. [Supported by the National Institute on Aging and the V. A. Medical Research.]

Temporal processing in the aging auditory system.

Measures of monaural temporal processing and binaural sensitivity were obtained from 12 young (mean age = 26.1 years) and 12 elderly (mean age = 70.9 years) adults with clinically normal hearing (pure-tone thresholds < or = 20 dB HL from 250 to 6000 Hz). Monaural temporal processing was measured by gap detection thresholds. Binaural sensitivity was measured by interaural time difference (ITD) thresholds. Gap and ITD thresholds were obtained at three sound levels (4, 8, or 16 dB above individual threshold). Subjects were also tested on two measures of speech perception, a masking level difference (MLD) task, and a syllable identification/discrimination task that included phonemes varying in voice onset time (VOT). Elderly listeners displayed poorer monaural temporal analysis (higher gap detection thresholds) and poorer binaural processing (higher ITD thresholds) at all sound levels. There were significant interactions between age and sound level, indicating that the age difference was larger at lower stimulus levels. Gap detection performance was found to correlate significantly with performance on the ITD task for young, but not elderly adult listeners. Elderly listeners also performed more poorly than younger listeners on both speech measures; however, there was no significant correlation between psychoacoustic and speech measures of temporal processing. Findings suggest that age-related factors other than peripheral hearing loss contribute to temporal processing deficits of elderly listeners.

Comparison of Interaural Time and Intensity Difference Discrimination in Patients with Temporal Lobe Lesions

We studied the sound lateralization in 15 patients with left (12 patients) or bilateral (3 patients) temporal lobe cerebrovascular lesions. The lesions included the auditory cortex. Interaural time difference (ITD) and interaural intensity difference (IID) discriminations were separately measured by using a self-recording apparatus. All 12 patients with left temporal lobe lesions could discriminate ITD, and their ITD thresholds were significantly higher than those in healthy volunteers. None of the 3 patients with bilateral temporal lobe lesions could discriminate ITD. On the other hand, all patients with either unilateral or bilateral temporal lobe lesions could discriminate IID and their IID thresholds were significantly higher than those in healthy volunteers. The present findings indicate that the auditory cortex plays an important role in discriminating both cues, but that the role may not be essential for IID.

A study of audiologic findings in the elderly

We divided people more than 65 years of age into groups, by 5- year age intervals, and performed hearing tests based on pure tone, speech sound, and inner ear and retrocochlear hearing functions. Thus, we studied the mechanism of hearing disorder progression with aging in an attempt to identify factors involved in decreased speech sound recognition. The following results were obtained. 1. The regression coefficient of speech discrimination ability and the mean hearing threshold level were greater in the elderly patients than in the younger patients with cochlear hearing loss. 2. Abnormal decreases in speech discrimination ability evaluated on the basis of the mean hearing threshold level occur more frequently with advancing age. 3. The SISI score increased rapidly in patients 80 years of age or older. 4. Directional hearing function (interaural time difference threshold) varied more markedly with age. 5. Little difference in SSB clipping speech articulation was seen among elderly subjects less than 80 years old, though scores were lower than those of the control group. However, it was decreased in patients 80 or more years old. These results show that the effect of a raised pure-tone hearing threshold due to decreased inner ear function and the effect of recruitment phenomenon on speech hearing were the same in patients from 65 to the late 70s; however, the effect of retrocochlear hearing loss clearly increased with age. Therefore, we consider patients 80 or more years old, to have an impaired inner ear function which is aggravated and that this, in association with advanced retrocochlear disturbance, leads to greatly decreased speech discrimination ability.