Interaural Intensity Discrimination Research Articles

Forward-masked monaural and interaural intensity discrimination

Intensity-discrimination thresholds were measured for a 25-ms, 6-kHz pure tone for pedestal levels from 40 to 90 dB sound pressure level (SPL) with and without a forward masker (100-ms narrowband Gaussian noise, N(0)=70 dB). When the masker was present, the masker and probe were separated by 100 ms of silence. Unmasked and masked thresholds were measured in a two-interval monaural procedure and, separately, in a single-interval interaural procedure in which the pedestal and incremented pedestals were presented simultaneously to opposite ears. While the monaural thresholds were elevated markedly by the forward masker for mid-level pedestals, interaural thresholds were nearly unaffected by the masker across pedestal levels. The results argue against the notion that the monaural elevation in forward-masked thresholds is due to degraded encoding of intensity information at early stages of auditory processing.

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両耳間音圧差を用いた自覚的および他覚的両耳機能検査の比較

両耳間音圧差を用いた自覚的および他覚的両耳機能検査の比較

Monaural and interaural intensity discrimination: level effects and the "binaural advantage".

This study examined whether the level effects seen in monaural intensity discrimination (Weber's law and the "near miss") in a two-interval task are also observed in discrimination of interaural intensity differences (IIDs) in a single-interval task. Both tasks were performed for various standard levels of 4-kHz pure tones and broadband noise. The Weber functions (10 log deltaI/I versus I in dB) in the monaural and binaural conditions were parallel. For noise, the Weber functions had slopes close to zero (Weber's law) while the Weber functions for the tones had a mean slope of -0.089 (near miss). The near miss for the monaural and binaural tasks with tones was eliminated when a high-pass masker was gated with the listening intervals. The near-miss was also observed for 250- and 1000-Hz tones in the binaural task despite overall decreased sensitivity to changes in IID at 1000 Hz. The binaural thresholds showed a small (about 2-dB) advantage over monaural thresholds only in the broadband noise conditions. More important, however, is the fact that the level effects seen monaurally are also seen binaurally. This suggests that the basic mechanisms responsible for Weber's law and the near miss are common to monaural and binaural processing.

Level effects in monaural and interaural intensity discrimination

Discrimination of interaural level differences (ILDs) is often assessed using a two-interval task in which ILD changes between intervals. In such a situation, overall level is usually roved in order to minimize monaural cues. This roving makes it difficult to assess level-dependent effects. In the present experiment, monaural intensity discrimination in a 2IFC task was compared to ILD discrimination in a single-interval task, thus producing an analogous situation in terms of number of ‘‘observations’’ and eliminating the need for an overall level rove. Monaural intensity DLs and ILD discrimination thresholds were measured as a function of level for 4-kHz tones and for broadband noise. The Weber functions (10<th>logΔI/I vs I in dB) in the monaural and binaural conditions were parallel. For the noise the Weber functions had slopes close to zero (Weber’s law); for the tones the slopes were −0.082 (near-miss to Weber’s law). Overall, the binaural thresholds showed a small, approximately 2 dB, advantage over the monaural thresholds. The important aspect, however, is that the level effects seen monaurally are also seen binaurally. This suggests that the basic processes responsible for Weber’s law and the near-miss occur prior to binaural interaction. [Work supported by NIDCD DC05343 and DC00683.]

Cross-frequency interaction for interaural time and intensity discrimination in listeners with normal hearing and impaired hearing

The ability to discriminate interaural differences in complex signals was examined in listeners with normal hearing and with bilateral sensorineural hearing impairment. Just noticeable differences (jnds) for interaural time and interaural intensity were obtained using 1/3-octave narrow band noise (NBN) stimuli centered at 500 and 4000 Hz in three conditions. In condition 1 the NBNs were played in isolation; in condition 2 the NBNs were played simultaneously with congruent interaural information across frequency; and in condition 3 the NBNs were played simultaneously with either the 500-Hz band dichotic and the 4000-Hz band diotic (3a) or the 500-Hz band diotic and the 4000-Hz band dichotic (3b). The best interaural time and intensity discrimination for the listeners with impaired hearing is seen when both bands contain congruent interaural information (condition 2). Subjects with impaired hearing and normal hearing show reduced sensitivity to interaural time differences when the 500-Hz band is diotic and discrimination is based on the 4000-Hz band (3b). The listeners with impaired hearing also demonstrate poorest interaural intensity discrimination for this condition (3b). Results suggest that subjects with impaired hearing may benefit from congruent interaural information in more than one frequency region. [Work supported by the Dept. of Veterans Affairs and NIH Grant No. DC00428.]

Comparison of adaptive procedures for obtaining psychophysical thresholds using computer simulation

In order to compare the efficacy and efficiency of various adaptive psychophysical procedures, a program that simulates these procedures has been developed and used to compare several adaptive methods for three experiments: binaural detection, interaural intensity discrimination, and interaural time discrimination. The simulation program takes as input a psychometric function, which specifies the probability of a correct response for any value of the adapting variable. A threshold is generated by choosing the next value of the variable according to the rules of the procedure being simulated, including the rules for stopping and calculating the threshold estimate. The software generates distributions of thresholds and statistical analyses are performed, such as bias and variance of the obtained distribution. The program has been applied to psychometric functions appropriate for normal-hearing and hearing-impaired listeners for each of the three experiments noted above, and has compared a number of adaptive procedures including the Transformed Up-Down (such as 2-down-1-up), PEST, BUDTIF, and Weighted Up-Down methods. [Work supported by NIDCD and Dept. of Education.]

Effects of roving level on binaural performance in normal-hearing and hearing-impaired listeners at 500 and 4000 Hz

Measurements of NoSπ thresholds and interaural time and intensity jnd's with and without a 10-dB roving level for both normal-hearing (4000 Hz) and hearing-impaired (500 and 4000 Hz) subjects have been obtained. This extends previous measurements of the effects of roving level on binaural detection and interaural discrimination at 500 Hz for normal-hearing listeners [J. Koehnke and H. S. Colburn, J. Acoust. Soc. Am. Suppl. 1 82, S10g (1987)]. All measurements were made using an adaptive, four-interval, two-alternative, forced-choice procedure and 1/3-oct noise bands. For detection, the target was a tone burst centered in the noise band. Binaural detection and interaural time discrimination show no effect of roving level at 500 Hz. However, interaural intensity jnd's at 500 Hz for the hearing impaired listeners are consistently larger with the roving level. In contrast to the data obtained at 500 Hz, at 4000-Hz performance of all the subjects for interaural intensity discrimination and NoSπ detection is worse with the roving level. For interaural time discrimination, there is no apparent effect of the roving level at 4000 Hz. [Work supported by NIH.]

Effects of roving level on binaural detection and discrimination on and off midline

NoSπ detection thresholds and interaural time and intensity jnd's were measured both with and without a 10-dB random rove applied to the overall level. The stimulus for discrimination and the masker for detection were a 1/3-oct noise band centered at 500 Hz; for detection, the target was a 500-Hz tone burst. Thresholds and jnd's were measured using an adaptive, four-interval, two-alternative forced-choice procedure for several interaural time and intensity reference conditions including differences of 0 and 24 dB and 0 and 600 μs. Results indicate no effect of roving level for the 0-dB, 0-μs reference condition for binaural detection or interaural discrimination. For NoSπ detection, there is no effect of roving level or reference condition on thresholds. For interaural time discrimination, jnd's are larger off midline and roving level variation has no consistent effect on performance. For interaural intensity discrimination, jnd's generally increase off midline and are usually larger when measured with roving level than without. [Work supported by NIH.]

Interaural intensity discrimination: insensitivity at 1000 Hz.

Recent data from three laboratories have replicated Mills' [J. Acoust. Soc. Am. 32, 132-134 (1960)] finding that interaural intensity discrimination is relatively poorer for tones of 1000 Hz than for tones of either higher or lower frequencies. To get a finer look at this frequency effect, interaural intensity difference thresholds were obtained from four subjects for tones of several frequencies around 1000 Hz. An adaptive two-interval forced-choice procedure was employed, in which the overall intensity of the signals was varied randomly in order to prevent subjects from listening to monaural loudness changes. Despite large intersubject differences in overall sensitivity to interaural intensity differences, all four subjects showed a local peak in their threshold functions at or near 1000 Hz. This curious "1000-Hz effect" might be explained by imagining that an interaural intensity comparator operates more efficiently as frequency increases, but that a peripheral interaural intensity difference to interaural-time difference conversion contributes to laterality judgments for low-frequency tones, thus acting to lower thresholds again for frequencies below 1000 Hz.

Binaural interaction in hearing impaired listeners

The binaural hearing abilities of four sensori‐neural loss listeners were tested in four experiments: (1) interaural time discrimination, (2) interaural intensity discrimination, (3) interaural correlation discrimination, and (4) binaural detection experiments. All tests were conducted on each subject using third‐octave bands of noise centered at 250, 500, 1000, 2000, and 4000 Hz. “Binaural Audiograms” can be obtained by plotting the ration of normal discrimination jnds to measured discrimination jnds versus frequency on a log‐log scale. In general, the frequency dependence of binaural hearing loss, so defined, appears unrelated to the frequency dependence of hearing loss as measured by current monaural audiometric techniques. Furthermore, impairements of interaural time discrimination are independent of impairments in interaural intensity discrimination. Finally, the MLD alone was not related to loss of interaural time or intensity sensitivity. Two of the subjects tested showed elevated NoSπ and NoSo thresholds, poor interaural time and intensity discrimination but a “normal” MLD. [Work supported by NIH Grant NS10916.]

Interaural intensity discrimination of noise as a function of center frequency, duration, and interaural correlation

Interaural intensity difference thresholds were determined for three bands of Gaussian noise: a wideband noise (100–5000 Hz) and two 0.4‐octave narrow bands (48 dB/octave skirts) centered at 500 and 4000 Hz. Thresholds were estimated from psychometric functions generated employing a two‐interval forced‐choice blocked procedure, in which the nominal interaural intensity difference on each trial was divided between the two ears. Reference intensity was 70 dB SPL (overall). StimUlus duration (30 or 500 ms) and interaural correlation of the noise (0.0 or 1.0) were parametrically varied. For the 500‐ms stimulus durations thresholds were near 1 dB (12‐dB change in either ear alone) for all types of noise and for the two values of interaural correlation employed. For the briefer (30 ms) stimuli thresholds varied from about 1.3 dB (for wideband, interaurally correlated noise) to about 3 dB (for narrow noise centered at 500 Hz, interaurally uncorrelated). The results will be discussed in terms of the tradability of bandwidth, duration, and interaurally redundant information available in noisy stimuli. [Supported by NSF.]

Brain stem auditory evoked potentials are related to interaural time discrimination in patients with multiple sclerosis

In a combined psychophysical-electrophysiological study on 29 patients with multiple sclerosis (MS), a compromised ability to make interaural time discriminations was nearly always found to be associated with ‘abnormal’ brain stem potentials evoked by clicks to at least one ear. However, no obvious relationships were found between evoked brain stem potentials and several other auditory behavioral measures such as interaural intensity discrimination, pure-tone thresholds and speech discrimination.

Interaural time discrimination in cochlear-impaired listeners

Interaural time just noticeable differences (jnds) were obtained on a group of normal-hearing listeners and four groups of listeners with sensorineural hearing losses. These groups consisted of persons with moderate hearing loss (1) only in the low frequencies, (2) only in the high frequencies, (3) across the frequency range, and (4) only in one ear. The stimuli consisted of computer-generated narrow bands of noise, 450–550 Hz and 3750–4250 Hz, with the paradigm being constant stimulus two interval forced choice. Interaural intensity jnds were also obtained for the same two bands of noise with hearing-impaired listeners who performed poorly on the time discrimination task. The results indicated that (1) the presence of a sensorineural hearing loss inhibits normal processing of interaural time differences in a lateralization paradigm; (2) there are large individual differences in the amount of deficit in lateralization due to a hearing loss; (3) based on a limited number of subjects employed, there is no clear relationship between configuration of loss and performance on interaural time discrimination; and (4) poor performance on interaural time discrimination tasks does not mean that performance on interaural intensity discrimination tasks will also be poor.