Interaural Time Difference Discrimination Research Articles

Sound lateralization test in adolescent blind individuals

Blind individuals require to compensate for the lack of visual information by other sensory inputs. In particular, auditory inputs are crucial to such individuals. To investigate whether blind individuals localize sound in space better than sighted individuals, we tested the auditory ability of adolescent blind individuals using a sound lateralization method. The interaural time difference discrimination thresholds of blind individuals were statistically significantly shorter than those of blind individuals with residual vision and controls. These findings suggest that blind individuals have better auditory spatial ability than individuals with visual cues; therefore, some perceptual compensation occurred in the former.

Observer weighting strategies in interaural time-difference discrimination and monaural level discrimination for a multi-tone complex

Two experiments measured listeners' abilities to weight information from different components in a complex of 553, 753, and 953 Hz. The goal was to determine whether or not the ability to adjust perceptual weights generalized across tasks. Weights were measured by binary logistic regression between stimulus values that were sampled from Gaussian distributions and listeners' responses. The first task was interaural time discrimination in which listeners judged the laterality of the target component. The second task was monaural level discrimination in which listeners indicated whether the level of the target component decreased or increased across two intervals. For both experiments, each of the three components served as the target. Ten listeners participated in both experiments. The results showed that those individuals who adjusted perceptual weights in the interaural time experiment could also do so in the monaural level discrimination task. The fact that the same individuals appeared to be analytic in both tasks is an indication that the weights measure the ability to attend to a particular region of the spectrum while ignoring other spectral regions.

Effects of different amounts of brief training and rest on the generalization of learning from interaural-level-difference to interaural-time-difference discrimination

Training-induced improvements on perceptual skills can be enhanced by increasing the amount of training and by resting between training and testing. However, how these two factors affect the generalization of learning from a briefly trained condition to an untrained one is unknown. Here, listeners were trained on an interaural-level-difference (ILD) discrimination condition (4-kHz tones), then were tested for generalization to an interaural-time-difference (ITD) discrimination condition (0.5-kHz tones). The amount of training and the time between training and testing differed across four groups. Listeners tested 10 hours after training had significantly lower ITD discrimination thresholds than naive listeners (n=94), regardless of whether training lasted for 20 min (n=14) or 2 hs (n=11). Thus, when there was a long time between training and testing, learning generalized from ILD to ITD discrimination regardless of the amount of training. In contrast, listeners tested immediately after training on ILD discrimination showed generalization to ITD discrimination with 20 min (n=14), but not 2 h (n=14), of training, suggesting that without rest between training and testing, longer training disrupted generalization. Hence, increased training interfered with generalization from ILD to ITD discrimination for short, but not long, delays between training and testing. [Work supported by NIH.]

Learning on auditory discrimination tasks in normal-hearing listeners: Implications for hearing rehabilitation

Hearing rehabilitation extends beyond simply fitting a hearing aid or cochlear implant. To improve the benefit of these devices, it must be established which auditory abilities can be improved with training. Toward this end, learning in normal-hearing listeners was examined on five auditory discrimination tasks: frequency, intensity, interaural-time-difference (ITD), interaural-level-difference (ILD), and duration. Because the same training regimen was used throughout, any differences in the learning patterns across these trained discriminations likely reflect differences in the plasticity of the underlying mechanisms, at least for that regimen. The influence of training was assessed by comparing the improvements in discrimination threshold on trained and untrained conditions between listeners who were given multiple-hour practice on a single discrimination condition and those who were not. Learning on the five tasks followed one of two general patterns. For ITD and intensity discrimination, multiple-hour practice did not lead to greater learning than that seen in untrained listeners. In contrast, for ILD, duration, and frequency discrimination, such practice yielded greater learning, but only on a subset of conditions. The differences in the plasticity across these auditory tasks in normal-hearing listeners imply that cochlear-implant users may benefit more from training on some tasks than others. [Work supported by NIH.]

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.

A physiologically based model of interaural time difference discrimination.

Interaural time difference (ITD) is a cue to the location of sounds containing low frequencies and is represented in the inferior colliculus (IC) by cells that respond maximally at a particular best delay (BD). Previous studies have demonstrated that single ITD-sensitive cells contain sufficient information in their discharge patterns to account for ITD acuity on the midline (ITD = 0). If ITD discrimination were based on the activity of the most sensitive cell available ("lower envelope hypothesis"), then ITD acuity should be relatively constant as a function of ITD. In response to broadband noise, however, the ITD acuity of human listeners degrades as ITD increases. To account for these results, we hypothesize that pooling of information across neurons is an essential component of ITD discrimination. This report describes a neural pooling model of ITD discrimination based on the response properties of ITD-sensitive cells in the IC of anesthetized cats. Rate versus ITD curves were fit with a cross-correlation model of ITD sensitivity, and the parameters were used to constrain a population model of ITD discrimination. The model accurately predicts ITD acuity as a function of ITD for broadband noise stimuli when responses are pooled across best frequency (BF). Furthermore, ITD tuning based solely on a system of internal delays is not sufficient to predict ITD acuity in response to 500 Hz tones, suggesting that acuity is likely refined by additional mechanisms. The physiological data confirms evidence from the guinea pig that BD varies systematically with BF, generalizing the observation across species.

Spatial discrimination of sound sources in the horizontal plane following an adapter sound

The effect of a preceding (adapter) sound on the spatial discrimination of two subsequent, successively presented (target) sounds was tested in the horizontal plane. The adapter and the first target were located in front of the subject or 30° to the right of the midline; both sounds were presented either at the same location or at different locations. The second target was located to the right or the left of the first. Sound spectra of the 3-s adapter and the 100-ms targets were either high (4.5–18 kHz) or low (1–4 kHz) in frequency. Fifteen subjects judged the position of the second target relative to the first in a two-alternative forced-choice paradigm. In comparison with a no-adapter control condition, in which no sound preceded, discrimination performance was increased when adapter and first target were presented at the same location and when both sounds consisted of the same frequency spectrum. No improvement occurred when adapter and targets differed in location or frequency. The results are consistent with previous results on post-adaptation discrimination of interaural time differences. Possibly, spatial adaptation of the underlying mechanisms of auditory localization may explain the discrimination aftereffect.

Training in binaural hearing: towards its therapeutic use in clinical practice

Background: Interaural level differences (ILD) and interaural time differences (ITD) are binaural cues used in sound localisation and in detection of signals in noisy backgrounds. People with asymmetric hearing loss have poor binaural processing skills. Discrimination performance in ILD and ITD tasks substantially improves with training. Training in binaural hearing may be of clinical benefit to certain populations. Objectives: (1) What is the role of perceptual learning in auditory training? (2) Which regime trains binaural cue discrimination most effectively? Methods: (1) Subjects were trained on an identical, adaptive ILD-discrimination task presented within different procedures in each of four groups, then tested on all procedures post-training. (2) Subjects underwent either blocked or distributed training in ILD or ITD discrimination and were tested at intervals post-training. Results: (1) There was no significant difference in task performance post-training between the groups. (2) ILD and ITD discrimination performance was not significantly different in the blocked and distributed training groups. Conclusions: Perceptual learning occurred rapidly in auditory training. Significant, lasting training effects were found for both ILD and ITD tasks, and both blocked and distributed regimes were equally effective. These findings will guide further work on the potential therapeutic use of auditory training.

The basic hearing abilities of absolute pitch possessors.

Generally, people tend to think that absolute pitch possessors or musical experts are superior in all of their auditory and musical abilities. However, not much data has been shown to prove whether any differences exist in the basic hearing abilities of absolute pitch possessors or musical experts for general sounds outside the context of music. In this study, we conducted four experiments to investigate if absolute pitch possessors or music experts are superior in their basic hearing abilities with respect to frequency resolution, temporal resolution, and spatial resolution. In Experiments 1 and 2, we measured frequency discrimination thresholds and thresholds to detect a tone in notched noise to examine the characteristics of frequency resolution. In Experiment 3, we conducted a gap detection task to measure the temporal resolution. Finally, in Experiment 4, we measured interaural time difference discrimination thresholds to examine the spatial resolution abilities. The overall results show that there were no significant differences in frequency, temporal and spatial resolutions among groups with different absolute pitch capability. This indicates that absolute-pitch possessors do not have particularly ‘good ears’ in terms of resolution.

Different patterns of human discrimination learning for two interaural cues to sound-source location.

Two of the primary cues used to localize the sources of sounds are interaural level differences (ILDs) and interaural time differences (ITDs). We conducted two experiments to explore how practice affects the human discrimination of values of ILDs and ongoing ITDs presented over headphones. We measured discrimination thresholds of 13 to 32 naive listeners in a variety of conditions during a pretest and again, 2 weeks later, during a posttest. Between those two tests, we trained a subset of listeners 1 h per day for 9 days on a single ILD or ITD condition. Listeners improved on both ILD and ITD discrimination. Improvement was initially rapid for both cue types and appeared to generalize broadly across conditions, indicating conceptual or procedural learning. A subsequent slower-improvement stage, which occurred solely for the ILD cue, only affected conditions with the trained stimulus frequency, suggesting that stimulus processing had fundamentally changed. These different learning patterns indicate that practice affects the attention to, or low-level encoding of, ILDs and ITDs at sites at which the two cue types are processed separately. Thus, these data reveal differences in the effect of practice on ILD and ITD discrimination, and provide insight into the encoding of these two cues to sound-source location in humans.

Perceptual learning and its generalization in the discrimination of interaural time differences.

Perceptual learning and its generalization in the discrimination of interaural time differences.

Perceptual learning in the discrimination of interaural time differences

Experiments were conducted to clarify whether spatial resolution improves with practice. Four adult subjects were involved in measuring a discrimination threshold in interaural time difference (ITD) discrimination tasks. The threshold was measured using adaptive, 2-alternative, forced-choice procedure. The subjects were pretested with 500-Hz pure tones which have ±300 (plus meaning that the right channel signal leads) or 0 μs ITDs. Then two subjects were repeatedly tested for 9 days (about 80 min per day) at 300 μs ITD and the others at −300 μs ITD, with feedback to their responses. Following the practice period they were tested at ±300 and 0 μs ITDs again (post-test). At the trained ITDs, the spatial resolution improved throughout practice period, and the threshold declined from 81 to 39 μs on average between pretest and post-test. At untrained ITDs, on the other hand, the resolution did not improve between two tests. The improvement was maintained even 74 days after the post-test. These results suggest that the spatial resolution based on ITD improves with practice, and the improvement occurs locally in a certain ITD region, at least within 300 μs around the trained ITD.

Auditory spatial attention influences discrimination of interaural time differences: Evidence from a probe-signal paradigm

Performance on a same–different ITD discrimination task was used as a metric for auditory spatial attention. In several variants of a probe-signal procedure, attention was directed as detailed below. In all experiments, performance was better for signals lateralized on the expected side of the head. In experiment 1, a lateralized auditory cue preceded the signal. Lest this functioned not simply as an attentional cue but also as a spatial landmark, experiment 2 used no cue but manipulated expectation by presenting trials more frequently on a particular side. To establish whether the effects in experiment 2 generalized to all sounds within the spatial focus or were attributable simply to greater familiarity with the expected stimulus, a rove was incorporated in the pitch (experiment 3) or the timbre (experiment 4) of the signals on each trial. Experiment 5 introduced trial-by-trial visual cueing which, necessarily endogenous, provided evidence for top–down attentional control. Further work established whether analogous effects are seen on a frequency discrimination task, and when sounds are lateralized by IID. [Work supported by UK BBSRC.]

方向感検査の臨床応用

Sound localization in the horizontal plane primarily depends on the interaural time difference (ITD) and interaural intensity difference (IID). When sounds with identical ITD or IID are presented to both ears, the sound image is lateralized to the side receiving the earlier or louder stimulus within the head. Currently, ITD and IID discrimination abilities are separately measured with a commercially available self recording apparatus.In this review, the history and basic science of sound localization is discussed in detail. Moreover, for future clinical application, sound lateralization abilities of patients with peripheral hearing loss, brain stem disorders, and auditory cortex disorders are described pertaining to discrimination of ITD or IID separately using a self recording sound lateralization system.

Sound Lateralization and Speech Discrimination in Patients with Sensorineural Hearing Loss

Sound lateralization and speech discrimination abilities are both closely related to central auditory system function. In this study, we investigated the relationship between sound lateralization and speech discrimination in patients with sensorineural hearing loss (SNHL) of unknown aetiology or presbycusis. Interaural intensity difference (IID) and interaural time difference (ITD) discriminations were measured using a self-recording apparatus for dichotic sound presentation, and the means of bilateral maximum scores in speech discrimination tests were calculated. Subjects with normal sound lateralization had good speech discrimination scores above 70%, while those with abnormal sound lateralization had poor scores below 70%. Some patients, however, had good speech discrimination but abnormal sound lateralization. These findings were obtained in both IID and ITD discrimination tests. Furthermore, some subjects could not discriminate ITD, although all could discriminate IID. These findings suggest that sound lateralization ability may be affected more than speech discrimination in SNHL of unknown aetiology or in presbycusis.

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.

Sound lateralization in patients with lesions including the auditory cortex: comparison of interaural time difference (ITD) discrimination and interaural intensity difference (IID) discrimination

We examined sound lateralization using dichotic presentation of noises in 15 patients with left unilateral (12 patients) or bilateral (3 patients) temporal lobe lesions, that included the auditory cortex, and evaluated their abilities to discriminate interaural time and intensity difference (ITD, IID) separately. On the ITD discrimination test, discrimination thresholds in patients with left unilateral lesions were significantly higher than those in normal subjects, but all patients with left unilateral lesions could detect ITD. However, none of 3 patients with bilateral lesions could detect ITD. On the 1113 discrimination test, all patients with either unilateral or bilateral lesions could detect IID. IID discrimination thresholds in these patients were significantly higher than those in normal subjects. The auditory cortex plays an important role in discriminating both cues, but appears to be necessary for discriminating ITD.

Auditory after-effects revealing adaptive representation of auditory space

After-effects refer to distortions of perception following prolonged exposure to a stimulus. In vision, various after-effects have been known for years and have proven to be useful tools in analyzing processing channels of the perceptual system. On the other hand, few after-effects have been reported in hearing. The authors have been studying auditory after-effects affecting sound localization in order to investigate dynamic aspects of auditory spatial representation. It was found that exposure to a sound alters the perceived localization of a subsequent sound, depending on the frequency relationship and interaural time difference (ITD) relationship between the two sounds. It was also found that adaptation to a sound affects the discrimination of ITDs of the following sounds: ITD discrimination is selectively improved in the neighborhood of the preceding sound, and reduced elsewhere. Together, these results indicate that the internal representation of auditory space is not fixed, but adaptive. The responsible mechansims are recalibrated during stimulation so that they can represent recently experienced sounds with the maximum resolution. Such adaptive representations of auditory space provide a basis for efficient information processing of auditory events in the ever-changing real-world environment.

Interaural time discrimination in the bottlenose dolphin

Binaural hearing is the advantage of two ears over one. Localization studies with marine mammals have shown that acoustic localization underwater is mediated by the same binaural mechanisms used by other mammals in air. Dolphin echolocation abilities are enhanced by the use of two separate receivers. Au and Moore [J. Acoust. Soc. Am. 75, 255–262 (1984)] modeled the animal’s hearing system as a two‐element array using receiving beam pattern measurements. The present study sought to measure the interaural phenomena of interaural time difference discrimination (ITD) and interaural intensity difference discrimination (IID). A dolphin was trained to accept contact hydrophones, attached by suction cups, to the outer surface of the lower jaw in the area of the pan‐bone and to perform detection threshold experiments. Audiometric data at several frequencies indicate that the dolphin minimum threshold for pure tones may be 20 dB different when measured with jaw phones. The first behavioral measures of ITD and IID discrimination and for a cetacean have been obtained. ITD and IID for clicks trains of various frequencies, presented with interclick intervals simulating various echolocation target ranges were collected. This method of collecting audiometric data represents a new approach for assessing bearing capabilities in cetaceans.

Auditory spatial resolution in the horizontal plane as a function of reference angle: Microstructure of the azimuth function

Auditory spatial resolution in the horizontal plane was measured in five normal-hearing subjects by determining the minimum audible angle (MAA) for brief 1.0-kHz tones. Using a left–right discrimination paradigm, MAAs were determined for 22 reference azimuths ranging from −62° to +63°. The gross pattern of the mean data was similar to that reported previously for more sparsely sampled azimuth functions: spatial resolution tended to be best when the reference sound was near midline, becoming worse as the reference moved toward the periphery. However, there were remarkable asymmetries and nonmonotonicities in individual subject’s MAA versus azimuth functions, and these nonmonotonic features were highly subject-dependent. Although such individual differences may be due partly to differences among subjects in head and pinna shape, it is possible that the highly individual and idiosyncratic MAA functions also reflect subject-dependent nonmonotonicities in the underlying interaural time difference (ITD) discrimination functions as reference ITD is varied in fine increments. [Work supported by NIH and by the U.S. Army.] a)Current address: ENT/Audiology (HSHJ-SET-A), Madigan Army Med. Ctr., Tacoma, WA 98431-5374.