Large Interaural Time Differences Research Articles

Potential Destructive Binaural Interaction Effects in Auditory Steady-State Response Measurements.

An aided sound-field auditory steady-state response (ASSR) has the potential to be used to objectively validate hearing-aid (HA) fittings in clinics. Each aided ear should ideally be tested independently, but it is suspected that binaural testing may be used by clinics to reduce test time. This study simulates dichotic ASSR sound-field conditions to examine the risk of making false judgments due to unchecked binaural effects. Unaided ASSRs were recorded with a clinical two-channel electroencephalography (EEG) system for 15 normal hearing subjects using a three-band CE-Chirp® stimulus. It was found that the noise corrected power of a response harmonic can be suppressed by up to 10 dB by introducing large interaural time differences equal to half the time period of the stimulus envelope, which may occur in unilateral HA users. These large interaural time differences also changed the expression of ASSR power across the scalp, resulting in dramatically altered topographies. This would lead to considerably lower measured response power and possibly nondetections, evidencing that even well fit HAs are fit poorly (false referral), whereas monaural ASSR tests would pass. No effect was found for simulated lateralizations of the stimulus, which is beneficial for a proposed aided ASSR approach. Full-scalp ASSR recordings match previously found 40 Hz topographies but demonstrate suppression of cortical ASSR sources when using stimuli in interaural envelope antiphase.

Extent of lateralization at large interaural time differences in simulated electric hearing and bilateral cochlear implant users.

Normal-hearing (NH) listeners are able to localize sound sources with extraordinary accuracy through interaural cues, most importantly interaural time differences (ITDs) in the temporal fine structure. Bilateral cochlear implant (CI) users are also able to localize sound sources, yet generally at lower accuracy than NH listeners. The gap in performance can in part be attributed to current CI systems not faithfully transmitting interaural cues, especially ITDs. With the introduction of binaurally linked CI systems, the presentation of ITD cues for bilateral CI users is foreseeable. The current study therefore investigated extent-of-lateralization percepts elicited in bilateral CI listeners when presented with single-electrode pulse-trains carrying controlled ITD cues. The results were compared against NH listeners listening to broadband stimuli as well as simulations of CI listening. Broadband stimuli in NH listeners were perceived as fully lateralized within the natural ITD range. Using simulated as well as real CI stimuli, however, only a fraction of the full extent of lateralization range was covered by natural ITDs. The maximum extent of lateralization was reached at ITDs as large as twice the natural limit. The results suggest that ITD-enhancement might be a viable option for improving localization abilities with future binaural CI systems.

Transformation from a Pure Time Delay to a Mixed Time and Phase Delay Representation in the Auditory Forebrain Pathway

Birds and mammals exploit interaural time differences (ITDs) for sound localization. Subsequent to ITD detection by brainstem neurons, ITD processing continues in parallel midbrain and forebrain pathways. In the barn owl, both ITD detection and processing in the midbrain are specialized to extract ITDs independent of frequency, which amounts to a pure time delay representation. Recent results have elucidated different mechanisms of ITD detection in mammals, which lead to a representation of small ITDs in high-frequency channels and large ITDs in low-frequency channels, resembling a phase delay representation. However, the detection mechanism does not prevent a change in ITD representation at higher processing stages. Here we analyze ITD tuning across frequency channels with pure tone and noise stimuli in neurons of the barn owl's auditory arcopallium, a nucleus at the endpoint of the forebrain pathway. To extend the analysis of ITD representation across frequency bands to a large neural population, we employed Fourier analysis for the spectral decomposition of ITD curves recorded with noise stimuli. This method was validated using physiological as well as model data. We found that low frequencies convey sensitivity to large ITDs, whereas high frequencies convey sensitivity to small ITDs. Moreover, different linear phase frequency regimes in the high-frequency and low-frequency ranges suggested an independent convergence of inputs from these frequency channels. Our results are consistent with ITD being remodeled toward a phase delay representation along the forebrain pathway. This indicates that sensory representations may undergo substantial reorganization, presumably in relation to specific behavioral output.

Age-related dissociation of sensory and decision-based auditory motion processing

Studies on the maturation of auditory motion processing in children have yielded inconsistent reports. The present study combines subjective and objective measurements to investigate how the auditory perceptual abilities of children change during development and whether these changes are paralleled by changes in the event-related brain potential (ERP). We employed the mismatch negativity (MMN) to determine maturational changes in the discrimination of interaural time differences (ITDs) that generate lateralized moving auditory percepts. MMNs were elicited in children, teenagers, and adults, using a small and a large ITD at stimulus offset with respect to each subject's discrimination threshold. In adults and teenagers large deviants elicited prominent MMNs, whereas small deviants at the behavioral threshold elicited only a marginal or no MMN. In contrast, pronounced MMNs for both deviant sizes were found in children. Behaviorally, however, most of the children showed higher discrimination thresholds than teens and adults. Although automatic ITD detection is functional, active discrimination is still limited in children. The lack of MMN deviance dependency in children suggests that unlike in teenagers and adults, neural signatures of automatic auditory motion processing do not mirror discrimination abilities. The study critically accounts for advanced understanding of children's central auditory development.

Tuning to Interaural Time Difference and Frequency Differs Between the Auditory Arcopallium and the External Nucleus of the Inferior Colliculus

Barn owls process sound-localization information in two parallel pathways, the midbrain and the forebrain pathway. Exctracellular recordings of neural responses to auditory stimuli from far advanced stations of these pathways, the auditory arcopallium in the forebrain and the external nucleus of the inferior colliculus in the midbrain, demonstrated that the representations of interaural time difference and frequency in the forebrain pathway differ from those in the midbrain pathway. Specifically, low-frequency representation was conserved in the forebrain pathway, while it was lost in the midbrain pathway. Variation of interaural time difference yielded symmetrical tuning curves in the midbrain pathway. By contrast, the typical forebrain-tuning curve was asymmetric with a steep slope crossing zero time difference and a less-steep slope toward larger contralateral time disparities. Low sound frequencies contributed sensitivity to contralateral leading sounds underlying these asymmetries, whereas high frequencies enhanced the steepness of slopes at small interaural time differences. Furthermore, the peaks of time-disparity tuning curves were wider in the forebrain than in the midbrain. The distribution of the steepest slopes of best interaural time differences in the auditory arcopallium, but not in the external nucleus of the inferior colliculus, was centered at zero time difference. The distribution observed in the auditory arocpallium is reminiscent of the situation observed in small mammals. We speculate that the forebrain representation may serve as a population code supporting fine discrimination of central interaural time differences and coarse indication of laterality of a stimulus for large interaural time differences.

Interaural time difference processing of broadband and narrow-band noise by inexperienced listeners

Recent functional magnetic resonance imaging (fMRI) data might be interpreted as being in disagreement with existing psychophysical data regarding the laterality of broadband noise stimuli presented with large interaural time differences (ITDs). This study investigated the possibility that lateral judgments made by inexperienced listeners who did not receive feedback might be different than those reported for experienced listeners, especially when the ITD is longer than that occurring in nature, and therefore data from inexperienced listeners presented unnaturally long ITDs for the first time might be more consistent with the possible interpretation of the fMRI results. The results from this study using inexperienced listeners were not basically different from those reported in the literature based on experienced listeners, suggesting a possible difference does exist between inferences drawn from fTMRI data and human psychophysical results.

Context effects in the lateralization of sine tones?

Low‐frequency sine tones can have large interaural time differences, but high‐frequency tones cannot. If listeners estimate the lateral positions of tones of different frequency, using a fixed lateralization response scale, the estimates may depend on the experimental blocking of the tones. A context where all frequencies appear equally often (mixed) serves as a standard. It was hypothesized that a context with tones blocked on frequency would lead to abnormally large lateral position estimates for high‐frequency tones because of a tendency to use the entire response scale in any stimulus‐range context. Four listeners estimated the lateral positions of sine tones with frequencies 200, 500, 750, and 1,000 Hz having all possible interaural phase differences (IPD) spaced by 30 deg. Fixed‐frequency blocks were done first, then mixed. Very little trace of the hypothesized effect was observed, either in the overall range of responses or in the slope of the response‐IPD function for small IPD values. The same listeners matched the laterality of the same tones using the interaural level difference (ILD) of a narrow noise band. Relative slopes of the ILD‐IPD functions agreed with slopes of the lateral position estimates. [Work supported by the NIDCD.]

Neural Sensitivity to Interaural Time Differences: Beyond the Jeffress Model

Interaural time differences (ITDs) are a major cue for localizing the azimuthal position of sounds. The dominant models for processing ITDs are based on the Jeffress model and predict neurons that fire maximally at a common ITD across their responsive frequency range. Such neurons are indeed found in the binaural pathways and are referred to as "peak-type." However, other neurons discharge minimally at a common ITD (trough-type), and others do not display a common ITD at the maxima or minima (intermediate-type). From recordings of neurons in the auditory cortex of the unanesthetized rabbit to low-frequency tones and envelopes of high-frequency sounds, we show that the different response types combine to form a continuous axis of best ITD. This axis extends to ITDs well beyond that allowed by the head width. In Jeffress-type models, sensitivity to large ITDs would require neural delay lines with large differences in path lengths between the two ears. Our results suggest instead that sensitivity to large ITDs is created with short delay lines, using neurons that display intermediate- and trough-type responses. We demonstrate that a neuron's best ITD can be predicted from (1) its characteristic delay, a rough measure of the delay line, (2) its characteristic phase, which defines the response type, and (3) its best frequency for ITD sensitivity. The intermediate- and trough-type neurons that have large best ITDs are predicted to be most active when sounds at the two ears are decorrelated and may transmit information about auditory space other than sound localization.

Responses of low-frequency cells in the inferior colliculus to interaural time differences of clicks: excitatory and inhibitory components.

1. We studied extracellular responses of low-frequency cells in the central nucleus of the inferior colliculus (ICC) to interaural time differences (ITDs) of clicks and compared their responses to ITDs of noise and tones. Most cells that displayed sensitivity to ITDs of clicks responded cyclically as a function of ITD with central peaks and troughs at the same ITDs as in response to noise. The positions of these peaks and troughs also matched those predicted from tonal ITD curves. Thus over the range of physiologically relevant ITDs, the binaural cells in the ICC showed similar sensitivity to ITDs of tones, noise, and clicks. 2. The transient nature of the response to a click allowed association of individual discharges with either the ipsilateral or contralateral stimulus when the binaural stimulus included a large ITD. We studied the influence of the click presented to one side on responses to the click presented to the other side. By examining responses to clicks with large ITDs, ranging from 2 to 3 up to 200 ms, we could identify both excitatory and inhibitory components in response to binaural clicks. 3. For many cells, there was evidence for a short-lasting excitation arising from one or both inputs of the binaural stimulus. Inhibitory interactions could also be demonstrated over a large range of ITDs. Long-lasting, late inhibitory components arose from both contralateral and ipsilateral inputs. In 87% of cells that were driven by the contralateral input, a late inhibitory component originating from the ipsilateral side was detected. In all cells that were driven by the ipsilateral side, a late inhibitory contralateral component was detected. This late inhibition of the excitatory response to one side by a leading stimulus to the other side could be evoked even when the leading stimulus was not effective in evoking an excitatory response. 4. Some cells also exhibited an early inhibitory component that preceded the excitation. An early contralateral inhibition was detected in 44% of cells that were driven by the ipsilateral input, whereas an early ipsilateral component was detected in 17% of cells driven by the contralateral input. 5. We confirmed hypotheses about the laterality and time course of the inhibitory and excitatory components by introducing interaural level differences (ILDs) into the binaural clicks and thus varying the strengths of the different components. 6. Inhibitory components may play a role in shaping the sensitivity of individual cells to ITDs of stimuli other than clicks; they were also apparent in responses to noise.(ABSTRACT TRUNCATED AT 400 WORDS)

Some new data on the lateralization of noise signals with large interaural time differences

Blodgett, Wilbanks, and Jeffress (1956) showed that the human auditory system can utilize interaural time differences considerably greater than those which can arise in a free sound field to lateralize noise signals. Some new data are reported on the effect of large interaural time differences upon judgments of sidedness. The results show that subjects can detect interaural time differences as large as 10 msec as differences in direction or position, and that subjects can distinguish between uncorrelated noise and noise signals with interaural correla- tions as small as r <.02 as differences in sidedness.

Differences of interaural phase and level in detection and lateralization: 1000 and 2000 Hz.

Detection and lateralization performance were measured using as a signal the same narrow band of noise that served as the masker. The center frequency of the noise band was either 1000 or 2000 Hz, in different experiments. Both diotic (N0-S0) and dichotic (N0-Sπ) data were taken at both frequencies. By varying the signal-to-masker ratio and the angle α at which the signal is added to the masker in the N0-Sπ condition, it is possible to control the magnitudes of the two binaural cues—interaural time difference and interaural level difference. The outcomes at 1000 Hz support our previous findings—subjects differ in their sensitivities to the two cues, and the cues do not cancel perfectly when the task is detection. At 2000 Hz, even relatively large interaural time differences were of little or no benefit either for detection or lateralization, and interaural level difference was the primary binaural cue. At 1000 Hz, sizeable masking-level differences (MLDs) were observed for all subjects at all values of α. At 2000 Hz, the MLDs were large at α = 0°; however, detectability was essentially the same in the conditions N0-Sπ, α = 90°, and N0-S0, α = 90° implying that in both of these conditions performance was based on the increment in level that occurred with signal onset.

Effect of Large Interaural Time Differences upon the Judgment of Sidedness

The maximal interaural time difference that could be introduced into one channel without loss of sidedness was determined for various noise bands. Through the presentation of stimuli by several methods it was found that time values vary with frequency of the noise bands, being greater for lower bands. Marked intersubject differences were found. For the lowest and most effective band used (106–212∼), times ranged from 7.5 to 20.6 msec.

The Effect of Large Interaural Time Differences upon the Judgment of Sidedness

Bands of noise presented to the two ears via earphones are perceived as lying to one side when that side leads in time. Roy Klumpp has shown that interaural time differences as small as ten microseconds can be detected. The present study is concerned with the other extreme—the largest time differences that can be heard as “sided.” The interaural time delay in the present experiments was obtained by displacing one head of a binaural tape reproducer carrying the noise. A slow acting gate turning the signal on in both channels simultaneously was used to prevent the subject's getting a cue from the onset of the signal. Results will be presented showing the effects on the threshold for sidedness, of level imbalance, of the band width and the frequency of the noise, and of individual differences. The thresholds varied from a fraction of a millisecond to about 20 milliseconds depending upon the parameters involved.