Binaural speech intelligibility in noise for hearing-impaired listeners

The effect of head-induced interaural time delay (ITD) and interaural level differences (ILD) on binaural speech intelligibility in noise was studied for listeners with symmetrical and asymmetrical sensorineural hearing losses. The material, recorded with a KEMAR manikin in an anechoic room, consisted of speech, presented from the front (0 degree), and noise, presented at azimuths of 0 degree, 30 degrees, and 90 degrees. Derived noise signals, containing either only ITD or only ILD, were generated using a computer. For both groups of subjects, speech-reception thresholds (SRT) for sentences in noise were determined as a function of: (1) noise azimuth, (2) binaural cue, and (3) an interaural difference in overall presentation level, simulating the effect of a monaural hearing acid. Comparison of the mean results with corresponding data obtained previously from normal-hearing listeners shows that the hearing impaired have a 2.5 dB higher SRT in noise when both speech and noise are presented from the front, and 2.6-5.1 dB less binaural gain when the noise azimuth is changed from 0 degree to 90 degrees. The gain due to ILD varies among the hearing-impaired listeners between 0 dB and normal values of 7 dB or more. It depends on the high-frequency hearing loss at the side presented with the most favorable signal-to-noise (S/N) ratio. The gain due to ITD is nearly normal for the symmetrically impaired (4.2 dB, compared with 4.7 dB for the normal hearing), but only 2.5 dB in the case of asymmetrical impairment. When ITD is introduced in noise already containing ILD, the resulting gain is 2-2.5 dB for all groups. The only marked effect of the interaural difference in overall presentation level is a reduction of the gain due to ILD when the level at the ear with the better S/N ratio is decreased. This implies that an optimal monaural hearing aid (with a moderate gain) will hardly interfere with unmasking through ITD, while it may increase the gain due to ILD by preventing or diminishing threshold effects.     

Click lateralization is related to the beta component of the dichotic brainstem auditory evoked potentials of human subjects

The changes in perception and in the binaural difference waveform (BD) for dichotic clicks with interaural time and level differences (ITDs and ILDs) are compared. Only beta, the first major peak of the BD, correlated with the perceptual measurements. Whenever beta is clearly present, the clicks are perceived as a unitary fused image. Whenever the clicks are perceived as not fused, beta is undetectable by our methods. The amplitude of beta remains nearly constant as the ITD is increased to about 1 ms, while the click's position is perceived as moving from midline toward the leading ear. Over about the next 0.2 ms, beta becomes undetectable, as the image stops moving and loses its fused quality. As the ILD is increased, beta amplitude decreases gradually, while the image remains unitary and moves toward the unattenuated earphone. Thus beta becomes undetectable for ILDs of 30 to 35 dB, and the dichotic clicks become indistinguishable from monotic clicks for ILDs of 44 to 53 dB. The ITD and ILD matching curve for beta latency is similar to the ITD and ILD psychophysical matching curve for lateralization. These results suggest that beta is a physiological correlate of the categorical percept, binaural fusion, and is generated by a brainstem structure essential for the preception of click lateralization.     

Dynamic-range compression affects the lateral position of sounds

Dynamic-range compression acting independently at each ear in a bilateral hearing-aid or cochlear-implant fitting can alter interaural level differences (ILDs) potentially affecting spatial perception. The influence of compression on the lateral position of sounds was studied in normal-hearing listeners using virtual acoustic stimuli. In a lateralization task, listeners indicated the leftmost and rightmost extents of the auditory event and reported whether they heard (1) a single, stationary image, (2) a moving/gradually broadening image, or (3) a split image. Fast-acting compression significantly affected the perceived position of high-pass sounds. For sounds with abrupt onsets and offsets, compression shifted the entire image to a more central position. For sounds containing gradual onsets and offsets, including speech, compression increased the occurrence of moving and split images by up to 57 percentage points and increased the perceived lateral extent of the auditory event. The severity of the effects was reduced when undisturbed low-frequency binaural cues were made available. At high frequencies, listeners gave increased weight to ILDs relative to interaural time differences carried in the envelope when compression caused ILDs to change dynamically at low rates, although individual differences were apparent. Specific conditions are identified in which compression is likely to affect spatial perception.     

Monaural and interaural temporal modulation transfer functions measured with 5-kHz carriers

Temporal modulation transfer functions (TMTFs) were measured for detection of monaural sinusoidal amplitude modulation and dynamically varying interaural level differences for a single set of listeners. For the interaural TMTFs, thresholds are the modulation depths at which listeners can just discriminate interaural envelope-phase differences of 0 and 180 degrees. A 5-kHz pure tone and narrowband noises, 30- and 300-Hz wide centered at 5 kHz, were used as carriers. In the interaural conditions, the noise carriers were either diotic or interaurally uncorrelated. The interaural TMTFs with tonal and diotic noise carriers exhibited a low-pass characteristic but the cutoff frequencies changed nonmonotonically with increasing bandwidth. The interaural TMTFs for the tonal carrier began rolling off approximately a half-octave lower than the tonal monaural TMTF (approximately 80 Hz vs approximately 120 Hz). Monaural TMTFs obtained with noise carriers showed effects attributable to masking of the signal modulation by intrinsic fluctuations of the carrier. In the interaural task with dichotic noise carriers, similar masking due to the interaural carrier fluctuations was observed. Although the mechanisms responsible for differences between the monaural and interaural TMTFs are unknown, the lower binaural TMTF cutoff frequency suggests that binaural processing exhibits greater temporal limitation than monaural processing.     

Separation of concurrent broadband sound sources by human listeners

The effect of spatial separation on the ability of human listeners to resolve a pair of concurrent broadband sounds was examined. Stimuli were presented in a virtual auditory environment using individualized outer ear filter functions. Subjects were presented with two simultaneous noise bursts that were either spatially coincident or separated (horizontally or vertically), and responded as to whether they perceived one or two source locations. Testing was carried out at five reference locations on the audiovisual horizon (0 degrees, 22.5 degrees, 45 degrees, 67.5 degrees, and 90 degrees azimuth). Results from experiment 1 showed that at more lateral locations, a larger horizontal separation was required for the perception of two sounds. The reverse was true for vertical separation. Furthermore, it was observed that subjects were unable to separate stimulus pairs if they delivered the same interaural differences in time (ITD) and level (ILD). These findings suggested that the auditory system exploited differences in one or both of the binaural cues to resolve the sources, and could not use monaural spectral cues effectively for the task. In experiments 2 and 3, separation of concurrent noise sources was examined upon removal of low-frequency content (and ITDs), onset/offset ITDs, both of these in conjunction, and all ITD information. While onset and offset ITDs did not appear to play a major role, differences in ongoing ITDs were robust cues for separation under these conditions, including those in the envelopes of high-frequency channels.     

Buildup and breakdown of echo suppression for stimuli presented over headphones-the effects of interaural time and level differences

The current study investigates buildup and breakdown of echo suppression for stimuli presented over headphones. The stimuli consisted of pairs of 120-micros clicks. The leading click (lead) and the lagging click (lag) in each pair were lateralized on opposite sides of the midline by means of interaural level differences (ILDs) of +/-10 dB or interaural time differences (ITDs) of +/-300 micros. Echo threshold was measured with an adaptive one-interval, two-alternative, forced-choice procedure with a subjective decision criterion, in which listeners had to report whether they heard a single, fused auditory event on one side of the midline, or two separate events on both sides. In the control conditions, referred to as the "single" conditions, echo threshold was measured for a single click pair, the test pair, presented in isolation. In addition to the control conditions, two kinds of test conditions were investigated, in which the test pair was preceded by 12 identical conditioning pairs: in the "same" conditions, the interaural configuration (ILDs or ITDs) of the conditioning pairs was identical to that of the test pair; in the "switch" conditions, the interaural configuration of lead and lag was reversed between the conditioning pairs and the test pair, in order to produce a switch in the lateralizations of the stimuli between the conditioning train and the test pair. No matter whether the lateralization of the clicks was produced by ILDs or by ITDs, most listeners experienced a buildup of echo suppression in the "same" conditions, manifested by a prolongation of echo threshold relative to the respective "single" conditions. However, the breakdown of echo suppression was much stronger in the ILD-switch than in the ITD-switch conditions. In five out of six listeners, the ITD switch had hardly any effect on echo threshold, although the ITDs (+/-300 micros) produced roughly the same degree of lateral displacement as the ILDs (+/-10 dB). These results suggest that the dynamic processes in echo suppression operate differentially in pathways responsible for the processing of interaural time and level differences.     

The role of head-induced interaural time and level differences in the speech reception threshold for multiple interfering sound sources

Three experiments investigated the roles of interaural time differences (ITDs) and level differences (ILDs) in spatial unmasking in multi-source environments. In experiment 1, speech reception thresholds (SRTs) were measured in virtual-acoustic simulations of an anechoic environment with three interfering sound sources of either speech or noise. The target source lay directly ahead, while three interfering sources were (1) all at the target's location (0 degrees,0 degrees,0 degrees), (2) at locations distributed across both hemifields (-30 degrees,60 degrees,90 degrees), (3) at locations in the same hemifield (30 degrees,60 degrees,90 degrees), or (4) co-located in one hemifield (90 degrees,90 degrees,90 degrees). Sounds were convolved with head-related impulse responses (HRIRs) that were manipulated to remove individual binaural cues. Three conditions used HRIRs with (1) both ILDs and ITDs, (2) only ILDs, and (3) only ITDs. The ITD-only condition produced the same pattern of results across spatial configurations as the combined cues, but with smaller differences between spatial configurations. The ILD-only condition yielded similar SRTs for the (-30 degrees,60 degrees,90 degrees) and (0 degrees,0 degrees,0 degrees) configurations, as expected for best-ear listening. In experiment 2, pure-tone BMLDs were measured at third-octave frequencies against the ITD-only, speech-shaped noise interferers of experiment 1. These BMLDs were 4-8 dB at low frequencies for all spatial configurations. In experiment 3, SRTs were measured for speech in diotic, speech-shaped noise. Noises were filtered to reduce the spectrum level at each frequency according to the BMLDs measured in experiment 2. SRTs were as low or lower than those of the corresponding ITD-only conditions from experiment 1. Thus, an explanation of speech understanding in complex listening environments based on the combination of best-ear listening and binaural unmasking (without involving sound-localization) cannot be excluded.     

Localization cues with bilateral cochlear implants

Selected subjects with bilateral cochlear implants (CIs) showed excellent horizontal localization of wide-band sounds in previous studies. The current study investigated localization cues used by two bilateral CI subjects with outstanding localization ability. The first experiment studied localization for sounds of different spectral and temporal composition in the free field. Localization of wide-band noise was unaffected by envelope pulsation, suggesting that envelope-interaural time difference (ITD) cues contributed little. Low-pass noise was not localizable for one subject and localization depended on the cutoff frequency for the other which suggests that ITDs played only a limited role. High-pass noise with slow envelope changes could be localized, in line with contribution of interaural level differences (ILDs). In experiment 2, processors of one subject were raised above the head to void the head shadow. If they were spaced at ear distance, ITDs allowed discrimination of left from right for a pulsed wide-band noise. Good localization was observed with a head-sized cardboard inserted between processors, showing the reliance on ILDs. Experiment 3 investigated localization in virtual space with manipulated ILDs and ITDs. Localization shifted predominantly for offsets in ILDs, even for pulsed high-pass noise. This confirms that envelope ITDs contributed little and that localization with bilateral CIs was dominated by ILDs.     

Binaural sensitivity as a function of interaural electrode position with a bilateral cochlear implant user

Experiments were conducted with a single, bilateral cochlear implant user to examine interaural level and time-delay cues that putatively underlie the design and efficacy of bilateral implant systems. The subject's two implants were of different types but custom equipment allowed presentation of controlled bilateral stimuli, particularly those with specified interaural time difference (ITD) and interaural level difference (ILD) cues. A lateralization task was used to measure the effect of these cues on the perceived location of the sensations elicited. For trains of fixed-amplitude, biphasic current pulses at 100 pps, the subject demonstrated sensitivity to an ITD of 300 micros, providing evidence of access to binaural information. The choice of bilateral electrode pair greatly influenced ITD sensitivity, suggesting that electrode pairings are likely to be an important consideration in the effort to provide binaural advantages. The selection of bilateral electrode pairs showing sensitivity to ITD was partially aided by comparisons of the pitch elicited by individual electrodes in each ear (when stimulated alone with fixed-amplitude current pulses at 813 pps): specifically, interaural electrodes with similar pitches were more likely (but not certain) to show ITD sensitivity. Significant changes in lateral position occurred with specific electrode pairs. With five bilateral electrode pairs of 14 tested, ITDs of 300 and 600 micros moved an auditory image significantly from right to left. With these same pairs, ILD changes of approximately 11% of the dynamic range (in microApp) moved an auditory image from the far left to the far right-significantly farther than the nine pairs not showing significant ITD sensitivity. However, even these nine pairs did show response changes as a function of the interaural (or confounding monaural) level cue. Overall, insofar as the access to bilateral cues demonstrated herein generalizes to other subjects, it provides hope that the normal binaural advantages for speech recognition and sound localization can be made available to bilateral implant users.     

Binaural modulation masking

Modulation thresholds were measured in three subjects for a sinusoidally amplitude-modulated (SAM) wideband noise (the signal) in the presence of a second amplitude-modulated wideband noise (the masker). In monaural conditions (Mm-Sm) masker and signal were presented to only one ear; in binaural conditions (M0-S pi) the masker was presented diotically while the phase of modulation of the SAM noise signal was inverted in one ear relative to the other. In experiment 1 masker modulation frequency (fm) was fixed at 16 Hz, and signal modulation frequency (fs) was varied from 2-512 Hz. For monaural presentation, masking generally decreased as fs diverged from fm, although there was a secondary increase in masking for very low signal modulation frequencies, as reported previously [Bacon and Grantham, J. Acoust. Soc. Am. 85, 2575-2580 (1989)]. The binaural masking patterns did not show this low-frequency upturn: binaural thresholds continued to improve as fs decreased from 16 to 2 Hz. Thus, comparing masked monaural and masked binaural thresholds, there was an average binaural advantage, or masking-level difference (MLD) of 9.4 dB at fs = 2 Hz and 5.3 dB at fs = 4 Hz. In addition, there were positive MLDs for the on-frequency condition (fm = fs = 16 Hz: average MLD = 4.4 dB) and for the highest signal frequency tested (fs = 512 Hz: average MLD = 7.3 dB). In experiment 2 the signal was a SAM noise (fs = 16 Hz), and the masker was a wideband noise, amplitude-modulated by a narrow band of noise centered at fs. There was no effect on monaural or binaural thresholds as masker modulator bandwidth was varied from 4 to 20 Hz (the average MLD remained constant at 8.0 dB), which suggests that the observed "tuning" for modulation may be based on temporal pattern discrimination and not on a critical-band-like filtering mechanism. In a final condition the masker modulator was a 10-Hz-wide band of noise centered at the 64-Hz signal modulation frequency. The average MLD in this case was 7.4 dB. The results are discussed in terms of various binaural capacities that probably play a role in binaural release from modulation masking, including detection of varying interaural intensity differences (IIDs) and discrimination of interaural correlation.     

Lateralization and detection of pulse trains with alternating interaural time delays

The effect of onset interaural time differences (ITDs) on lateralization and detection was investigated for broadband pulse trains 250 ms long with a binaural fundamental frequency of 250 Hz. Within each train, ITDs of successive binaural pulse pairs alternated between two of three values (0 micros, 500 micros left-leading, and 500 micros right-leading) or were invariant. For the alternating conditions, the experimental manipulation was the choice of which of two ITDs was presented first (i.e., at stimulus onset). Lateralization, which was estimated using a broadband noise pointer with a listener adjustable interaural delay, was determined largely by the onset ITD. However, detection thresholds for the signals in left-leading or diotic continuous broadband noise were not affected by where the signals were lateralized. A quantitative analysis suggested that binaural masked thresholds for the pulse trains were well accounted for by the level and phase of harmonic components at 500 and 750 Hz. Detection thresholds obtained for brief stimuli (two binaural pulse or noise burst pairs) were also independent of which of two ITDs was presented first. The control of lateralization by onset cues appears to be based on mechanisms not essential for binaural detection.     

Statistical bias in the assessment of binaural benefit relative to the better ear

The comparison of measured binaural performance with the better of two monaural measures (one from each ear) may lead to underestimated binaural benefit due to statistical sampling bias that favors the monaural condition. The mathematical basis of such bias is reviewed and applied to speech reception thresholds measured in 32 bilateral cochlear implant (CI) users for coincident and spatially separated speech and noise. It is shown that the bias increases with test-retest variation and is maximal for uncorrelated samples of identical underlying performance in each ear. When measured differences between ears were assumed to reflect actual underlying performance differences, the bias averaged across the CI users was about 0.2 dB for coincident target and noise, and 0.1 dB for spatially separated conditions. An upper-bound estimate of the bias, based on the assumption that both ears have the same underlying performance and observed differences were due to test-retest variation, was about 0.7 dB regardless of noise location. To the extent that the test-retest variation in these data is comparable to other studies, the results indicate that binaural benefits in bilateral cochlear implant users are not substantially underestimated (on for average) when binaural performance is compared with the better ear in each listening configuration.     

The effect of head-induced interaural time and level differences on speech intelligibility in noise

A study was made of the effect of interaural time delay (ITD) and acoustic headshadow on binaural speech intelligibility in noise. A free-field condition was simulated by presenting recordings, made with a KEMAR manikin in an anechoic room, through earphones. Recordings were made of speech, reproduced in front of the manikin, and of noise, emanating from seven angles in the azimuthal plane, ranging from 0 degree (frontal) to 180 degrees in steps of 30 degrees. From this noise, two signals were derived, one containing only ITD, the other containing only interaural level differences (ILD) due to headshadow. Using this material, speech reception thresholds (SRT) for sentences in noise were determined for a group of normal-hearing subjects. Results show that (1) for noise azimuths between 30 degrees and 150 degrees, the gain due to ITD lies between 3.9 and 5.1 dB, while the gain due to ILD ranges from 3.5 to 7.8 dB, and (2) ILD decreases the effectiveness of binaural unmasking due to ITD (on the average, the threshold shift drops from 4.6 to 2.6 dB). In a second experiment, also conducted with normal-hearing subjects, similar stimuli were used, but now presented monaurally or with an overall 20-dB attenuation in one channel, in order to simulate hearing loss. In addition, SRTs were determined for noise with fixed ITDs, for comparison with the results obtained with head-induced (frequency dependent) ITDs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Binaural effects in center-frequency modulation detection interference for vowel formants

The detection of slow (5 Hz) center-frequency modulations of formants (signals) can be impaired by the simultaneous presentation of off-frequency modulated formants (maskers) to the same ear [J. Lyzenga and R. P. Carlyon, J. Acoust. Soc. Am. 105, 2792-2806 (1999)]. In the present study we examine this "formant-frequency modulation detection interference (FMDI)" for various binaural masker presentation schemes. Signals and maskers were formantlike complex tones, centered around 1500 and 3000 Hz, respectively. Fundamentals of 80 and 240 Hz were used. The signals were presented to the right ear. The maskers were presented either to the right, the left, or to both ears, and they were either unmodulated or modulated at a slow rate (10 Hz). They had the same fundamental as the signals. Hardly any interference was found for the unmodulated maskers. For modulated maskers, the amount of FMDI depended strongly on the binaural masker presentation scheme. Substantial interference was found for the ipsilateral maskers. Interference was smaller for the contralateral maskers. In both cases the FMDI increased with increasing masker level. Substantial interference was also found for the binaural maskers. Imposing different interaural time and level differences (ITDs and ILDs) on maskers and signals did not affect FMDI. The same was true for the ITD condition when the maskers had different fundamentals than the signals, though FMDI was slightly smaller here. The amount of interference for the binaural maskers was roughly equal to that of the corresponding monaural masker with the largest effect. The data could not be described accurately using a model based on the loudness of the maskers. On the other hand, they were well described by a model in which the amount of FMDI was predicted from a "weighted combination" of the monaural masker levels.     

Measurements of binaural echo suppression

The notion of binaural echo suppression that has persisted through the years states that when listening binaurally, the effects of reverberation (spectral modulation or coloration) are less noticeable than when listening with one ear only. This idea was tested in the present study by measuring thresholds for detection of an echo of a diotic noise masker with the echo presented with either a zero or a 500-musec interaural delay. With echo delays less than 5-10 msec, thresholds for the diotic echo were about 10 dB lower than for the dichotic signal, a finding opposite that of the usual binaural masking-level difference but consistent with the notion of binaural echo suppression. Additional echo-threshold measurements were made with echoes of interaurally reversed polarity, producing out-of-phase spectral modulations. The 10-15 dB increase in thresholds for the reverse-polarity echo, over those for the same-polarity echo, indicated that the apparent "hollowness" associated with spectral modulations can be partially canceled centrally. Overall, the results of this study are consistent with a model in which: (1) the monaural representations of spectral magnitude are nonlinearly compressed prior to being combined centrally; and (2) neither monaural channel can be isolated in order to perform the detection task.     

NoSo and NoS pi detection as a function of masker bandwidth in normal-hearing and cochlear-impaired listeners

NoSo and NoS pi detection thresholds for a 500-Hz pure-tone signal were measured as a function of masking noise bandwidth in normal-hearing and cochlear hearing-impaired subjects. NoSo and NoS pi critical bands were derived from the bandlimited noise functions. A notched noise measure of the monaural critical band was also obtained for each ear. One hypothesis tested was that an asymmetrical monaural critical band would result in a relatively steep improvement of the NoS pi detection threshold as a function of decreasing masker bandwidth and would, therefore, be associated with a wider binaural critical band. This was hypothesized because the outputs of the left and right auditory filters would be more decorrelated the greater the interaural difference in the monaural critical band. However, as the noise bandwidth was narrowed, the decorrelation would lessen, resulting in a relatively steep improvement in NoS pi detection. Results indicated that the masking level difference (MLD) was smaller and that the monaural critical bands were generally wider in cochlear-impaired listeners. NoSo and NoS pi critical bands were somewhat larger in the cochlear hearing-impaired listeners having relatively wide monaural critical bands. There was a significant correlation between monaural critical band asymmetry and the NoS pi critical band; however, this correlation was insignificant when a control was employed for the critical band in the worse ear. Therefore, the present results did not support a strong association between monaural critical band asymmetry and the width of the NoS pi critical band.     

The effect of interaural differences in envelope shape on the perceived location of sounds (L)

Users of bilateral cochlear implants and a cochlear implant combined with a contralateral hearing aid are sensitive to interaural time differences (ITDs). The way cochlear implant speech processors work and differences between modalities may result in interaural differences in shape of the temporal envelope presented to the binaural system. The effect of interaural differences in envelope shape on ITD sensitivity was investigated with normal-hearing listeners using a 4?kHz pure tone modulated with a periodic envelope with a trapezoid shape in each cycle. In one ear the onset segment of the trapezoid was transformed by a power function. No effect on the just noticeable difference in ITD was found with an interaural difference in envelope shape, but the ITD for a centered percept was significantly different across envelope shape conditions.     

Lateralization of bands of noise and sinusoidally amplitude-modulated tones: effects of spectral locus and bandwidth

Lateralization of narrow bands of noise was investigated while varying interaural temporal disparity (ITD), center frequency, and bandwidth, utilizing an acoustic pointing task. Stimuli were narrow bands of noise centered at octave intervals between 500 Hz and 4 kHz with bandwidths ranging from 50-400 Hz. In a second experiment, lateralization for bands of noise and sinusoidally amplitude-modulated (SAM) tones, whose spectral content was constrained to be no lower than 3.8 kHz, was assessed. Overall, relatively large extents of laterality were obtained from all four listeners for ITDs of low-frequency bands of noise. Increasing the bandwidth of these noises did not yield consistent changes in the extent of laterality across ITDs and listeners. Most targets centered at high frequencies were lateralized near the midline. However, three of the four listeners did exhibit rather large displacements of the intracranial image when the bandwidth of the high-frequency noises was 400 Hz or greater. Interestingly, ITDs within high-frequency SAM tones were relatively ineffective. Thus, it appears that ITDs of relatively wide-band, high-frequency stimuli can mediate rather substantial extents of laterality. However, these effects are highly listener-dependent.     

Perception of across-frequency interaural level differences

The interaural level difference (ILD) is an important cue for the localization of sound sources. Just noticeable differences (JND) in ILD were measured in 12 normal hearing subjects for uncorrelated noise bands with a bandwidth of 13 octave and a different center frequency in both ears. In one ear the center frequency was either 250, 500, 1000, or 4000 Hz. In the other ear, a frequency shift of 0, 16, 13, or 1 octave was introduced. JNDs in ILD for unshifted, uncorrelated noise bands of 13 octave width were 2.6, 2.6, 2.5, and 1.4 dB for 250, 500, 1000, and 4000 Hz, respectively. Averaged over all shifts, JNDs decreased significantly with increasing frequency. For the shifted conditions, JNDs increased significantly with increasing shift. Performance on average worsened by 0.5, 0.9, and 1.5 dB for shifts of 16, 13, and 1 octave. Though performance decreases, the just noticeable ILDs for the shifted conditions were still in a range usable for lateralization. This has implications for signal processing algorithms for bilateral bimodal hearing instruments and the fitting of bilateral cochlear implants.     

The acoustical cues to sound location in the rat: measurements of directional transfer functions

The acoustical cues for sound location are generated by spatial- and frequency-dependent filtering of propagating sound waves by the head and external ears. Although rats have been a common model system for anatomy, physiology, and psychophysics of localization, there have been few studies of the acoustical cues available to rats. Here, directional transfer functions (DTFs), the directional components of the head-related transfer functions, were measured in six adult rats. The cues to location were computed from the DTFs. In the frontal hemisphere, spectral notches were present for frequencies from approximately 16 to 30 kHz; in general, the frequency corresponding to the notch increased with increases in source elevation and in azimuth toward the ipsilateral ear. The maximum high-frequency envelope-based interaural time differences (ITDs) were 130 mus, whereas low-frequency (<3.5 kHz) fine-structure ITDs were 160 mus; both types of ITDs were larger than predicted from spherical head models. Interaural level differences (ILDs) strongly depended on location and frequency. Maximum ILDs were <10 dB for frequencies <8 kHz and were as large as 20-40 dB for frequencies >20 kHz. Removal of the pinna eliminated the spectral notches, reduced the acoustic gain and ILDs, altered the acoustical axis, and reduced the ITDs.