Advances in the Field of Bone Conduction Hearing Implants.

The number of marketed bone-conduction hearing implants (BCHIs) has been steadily growing, with multiple percutaneous devices and transcutaneous devices now available. However, studies assessing efficacy often have small sample sizes and employ different assessment methodologies. Thus, there is a paucity of evidence to guide clinicians to the most appropriate device for each patient. This paper outlines audiological guidelines for the latest devices, as well as research from the most up-to-date clinical trials. We also outline the evidence base for some potentially contentious issues in the field of bone conduction, including bilateral fitting of BCHIs in those with bilateral conductive hearing loss as well as the use of BCHIs in single-sided deafness (SSD). Bilateral fitting of BCHIs have been found to significantly increase the hearing thresholds in quiet and improve sound localization, but to give limited benefits in background noise. Studies conducted via multiple assessment questionnaires have found strong evidence of subjective benefits for the use of BCHIs in SSD. However, there is little objective evidence of benefit for SSD patients from sound localization and speech in noise tests.

Interaural correlation sensitivity.

Sensitivity to differences in interaural correlation was measured for 1.3-ERB-wide bands of noise using a 2IFC task at six frequencies: 250, 500, 750, 1000, 1250, and 1500 Hz. The sensitivity index, d', was measured for discriminations between a number of fixed pairs of correlation values. Cumulative d' functions were derived for each frequency and condition. The d' for discriminating any two values of correlation may be recovered from the cumulative d' function by the difference between cumulative d''s for these values. Two conditions were employed: the noisebands were either presented in isolation (narrow-band condition) or in the context of broad, contiguous flanking bands of correlated noise (fringed condition). The cumulative d' functions showed greater sensitivity to differences in correlation close to 1 than close to 0 at low frequencies, but this difference was less pronounced in the fringed condition. Also, a more linear relationship was observed when cumulative d' was plotted as a function of the equivalent signal-to-noise ratio (SNR) in dB for each correlation value, rather than directly against correlation. The equivalent SNR was the SNR at which the interaural correlation in an NoS(pi) stimulus would equal the interaural correlation of the noise used in the experiment. The maximum cumulative d' declined above 750 Hz. This decline was steeper for the fringed than for the narrow-band condition. For the narrow-band condition, the total cumulative d' was variable across listeners. All cumulative d' functions were closely fitted using a simple two-parameter function. The complete data sets, averaged across listeners, from the fringed and narrow-band conditions were fitted using functions to describe the changes in these parameters over frequency, in order to produce an interpolated family of curves that describe sensitivity at frequencies between those tested. These curves predict the spectra recovered by the binaural system when complex sounds, such as speech, are masked by noise.

Auditory motion segregation: a limited analogy with vision.

Interaural time delay (ITD) is the dominant cue to sound-source location. When one component of a complex sound changes rapidly but smoothly in ITD, it perceptually segregates from the complex. When different components were changed in ITD in succession, a recognizable melody was heard. Each note was more detectable from the transition than from a distinct ITD with respect to the complex. However, a transition relative to a coherently changing complex produced no segregation, whereas cyclic modulation of ITD, too rapid to be perceived as movement, did produce segregation. These two results suggest that the relevant cue is not movement per se but rather the lack of a well-defined ITD during the transition. Segregation largely disappeared when the transition in ITD was replaced by a temporal gap in the complex of the order of 100 ms. The effect seems similar to visual motion segregation but is best explained by the mechanism of binaural unmasking.

Dichotic pitches as illusions of binaural unmasking. III. The existence region of the Fourcin pitch.

Two experiments explored the existence region of the Fourcin pitch. In each experiment, detectability was assessed by measuring listeners' ability to discriminate pitch changes. In the first experiment, the detectability of the pitch was measured as a function of the number of noises used to generate it. In the second experiment, the pitch was generated using two noises with equal and opposite interaural delays and detectability was measured as a function of the difference between these two delays, and thus of the perceived pitch height. In each case, the experimental results were compared with the predictions produced by a model of binaural unmasking, based on equalization cancellation, that had been designed to recover broadband sounds, such as speech, from interfering noise [Culling and Summerfield, J. Acoust. Soc. Am. 98, 785-797 (1995)]. The model accurately predicted the results from experiment 1, but failed to show an adequate decline in performance for small differences in interaural delay (corresponding to higher perceived pitches) in experiment 2. A revised model, based on similar principles, but using data on listeners' sensitivity to interaural decorrelation, rather than an equalization-cancellation mechanism, was able to predict the results of both experiments successfully.

Binaural sluggishness in the perception of tone sequences and speech in noise.

The binaural system is well-known for its sluggish response to changes in the interaural parameters to which it is sensitive. Theories of binaural unmasking have suggested that detection of signals in noise is mediated by detection of differences in interaural correlation. If these theories are correct, improvements in the intelligibility of speech in favorable binaural conditions is most likely mediated by spectro-temporal variations in interaural correlation of the stimulus which mirror the spectro-temporal amplitude modulations of the speech. However, binaural sluggishness should limit the temporal resolution of the representation of speech recovered by this means. The present study tested this prediction in two ways. First, listeners' masked discrimination thresholds for ascending vs descending pure-tone arpeggios were measured as a function of rate of frequency change in the NoSo and NoSpi binaural configurations. Three-tone arpeggios were presented repeatedly and continuously for 1.6 s, masked by a 1.6-s burst of noise. In a two-interval task, listeners determined the interval in which the arpeggios were ascending. The results showed a binaural advantage of 12-14 dB for NoSpi at 3.3 arpeggios per s (arp/s), which reduced to 3-5 dB at 10.4 arp/s. This outcome confirmed that the discrimination of spectro-temporal patterns in noise is susceptible to the effects of binaural sluggishness. Second, listeners' masked speech-reception thresholds were measured in speech-shaped noise using speech which was 1, 1.5, and 2 times the original articulation rate. The articulation rate was increased using a phase-vocoder technique which increased all the modulation frequencies in the speech without altering its pitch. Speech-reception thresholds were, on average, 5.2 dB lower for the NoSpi than for the NoSo configuration, at the original articulation rate. This binaural masking release was reduced to 2.8 dB when the articulation rate was doubled, but the most notable effect was a 6-8 dB increase in thresholds with articulation rate for both configurations. These results suggest that higher modulation frequencies in masked signals cannot be temporally resolved by the binaural system, but that the useful modulation frequencies in speech are sufficiently low (<5 Hz) that they are invulnerable to the effects of binaural sluggishness, even at elevated articulation rates.

The existence region of Huggins' pitch.

Huggins' pitch (HP) can be heard when listening to white noise which is diotic at all frequencies except for a narrow band over which the interaural phase of the noise changes progressively through 360 degrees. The detectability (d') of HP was measured for 11 center frequencies between 100 Hz and 3200 Hz in half-octave steps. The listeners' task was to discriminate HP stimuli from diotic noises in a single-interval, YES/NO task. Detectability was simultaneously compared with that of pure tones presented interaurally out of phase and masked by diotic white noise (NoS pi). The levels of the tones were set so that they would fall below masked threshold monaurally at the higher frequencies tested. All four listeners showed similar patterns of detectability at high frequencies for both the Huggins-pitch stimuli and the masked tones. The results showed that HP can be detected at higher frequencies than previously reported; all four listeners could detect both the tones and HP to some extent at 2256 Hz and one listener could also detect HP at 3200 Hz. The results demonstrate that both HP and NoS pi stimuli show a sharp drop in detectability at around 1500 Hz, but remain detectable at higher frequencies.

Lateralization of large interaural delays.

Two experiments explored the limits of listeners' abilities to interpret large interaural time delays (ITDs) in terms of laterality. In experiment 1, just-noticeable differences (jnd's) were measured, using an adaptive procedure, for various reference ITDs of Gaussian noise between 0 and 3000 microseconds. The jnd's increased gradually with reference ITD for reference ITDs between 0 microsecond and 700 microseconds, then rose sharply to plateau at much higher jnd's for the remainder of the standard ITDs tested (1000-3000 microseconds). The second experiment tested left/right discrimination of Gaussian noise that was interaurally delayed up to 10,000 microseconds, and high-pass filtered to cutoff frequencies between 0 Hz (broadband) and 3000 Hz. There was good discrimination (62%; significantly above chance, p < 0.05) for broadband and 500-Hz high-pass cutoff stimuli for all ITDs up to 10,000 microseconds, and for ITDs up to at least 3000 microseconds for higher high-pass cutoff frequencies. These results indicate that laterality cues are discriminable at much larger ITDs than are experienced in free-field listening, even in the absence of energy below 3 kHz.

Dichotic pitches as illusions of binaural unmasking. I. Huggins' pitch and the "binaural edge pitch".

The two most salient dichotic pitches, the Huggins pitch (HP) and the binaural edge pitch (BEP), are produced by applying interaural phase transitions of 360 and 180 degrees, respectively, to a broadband noise. This paper examines accounts of these pitches, concentrating on a "central activity pattern" (CAP) model and a "modified equalization-cancellation" (mE-C) model. The CAP model proposes that a dichotic pitch is heard at frequency f when an individual across-frequency scan in an interaural cross-correlation matrix contains a sharp peak at f. The mE-C model proposes that a dichotic pitch is heard when a plot of interaural decorrelation against frequency contains a peak at f. The predictions of the models diverge for the BEP at very narrow transition bandwidths: the mE-C model predicts that salience is sustained, while the CAP model predicts that salience declines and that the dominant percept is of the in-phase segment of the noise. Experiment 1 showed that the salience of the BEP was sustained at the narrowest bandwidths that could be generated (0.5% of the transition frequency). Experiment 2 confirmed that the pitch of a BEP produced by a 0.5% transition bandwidth was close to the frequency of the transition band. Experiment 3 showed that pairs of simultaneous narrow 180-degree transitions, whose frequencies corresponded to vowel formants, were perceived as the intended vowels. Moreover, the same vowels were perceived whether the in-phase portion of the noise lay between the two transition frequencies or on either side of them. In contrast, different patterns of identification responses were made to diotic band-pass and band-stop noises whose cutoff frequencies corresponded to the same formants. Thus, the vowel-identification responses made to the dichotic stimuli were not based on hearing the in-phase portions of the noise as formants. These results are not predicted by the CAP model but are consistent with the mE-C model. It is argued that the mE-C model provides a more coherent and parsimonious account of many aspects of the HP and the BEP than do alternative models.

Dichotic pitches as illusions of binaural unmasking. II. The Fourcin pitch and the dichotic repetition pitch.

The predictions of three models are compared with respect to existing experimental data on the perception of the Fourcin pitch (FP) and the dichotic repetition pitch (DRP). Each model generates a central spectrum (CS), which is examined for peaks at frequencies consistent with the perceived pitches. A modified equalization-cancellation (mE-C) model of binaural unmasking [Culling and Summerfield, J. Acoust. Soc. Am. 98, 785-797 (1995)] generates a CS which reflects the degree of interaural decorrelation present in each frequency channel. This model accounts for the perceived frequencies of FPs, but produces no output for DRP stimuli. A restricted equalization-cancellation (rE-C) model [Bilsen and Goldstein, J. Acoust. Soc. Am. 55, 292-296 (1974)] sums the time-varying excitation in corresponding frequency channels, without equalization, to form a CS. A central activity pattern (CAP) model [Raatgever and Bilsen, J. Acoust. Soc. Am. 80, 429-441 (1986)] generates a CS by scanning an interaural cross-correlation matrix across frequency. The rE-C and CAP models yield inaccurate predictions of the perceived frequencies of FPs, but predict the occurrence of the DRP and its correct pitch. The complementary predictions of the mE-C model compared to the rE-C and CAP models, together with the evidence that the FP is clearly audible for the majority of listeners, while the DRP is faintly heard by a minority of listeners, suggest that the mE-C model provides the best available account of the FP, and that the DRP is produced by a separate mechanism.

Changes in lateralization and loudness judgements during one week of unilateral ear plugging.

The aim of this study was to determine whether lateralization judgements show adaptation during a period of unilateral ear plugging. Six normally hearing young adults were tested repeatedly using pure tone stimuli of 500 and 4000 Hz to determine (i) the threshold in each ear, (ii) the interaural sensation level difference (ISLD) at which sounds presented alternately to the two ears were of equal loudness, and (iii) the ISLD at which sounds presented simultaneously to the two ears produced a centered internal sound image. Subjects were tested 9-14 times over one week before an ear plug, producing a nominal attenuation of 21 dB (at both frequencies), was placed in one ear. Subjects wore the plug continuously for a further week, and were tested daily during this period, with the plug in place. After unplugging, subjects were tested less frequently for one final week. Net changes in binaural hearing were measured by subtracting the equal loudness ISLD from the centering ISLD. Four subjects showed no net change, either during or after plugging, but a small (< 3 dB) adaptation occurred during plugging in two subjects.

Perceptual separation of concurrent speech sounds: absence of across-frequency grouping by common interaural delay.

Three experiments and a computational model explored the role of within-channel and across-channel processes in the perceptual separation of competing, complex, broadband sounds which differed in their interaural phase spectra. In each experiment, two competing vowels, whose first and second formants were represented by two discrete bands of noise, were presented concurrently, for identification. Experiments 1 and 2 showed that listeners were able to identify the vowels accurately when each was presented to a different ear, but were unable to identify the vowels when they were presented with different interaural time delays (ITDs); i.e. listeners could not group the noisebands in different frequency regions with the same ITD and thereby separate them from bands in other frequency regions with a different ITD. Experiment 3 demonstrated that while listeners were unable to exploit a difference in interaural delay between the pairs of noisebands, listeners could identify a vowel defined by interaurally decorrelated noisebands when the other two noisebands were interaurally correlated. A computational model based upon that of Durlach [J. Acoust. Soc. Am. 32, 1075-1076 (1960)] showed that the results of these and other experiments can be interpreted in terms of a within-channel mechanism, which is sensitive to interaural decorrelation. Thus the across-frequency integration which occurs in the lateralization of complex sounds may play little role in segregating concurrent sounds.

The role of frequency modulation in the perceptual segregation of concurrent vowels.

Two experiments investigated the effect of frequency modulation on the identification of vowel sounds presented concurrently with interfering vowels. In experiment 1, identification thresholds were measured for each of five target vowels, masked, in each trial, by one of ten masking vowels. Both target and masking vowels were synthesized using harmonically spaced frequency components. Inharmonic spacing was used in order to prevent powerful grouping processes which exploit fundamental frequency from dominating the results. The target vowels were synthesized with sinusoidal frequency modulation on each frequency component which was either coherent (same phase) or incoherent (random phases). The masking vowels were synthesized with components which were either modulated in the same way as the target vowel or were unmodulated. Identification thresholds were lower when the masking vowel had no modulation. The effect occurred for both coherent and incoherent frequency modulation, indicating that it is mediated by the movement of each component independently, rather than by grouping of coherently modulated components. This result is consistent in some respects with judgments of the prominence of competing vowels [S. E. McAdams, J. Acoust. Soc. Am. 85, 2148-2159 (1989)], which show that modulated vowels are more prominent than unmodulated vowels regardless of the type of modulation applied to the competing vowels. Experiment 2 used a paradigm similar to that developed by McAdams, in order to compare more directly the effect of FM on vowel identification and vowel prominence. On each trial, three vowels were presented concurrently. Either none, one, or two of the vowels were modulated throughout, while modulation was applied to another vowel (the target) halfway through the stimulus. The vowels were either harmonic (with different fundamental frequencies) and coherently modulated or inharmonic and incoherently modulated. Accuracy of identification of the target vowel was not significantly different in the harmonic/coherent and inharmonic/incoherent conditions and declined, in each case, as the number of modulated background vowels increased. Overall, the results of experiments 1 and 2, and of McAdams' prominence judgment data, suggest that there is an auditory mechanism for detecting frequency modulation which can alert the listener to the presence of frequency modulated sounds, but which is insensitive to across-frequency differences in the pattern of that modulation.

Perceptual and computational separation of simultaneous vowels: cues arising from low-frequency beating.

Identification of simultaneous speech sounds, such as pairs of steady-state vowels (double vowels), is more accurate when there is a difference in fundamental frequency (F0). Accuracy of identification for double vowels increases with increasing F0 difference (delta F0) asymptoting above 1 semitone. The experiment described here attempts to distinguish two mechanisms underlying this effect: first, perceptual separation by grouping together harmonic components of a common F0; and, second, exploitation of the fluctuations in the spectral envelope of the composite stimulus that result from beating between unresolved components. The beating is mainly caused by interactions between corresponding harmonics of the two vowels with a small delta F0. Identification accuracy for normal, harmonically excited double vowels was compared with that for double vowels composed from the same components, but whose constituent vowels were excited by a mixture of the two harmonic series. These double vowels were designed to produce similar beating patterns to the normal double vowels. Both harmonically and inharmonically excited constituents improved identification with increasing delta F0, but the increase was larger for harmonically excited vowels. A computational model based upon psychophysical measurements of auditory frequency and temporal resolution correctly predicted an increase in accuracy of identification with increasing delta F0 which was attributable to beating. The results are interpreted in terms of a spectral change cue in the identification of double vowels with delta F0's which complements grouping by F0, and which plays a dominant role for delta F0's smaller than 1 semitone.

The role of timbre in the segregation of simultaneous voices with intersecting F0 contours.

When the fundamental frequency (F0) contours of two speakers' voices intersect, the listener is presented with a problem. The listener must decide which of the F0 contours emerging from the intersection is a continuation of which contour entering the intersection: have the F0 contours crossed or merely approached and parted? In the present experiment, subjects listened to two simultaneous diphthong-like sounds with F0 contours that either approached and diverged or crossed over. The task was to report whether the pitches "crossed" or "bounced" away from each other. Despite the changing timbres of the two sounds, the subjects were able to discriminate crossing and bouncing F0s, provided that the timbres of the vowels differed at the moment when their F0s were the same. When the timbres were the same, the subjects could not make the discrimination and tended to hear a bouncing percept. These results are consistent with the idea that listeners use continuity of timbre rather than continuity of F0 movement to disambiguate F0 intersections.

Perceptual separation of simultaneous vowels: within and across-formant grouping by F0.

Six experiments explored why the identification of the two members of a pair of diotic, simultaneous, steady-state vowels improves with a difference in fundamental frequency (delta F0). Experiment 1 confirmed earlier reports that a delta F0 improves identification of 200-ms but not 50-ms duration "double vowels"; identification improves up to 1 semitone delta F0 and then asymptotes. In such stimuli, all the formants of a given vowel are excited by the same F0, providing listeners with a potential grouping cue. Subsequent experiments asked whether the improvement in identification with delta F0 for the longer vowels was due to listeners using the consistent F0 within each vowel of a pair to group formants appropriately. Individual vowels were synthesized with a different F0 in the region of the first formant peak from in the region of the higher formant peaks. Such vowels were then paired so that the first formant of one vowel bore the same F0 as the higher formants of the other vowel. These across-formant inconsistencies in F0 did not substantially reduce the previous improvement in identification rates with increasing delta F0's of up to 4 semitones (experiment 2). The subjects' improvement with increasing delta F0 in the inconsistent condition was not produced by identifying vowels on the basis of information in the first-formant or higher-formant regions alone, since stimuli which contained either of these regions in isolation were difficult for subjects to identify. In addition, the inconsistent condition did produce poorer identification for larger delta F0's (experiment 3). The improvement in identification with delta F0 found for the inconsistent stimuli persisted when the delta F0 between vowel pairs was confined to the first formant region (experiment 4) but not when it was confined to the higher formants (experiment 6). The results replicate at different overall presentation levels (experiment 5). The experiments show that at small delta F0's only the first-formant region contributes to improvements in identification accuracy, whereas with larger delta F0's the higher formant region may also contribute. This difference may be related to other results that demonstrate the superiority of resolved rather than unresolved harmonics in coding pitch.

Auditory segregation of competing voices: absence of effects of FM or AM coherence.

Four experiments sought evidence that listeners can use coherent changes in the frequency or amplitude of harmonics to segregate concurrent vowels. Segregation was not helped by giving the harmonics of competing vowels different patterns of frequency or amplitude modulation. However, modulating the frequencies of the components of one vowel was beneficial when the other vowel was not modulated, provided that both vowels were composed of components placed randomly in frequency. In addition, staggering the onsets of the two vowels, so that the amplitude of one vowel increased abruptly while the amplitude of the other was stationary, was also beneficial. Thus, the results demonstrate that listeners can group changing harmonics and can segregate them from stationary harmonics, but cannot use coherence of change to separate two sets of changing harmonics.