Perception of Across-Frequency Asynchrony by Listeners with Cochlear Hearing Loss

Cochlear hearing loss is often associated with broader tuning of the cochlear filters. Cochlear response latencies are dependent on the filter bandwidths, so hearing loss may affect the relationship between latencies across different characteristic frequencies. This prediction was tested by investigating the perception of synchrony between two tones exciting different regions of the cochlea in listeners with hearing loss. Subjective judgments of synchrony were compared with thresholds for asynchrony discrimination in a three-alternative forced-choice task. In contrast to earlier data from normal-hearing (NH) listeners, the synchronous-response functions obtained from the hearing-impaired (HI) listeners differed in patterns of symmetry and often had a very low peak (i.e., maximum proportion of “synchronous” responses). Also in contrast to data from NH listeners, the quantitative and qualitative correspondence between the data from the subjective and the forced-choice tasks was often poor. The results do not provide strong evidence for the influence of changes in cochlear mechanics on the perception of synchrony in HI listeners, and it remains possible that age, independent of hearing loss, plays an important role in temporal synchrony and asynchrony perception.     

The Intelligibility of Interrupted Speech: Cochlear Implant Users and Normal Hearing Listeners

Compared with normal-hearing listeners, cochlear implant (CI) users display a loss of intelligibility of speech interrupted by silence or noise, possibly due to reduced ability to integrate and restore speech glimpses across silence or noise intervals. The present study was conducted to establish the extent of the deficit typical CI users have in understanding interrupted high-context sentences as a function of a range of interruption rates (1.5 to 24 Hz) and duty cycles (50 and 75 %). Further, factors such as reduced signal quality of CI signal transmission and advanced age, as well as potentially lower speech intelligibility of CI users even in the lack of interruption manipulation, were explored by presenting young, as well as age-matched, normal-hearing (NH) listeners with full-spectrum and vocoded speech (eight-channel and speech intelligibility baseline performance matched). While the actual CI users had more difficulties in understanding interrupted speech and taking advantage of faster interruption rates and increased duty cycle than the eight-channel noise-band vocoded listeners, their performance was similar to the matched noise-band vocoded listeners. These results suggest that while loss of spectro-temporal resolution indeed plays an important role in reduced intelligibility of interrupted speech, these factors alone cannot entirely explain the deficit. Other factors associated with real CIs, such as aging or failure in transmission of essential speech cues, seem to additionally contribute to poor intelligibility of interrupted speech.     

Encoding Intensity in Ventral Cochlear Nucleus Following Acoustic Trauma: Implications for Loudness Recruitment

Loudness recruitment, an abnormally rapid growth of perceived loudness with sound level, is a common symptom of sensorineural hearing loss. Following acoustic trauma, auditory-nerve rate responses are reduced, and rate grows more slowly with sound level, which seems inconsistent with recruitment (Heinz et al., J. Assoc. Res. Otolaryngol. 6:91–105, 2005). However, rate-level functions (RLFs) in the central nervous system may increase in either slope or saturation value following trauma (e.g., Salvi et al., Hear. Res. 147:261–274, 2000), suggesting that recruitment may arise from central changes. In this paper, we studied RLFs of neurons in ventral cochlear nucleus (VCN) of the cat after acoustic trauma. Trauma did not change the general properties of VCN neurons, and the usual VCN functional classifications remained valid (chopper, primary-like, onset, etc.). After trauma, non-primary-like neurons, most noticeably choppers, exhibited elevated maximum discharge rates and steeper RLFs for frequencies at and near best frequency (BF). Primary-like neurons showed the opposite changes. To relate the neurons’ responses to recruitment, rate-balance functions were computed; these show the sound level required to give equal rates in a normal and a traumatized ear and are analogous to loudness balance functions that show the sound levels giving equal perceptual loudness in the two ears of a monaurally hearing-impaired person. The rate-balance functions showed recruitment-like steepening of their slopes in non-primary-like neurons in all conditions. However, primary-like neurons showed recruitment-like behavior only when rates were summated across neurons of all BFs. These results suggest that the non-primary-like, especially chopper, neurons may be the most peripheral site of the physiological changes in the brain that underlie recruitment.     

Acoustic Temporal Modulation Detection in Normal-Hearing and Cochlear Implanted Listeners: Effects of Hearing Mechanism and Development

Temporal modulation detection ability matures over many years after birth and may be particularly sensitive to experience during this period. Profound hearing loss during early childhood might result in greater perceptual deficits than a similar loss beginning in adulthood. We tested this idea by measuring performance in temporal modulation detection in profoundly deaf children and adults fitted with cochlear implants (CIs). At least two independent variables could constrain temporal modulation detection performance in children with CIs: altered encoding of modulation information due to the CI-auditory nerve interface, and atypical development of central processing of sound information provided by CIs. The effect of altered encoding was investigated by testing subjects with one of two different hearing mechanisms (normal hearing vs. CI) and the effect of atypical development was studied by testing two different age groups. All subjects were tested for their ability to detect acoustic temporal modulations of sound amplitude. A comparison of the slope, or cutoff frequency, of the temporal modulation transfer functions (TMTFs) among the four subject groups revealed that temporal resolution was mainly constrained by hearing mechanism: normal-hearing listeners could detect smaller amplitude modulations at high modulation frequencies than CI users. In contrast, a comparison of the height of the TMTFs revealed a significant interaction between hearing mechanism and age group on overall sensitivity to temporal modulation: sensitivity was significantly poorer in children with CIs, relative to the other three groups. Results suggest that there is an age-specific vulnerability of intensity discrimination or non-sensory factors, which subsequently affects sensitivity to temporal modulation in prelingually deaf children who use CIs.     

Vowel Perception in Listeners With Normal Hearing and in Listeners With Hearing Loss: A Preliminary Study

ObjectivesTo determine the influence of hearing loss on perception of vowel slices.MethodsFourteen listeners aged 20-27 participated; ten (6 males) had hearing within normal limits and four (3 males) had moderate-severe sensorineural hearing loss (SNHL). Stimuli were six naturally-produced words consisting of the vowels /i a u æ ɛ ʌ/ in a /b V b/ context. Each word was presented as a whole and in eight slices: the initial transition, one half and one fourth of initial transition, full central vowel, one-half central vowel, ending transition, one half and one fourth of ending transition. Each of the 54 stimuli was presented 10 times at 70 dB SPL (sound press level); listeners were asked to identify the word. Stimuli were shaped using signal processing software for the listeners with SNHL to mimic gain provided by an appropriately-fitting hearing aid.ResultsListeners with SNHL had a steeper rate of decreasing vowel identification with decreasing slice duration as compared to listeners with normal hearing, and the listeners with SNHL showed different patterns of vowel identification across vowels when compared to listeners with normal hearing.ConclusionAbnormal temporal integration is likely affecting vowel identification for listeners with SNHL, which in turn affects vowel internal representation at different levels of the auditory system.     

Informational Masking in Listeners with Sensorineural Hearing Loss

Measures of energetic and informational masking were obtained from 46 listeners with sensorineural hearing loss. The task was to detect the presence of a sequence of eight contiguous 60-ms bursts of a pure tone embedded in masker bursts that were played synchronously with the signal. The masker was either a sequence of Gaussian noise bursts (energetic masker) or a sequence of random-frequency 2-tone bursts (informational masker). The 2-tone maskers were of two types: one type that normally tends to produce large amounts of informational masking and a second type that normally tends to produce very little informational masking. The two informational maskers are called "multiple-bursts same" (MBS), because the same frequency components are present in each burst of a sequence, and "multiple-bursts different" (MBD), because different frequency components are presented in each burst of a sequence. The difference in masking observed for these two maskers is thought to occur because the signal perceptually segregates from the masker in the MBD condition but fuses with the masker in MBS. In the present study, the effectiveness of the MBD masker, measured as the signal-to-masker ratio at masked threshold, increased with increasing hearing loss. In contrast, the signal-to-masker ratio at masked threshold for the MBS masker changed much less as a function of hearing loss. These results suggest that sensorineural hearing loss interferes with the ability of the listener to perceptually segregate individual components of complex sounds. The results from the energetic masking condition, which included critical ratio estimates for all listeners and auditory filter characteristics for a subset of the listeners, indicated that increasing hearing loss also reduced frequency selectivity at the signal frequency. Overall, these results suggest that the increased susceptibility to masking observed in listeners with sensorineural hearing loss is a consequence of both peripheral and central processes.     

Amplitude Modulation and Loudness in Cochlear Implantees

The effect of amplitude modulation of pulse trains on the loudness perceived by cochlear implantees was investigated for different overall levels of the signal, modulation depth and the carrier rate of the pulse train. Equally loud and threshold levels were determined for a variety of signal levels, modulation depths and carrier rates in six cochlear implantees. The pattern of results was consistent with the predictions of a previously published loudness model of McKay et al. (J Acoust Soc Am 113:2054–2063, 2003). The degree to which the loudness of modulated stimuli differed from the loudness elicited by an unmodulated pulse train with equivalent average current depended on the modulation depth and the absolute current level of the unmodulated stimulus. The effect of carrier rate on this measure was predictable solely from the effect of rate on absolute current level for equal loudness. The results have important implications for the interpretation of experiments measuring modulation detection that do not control loudness cues. We show that several previously published results regarding the effect of carrier rate and added noise on modulation detection could be reinterpreted in the light of these findings.     

The Acoustic Reflex Threshold: Not Predictive for Loudness Perception in Normally-hearing Listeners

The working hypothesis of an ongoing study is that the quick and reliable procedure of acoustic reflex threshold (ART) determination in conjunction with measurements of HTL may yield accurate estimates of loudness. The aim of this study was to investigate whether differences in loudness in normally-hearing subjects are reflected in the ARTs and to collect normal material with respect to pure-tone elicited ART and loudness categories. Categorical loudness scaling (CLS) and ART measurements were performed at frequencies of 0.5, 1, 2 and 4 kHz in 60 normally-hearing subjects (HTL<20 dB HL, 26 males, 34 females, aged 21–63 years) with no history or sequelae of middle ear disease. Subjects reporting disturbing tinnitus were excluded. The results show that the ART is not a predictor of individual loudness perception for normally-hearing subjects. Using a numerical scale (HTL = 0, ‘very soft’ = 5, ‘soft’ = 15, ‘OK’ =25, ‘loud’ = 35, ‘very loud’ =45 and ‘too loud’ =50) loudness for pure tones grows almost linearly at approximately 0.4 arbitrary loudness units per dB below the ‘loud’ category. Above the ‘loud’ category the slope is around 1 unit per dB. The median ART was 85 dB HL at frequencies of 0.5, 1, 2 and 4 kHz. No differences in loudness perception across frequencies were found.     

Just Audible Pitch and Loudness Modulations with Cochlear Impairment

Just noticeable combined modulations in amplitude and frequency were determined for an observer (DP) with a bowl-shaped sensorineural hearing loss of about 40 dB between 1 and 2 kHz. In all stimulus conditions, interactions were found between the detection of amplitude modulations and frequency modulations. These interactions appear to correlate quantitatively with the effect of frequency on loudness (as given by the slope of the equal-loudness contour). When this effect of frequency on loudness is taken into account, the data support the hypothesis of independent detection of pitch and loudness modulations. The experimental data are also discussed in relation to the single-mechanism hypothesis for the detection of changes in amplitude and frequency (Zwicker, Maiwald). This model could not provide a satisfactory explanation of interactions between amplitude and frequency.

Les modulations juste audibles d'un son modulé simultanément en amplitude et en fréquence ont été déterminées chez un observateur souffrant d'une hypoacousie de type oreille interne avec une courbe audiométrique en forme de creux d'environ 40 dB entre 1 et 2 kHz. Dans toutes les conditions de stimulus, nous avons trouvé des interactions entre la détection des modulations d'amplitude et de fréquence. Ces interactions semblaient ětre liées quantitativement à l'effet de la fréquence sur la sonie (comme le montre la pente des lignes isosoniques de l'observateur). En tenant compte de cet effet de la fréquence sur la sonie, les résultats des mesures étaient en bon accord avec l'hypothèse selon laquelle la détection des différences d'amplitude et de fréquence reposerait sur deux mécanismes séparés.

Les résultats expérimentaux ont aussi été discutés à la lumière de l'hypothèse selon laquelle il existerait un mécanisme unique permettant la détection de changements en amplitude et en fréquence (Zwicker, Maiwald). Ce modèle n'était pas à měme de fournir une explication satisfaisante des interactions de l'amplitude et de la fréquence.     

Loudness Adaptation in Acoustic and Electric Hearing

The present study is aimed to evaluate and compare loudness adaptation between normal hearing and cochlear-implant subjects. Loudness adaptation for 367-s pure tones was measured in five normal-hearing subjects at three frequencies (125, 1,000, and 8,000 Hz) and three levels (30, 60, and 90 dB SPL). In addition, loudness adaptation for 367-s pulse trains was measured in five Clarion cochlear-implant subjects at three stimulation rates (100, 991, and 4,296 Hz), three levels (10, 50, and 90% of the electric dynamic range), three stimulation positions (apical, middle and basal), and two stimulation modes (monopolar and bipolar). The method of successive magnitude estimation was used to quantify loudness adaptation. Similar to the previous results, we found that loudness adaptation in normal-hearing subjects increases with decreasing level and increasing frequency. However, we also found a small but significant loudness enhancement at 90 dB SPL in acoustic hearing. Despite large individual variability, we found that loudness adaptation in cochlear-implant subjects increases with decreasing levels, but is not significantly affected by the rate, place and mode of stimulation. A phenomenological model was proposed to predict loudness adaptation as a function of stimulus frequency and level in acoustic hearing. The present results were not fully compatible with either the restricted excitation hypothesis or the neural adaptation hypothesis. Loudness adaptation may have a central component that is dependent on the peripheral excitation pattern.     

Gap Duration Discrimination in Listeners with Cochlear Hearing Loss: Effects of Gap and Marker Duration, Frequency Separation, and Mode of Presentation

This study examined the effects of cochlear hearing loss on gap duration discrimination (GDD), with particular interest in whether cochlear hearing loss results in increased difficulty for across-channel temporal judgments. The hypothesis being tested was that listeners with cochlear loss would perform as well as normal-hearing listeners for all within-channel conditions but would exhibit relatively greater performance deficits in the across-channel conditions. A subsidiary aim was to determine whether, in normal-hearing listeners, the across-channel effects previously observed for minimal-duration standard gaps also existed for relatively long standard gaps. Two experiments were undertaken, one dealing with monaural conditions and one dealing with dichotic conditions. The monaural results indicated that across-frequency GDD was poorer than isofrequency GDD, even for the longer gap durations of 35 and 250 ms examined here. However, the results showed no effect of hearing loss on GDD. Rather, GDD appeared to be sensitive to listener age, with younger listeners showing better performance in both within-channel and across-channel conditions. In addition, both within-channel and across-channel performance was sensitive to the duration of the leading gap marker. Finally, the pattern of dichotic "across-ear" performance was similar, but not equivalent, to that of monaural across-frequency performance.     

Across- and Within-Channel Envelope Interactions in Cochlear Implant Listeners

The effects of modulated maskers on detection thresholds of a 50-Hz sinusoidal amplitude modulation (SAM) in a signal carrier were measured in nine cochlear implant (CI) listeners as a function of masker envelope type and for different masker–signal electrode separations. Both signal and masker were 200-ms-long pulse trains, presented concurrently in an interleaved stimulation mode. Masker envelopes were SAM at 20, 50, (0- and -phase re: the signal modulator), and 125 Hz, as well as noise amplitude modulated (NAM), all with a fixed 20% modulation depth. Comparisons were made against steady-state maskers that had an amplitude equal to the mean amplitude of the modulated maskers or to their peak amplitude (SS_peak). Modulation thresholds were larger in the presence of the dynamic maskers versus the SS_peak maskers; however, there was significant intersubject variability in the pattern of results. Effects of relative phase between masker and signal were not consistent across subjects. Envelope masking (the dB difference in modulation detection thresholds between modulated and SS_peak maskers) was generally larger for the lower-modulation-frequency maskers than the 125-Hz masker. The spatial distribution of masked modulation detection thresholds was found to be considerably different from spatial forward-masking patterns obtained in the same subjects. Finally, modulation thresholds measured for a very wide separation between the masker and signal showed significant envelope masking. These results suggest that, as has been shown in acoustic stimulation, central, across-channel temporal processing mechanisms also occur in electrical stimulation.     

Effect of Pulse Polarity on Thresholds and on Non-monotonic Loudness Growth in Cochlear Implant Users

Most cochlear implants (CIs) activate their electrodes non-simultaneously in order to eliminate electrical field interactions. However, the membrane of auditory nerve fibers needs time to return to its resting state, causing the probability of firing to a pulse to be affected by previous pulses. Here, we provide new evidence on the effect of pulse polarity and current level on these interactions. In experiment 1, detection thresholds and most comfortable levels (MCLs) were measured in CI users for 100-Hz pulse trains consisting of two consecutive biphasic pulses of the same or of opposite polarity. All combinations of polarities were studied: anodic-cathodic-anodic-cathodic (ACAC), CACA, ACCA, and CAAC. Thresholds were lower when the adjacent phases of the two pulses had the same polarity (ACCA and CAAC) than when they were different (ACAC and CACA). Some subjects showed a lower threshold for ACCA than for CAAC while others showed the opposite trend demonstrating that polarity sensitivity at threshold is genuine and subject- or electrode-dependent. In contrast, anodic (CAAC) pulses always showed a lower MCL than cathodic (ACCA) pulses, confirming previous reports. In experiments 2 and 3, the subjects compared the loudness of several pulse trains differing in current level separately for ACCA and CAAC. For 40 % of the electrodes tested, loudness grew non-monotonically as a function of current level for ACCA but never for CAAC. This finding may relate to a conduction block of the action potentials along the fibers induced by a strong hyperpolarization of their central processes. Further analysis showed that the electrodes showing a lower threshold for ACCA than for CAAC were more likely to yield a non-monotonic loudness growth. It is proposed that polarity sensitivity at threshold reflects the local neural health and that anodic asymmetric pulses should preferably be used to convey sound information while avoiding abnormal loudness percepts.     

Temporal Resolution of the Normal Ear in Listeners with Unilateral Hearing Impairment

Unilateral hearing loss (UHL) leads to an imbalanced input to the brain and results in cortical reorganization. In listeners with unilateral impairments, while the perceptual deficits associated with the impaired ear are well documented, less is known regarding the auditory processing in the unimpaired, clinically normal ear. It is commonly accepted that perceptual consequences are unlikely to occur in the normal ear for listeners with UHL. This study investigated whether the temporal resolution in the normal-hearing (NH) ear of listeners with long-standing UHL is similar to those in listeners with NH. Temporal resolution was assayed via measuring gap detection thresholds (GDTs) in within- and between-channel paradigms. GDTs were assessed in the normal ear of adults with long-standing, severe-to-profound UHL (N = 13) and age-matched, NH listeners (N = 22) at two presentation levels (30 and 55 dB sensation level). Analysis indicated that within-channel GDTs for listeners with UHL were not significantly different than those for the NH subject group, but the between-channel GDTs for listeners with UHL were poorer (by greater than a factor of 2) than those for the listeners with NH. The hearing thresholds in the normal or impaired ears were not associated with the elevated between-channel GDTs for listeners with UHL. Contrary to the common assumption that auditory processing capabilities are preserved for the normal ear in listeners with UHL, the current study demonstrated that a long-standing unilateral hearing impairment may adversely affect auditory perception—temporal resolution—in the clinically normal ear. From a translational perspective, these findings imply that the temporal processing deficits in the unimpaired ear of listeners with unilateral hearing impairments may contribute to their overall auditory perceptual difficulties.     

Post-operative Stapedius Reflex Tests with Simultaneous Loudness Scaling in Patients Supplied with Cochlear Implants

The estimation of the maximum comfort loudness levels (MCL) by measurements of the electrically elicited stapedius reflex was examined in six experienced cochlear implant users supplied with the COMBI 40 implant system. The stapedius reflex was tested and loudness scaling was performed simultaneously using an up/down stimulation protocol close to the reflex threshold with automated recording of both test procedures. The electrical stapedius reflex threshold (ESRT) and loudness scaling were evaluated separately. Scaling at the reflex threshold ranged between normal and loud. The range of stimulus intensities corresponding to ESRT is much smaller than that at a particular loudness category. The overall correlation between ESRT and MCL was high (r=0.92), with a similar dependence of ESRT and MCL on the channel stimulated. Thus, when the stapedius reflex can be detected post-op-eratively, the ESRT can be applied successfully for the fitting procedure of the speech processor. Simultaneous loudness scaling during the entire reflex test showed that overstimulation via the implant can be avoided effectively.