Effects of Stimulation Rate, Mode and Level on Modulation Detection by Cochlear Implant Users

In cochlear implant (CI) patients, temporal processing is often poorest at low listening levels, making perception difficult for low-amplitude temporal cues that are important for consonant recognition and/or speech perception in noise. It remains unclear how speech processor parameters such as stimulation rate and stimulation mode may affect temporal processing, especially at low listening levels. The present study investigated the effects of these parameters on modulation detection by six CI users. Modulation detection thresholds (MDTs) were measured as functions of stimulation rate, mode, and level. Results show that for all stimulation rate and mode conditions, modulation sensitivity was poorest at quiet listening levels, consistent with results from previous studies. MDTs were better with the lower stimulation rate, especially for quiet-to-medium listening levels. Stimulation mode had no significant effect on MDTs. These results suggest that, although high stimulation rates may better encode temporal information and widen the electrode dynamic range, CI patients may not be able to access these enhanced temporal cues, especially at the lower portions of the dynamic range. Lower stimulation rates may provide better recognition of weak acoustic envelope information.     

Temporal Modulation Detection Depends on Sharpness of Spatial Tuning

Prior research has shown that in electrical hearing, cochlear implant (CI) users’ speech recognition performance is related in part to their ability to detect temporal modulation (i.e., modulation sensitivity). Previous studies have also shown better speech recognition when selectively stimulating sites with good modulation sensitivity rather than all stimulation sites. Site selection based on channel interaction measures, such as those using imaging or psychophysical estimates of spread of neural excitation, has also been shown to improve speech recognition. This led to the question of whether temporal modulation sensitivity and spatial selectivity of neural excitation are two related variables. In the present study, CI users’ modulation sensitivity was compared for sites with relatively broad or narrow neural excitation patterns. This was achieved by measuring temporal modulation detection thresholds (MDTs) at stimulation sites that were significantly different in their sharpness of the psychophysical spatial tuning curves (PTCs) and measuring MDTs at the same sites in monopolar (MP) and bipolar (BP) stimulation modes. Nine postlingually deafened subjects implanted with Cochlear Nucleus® device took part in the study. Results showed a significant correlation between the sharpness of PTCs and MDTs, indicating that modulation detection benefits from a more spatially restricted neural activation pattern. There was a significant interaction between stimulation site and mode. That is, using BP stimulation only improved MDTs at stimulation sites with broad PTCs but had no effect or sometimes a detrimental effect on MDTs at stimulation sites with sharp PTCs. This interaction could suggest that a criterion number of nerve fibers is needed to achieve optimal temporal resolution, and, to achieve optimized speech recognition outcomes, individualized selection of site-specific current focusing strategies may be necessary. These results also suggest that the removal of stimulation sites measured with poor MDTs might improve both temporal and spectral resolution.     

Temporal modulation transfer functions in cochlear implantees using a method that limits overall loudness cues

Temporal modulation transfer functions (TMTFs) were measured for six users of cochlear implants, using different carrier rates and levels. Unlike most previous studies investigating modulation detection, the experimental design limited potential effects of overall loudness cues. Psychometric functions (percent correct discrimination of modulated from unmodulated stimuli versus modulation depth) were obtained. For each modulation depth, each modulated stimulus was loudness balanced to the unmodulated reference stimulus, and level jitter was applied in the discrimination task. The loudness-balance data showed that the modulated stimuli were louder than the unmodulated reference stimuli with the same average current, thus confirming the need to limit loudness cues when measuring modulation detection. TMTFs measured in this way had a low-pass characteristic, with a cut-off frequency (at comfortably loud levels) similar to that for normal-hearing listeners. A reduction in level caused degradation in modulation detection efficiency and a lower-cut-off frequency (i.e. poorer temporal resolution). An increase in carrier rate also led to a degradation in modulation detection efficiency, but only at lower levels or higher modulation frequencies. When detection thresholds were expressed as a proportion of dynamic range, there was no effect of carrier rate for the lowest modulation frequency (50 Hz) at either level.     

Relationship between gap detection thresholds and loudness in cochlear-implant users

Gap detection threshold (GDT) is a commonly used measure of temporal acuity in cochlear-implant (CI) recipients. This measure, like other measures of temporal acuity, shows considerable variation across subjects and also varies across stimulation sites within subjects. The aims of this study were (1) to determine whether across-site variation in GDTs would be reduced or maintained with increased stimulation levels; (2) to determine whether across-site variation in GDTs at low stimulation levels was related to differences in loudness percepts at those same levels; and (3) to determine whether matching loudness levels could reduce across-site differences in GDTs. Thresholds and maximum comfortable loudness levels were measured in postlingually deaf adults using all available sites in their electrode arrays. All sites were then surveyed at 30% of the dynamic range (DR) to examine across-site variation. Two sites with the largest difference in GDTs were then selected and for those two sites GDTs were measured at multiple levels of the DR (10%, 30%, 50%, 70%, and 90%). Stimuli consisted of 500 ms trains of symmetric-biphasic pulses, 40 μs/phase, presented at a rate of 1000 pps using a monopolar (MP1+2) electrode configuration. To examine perceptual differences in loudness, the selected sites were loudness-matched at the same levels of the DR. Variations in GDTs and loudness patterns were observed across stimulation sites and across subjects. Variations in GDTs across sites tended to decrease with increasing stimulation levels. For the majority of the subjects, stimuli at a given level in %DR were perceived louder at sites with better GDTs than those presented at the same level in %DR at sites with poorer GDTs. These results suggest that loudness is a contributing factor to across-site variation in GDTs and that CI fittings based on more detailed loudness matching could reduce across-site variation and improve perceptual acuity.     

Temporal Processing in the Auditory System

Central auditory processing in humans was investigated by comparing the perceptual effects of temporal parameters of electrical stimulation in auditory midbrain implant (AMI) and cochlear implant (CI) users. Four experiments were conducted to measure the following: effect of interpulse intervals on detection thresholds and loudness; temporal modulation transfer functions (TMTFs); effect of duration on detection thresholds; and forward masking decay. The CI data were consistent with a phenomenological model that based detection or loudness decisions on the output of a sliding temporal integration window, the input to which was the hypothetical auditory nerve response to each stimulus pulse. To predict the AMI data, the model required changes to both the neural response input (i.e., midbrain activity to AMI stimuli, compared to auditory nerve activity to CI stimuli) and the shape of the integration window. AMI data were consistent with a neural response that decreased more steeply compared to CI stimulation as the pulse rate increased or interpulse interval decreased. For one AMI subject, the data were consistent with a significant adaptation of the neural response for rates above 200 Hz. The AMI model required an integration window that was significantly wider (i.e., decreased temporal resolution) than that for CI data, the latter being well fit using the same integration window shape as derived from normal-hearing data. These models provide a useful way to conceptualize how stimulation of central auditory structures differs from stimulation of the auditory nerve and to better understand why AMI users have difficulty processing temporal cues important for speech understanding.     

Channel Interaction and Current Level Affect Across-Electrode Integration of Interaural Time Differences in Bilateral Cochlear-Implant Listeners

Sensitivity to interaural time differences (ITDs) is important for sound localization. Normal-hearing listeners benefit from across-frequency processing, as seen with improved ITD thresholds when consistent ITD cues are presented over a range of frequency channels compared with when ITD information is only presented in a single frequency channel. This study aimed to clarify whether cochlear-implant (CI) listeners can make use of similar processing when being stimulated with multiple interaural electrode pairs transmitting consistent ITD information. ITD thresholds for unmodulated, 100-pulse-per-second pulse trains were measured in seven bilateral CI listeners using research interfaces. Consistent ITDs were presented at either one or two electrode pairs at different current levels, allowing for comparisons at either constant level per component electrode or equal overall loudness. Different tonotopic distances between the pairs were tested in order to clarify the potential influence of channel interaction. Comparison of ITD thresholds between double pairs and the respective single pairs revealed systematic effects of tonotopic separation and current level. At constant levels, performance with double-pair stimulation improved compared with single-pair stimulation but only for large tonotopic separation. Comparisons at equal overall loudness revealed no benefit from presenting ITD information at two electrode pairs for any tonotopic spacing. Irrespective of electrode-pair configuration, ITD sensitivity improved with increasing current level. Hence, the improved ITD sensitivity for double pairs found for a large tonotopic separation and constant current levels seems to be due to increased loudness. The overall data suggest that CI listeners can benefit from combining consistent ITD information across multiple electrodes, provided sufficient stimulus levels and that stimulating electrode pairs are widely spaced.     

Spectral loudness summation for electrical stimulation in cochlear implant users

Objective: This study investigates the effect of spectral loudness summation (SLS) in the electrical domain as perceived by cochlear implant (CI) users. Analogous to SLS in the acoustical domain, SLS was defined as the effect of electrode separation at a fixed overall stimulation rate. Design: Categorical loudness scaling (CLS) was conducted at three overall stimulation rates using single-electrode stimuli and multi-electrode stimuli presented interleaved on two or four electrodes. The specific loudness of the pulses in the multi-electrode stimuli were equalized based on single-electrode measurements at the same overall stimulation rate. At a fixed overall stimulation rate and a fixed loudness perception, SLS was calculated as the difference in mean current between single-electrode and multi-electrode stimuli. Study sample: Ten postlingually deafened adult CI users. Results: The amount of SLS varied between subjects and between the number and location of the stimulated electrodes in the multi-electrode configuration. SLS was significantly higher than 0 for a subset of the subjects. Conclusions: For a subpopulation of CI users, loudness models should account for nonlinear interactions between electrodes (in the perceptual domain). Similarly, SLS should be accounted for when using CLS outcomes for fitting purposes, at least in a subpopulation of CI users.     

Relation between neural response telemetry thresholds, T- and C-levels, and loudness judgments in 12 adult nucleus 24 cochlear implant recipients

OBJECTIVE: The primary purpose of this study was to determine if the contour of visual (vNRT) or predicted (tNRT) neural response telemetry (NRT) thresholds across electrodes could predict the contour of behaviorally programmed T-levels (minimum stimulation) and/or C-levels (maximum stimulation) across electrodes for well-fit MAPs. The secondary purpose was to determine the relation between NRT thresholds and loudness judgments obtained at the subject's MAP rate (250, 900, 1200, or 1800 pulses per second [pps]) and the NRT stimulus rate (80 pps). DESIGN: Twelve adult Nucleus 24 cochlear implant recipients participated in the study. The T- and C-levels from a preferred MAP, which had been worn for a minimum of 3 mo, were used in this study. Electrically evoked compound action potentials were measured on 11 active electrodes with NRT software (v3.0). Ascending loudness judgments from first hearing to maximum acceptable loudness were completed on these electrodes with the subject's preferred MAP rate stimulus, using the R126 (v.2.0) software and with an 80 pps rate stimulus, using the NRT software (v3.0). All measures were repeated approximately 1 mo later to determine their reliability. RESULTS: The reliability of the behavioral and objective measures was very high from the first to the second half of the study. The mean tNRT thresholds had a lower reliability (r = 0.73) than vNRT thresholds (r = 0.91). The loudness judgment dynamic range was notably different between rates. The NRT rate (80 pps) stimulus resulted in the narrowest dynamic range followed by increasingly wider dynamic range as the MAP rate increased. The NRT thresholds had a stronger correlation with loudness judgments made with the NRT rate stimulus than with the MAP rate stimulus. The group mean NRT thresholds were significantly correlated with C-levels (vNRT r = 0.69) (tNRT r = 0.66) but not T-levels. The relation between NRT thresholds and T- and C-levels varied for different MAP rates, with the NRT thresholds being closest to the C-levels for the 250 pps MAP rate. Each subject's vNRT thresholds and MAP levels were examined by fitting a third-order polynomial to the data. This analysis revealed significant variability demonstrating that no one fit predicts T- and C-levels well for all subjects. CONCLUSIONS: The results of this study provide important insight into the relation between NRT thresholds and loudness judgments for different stimulation rates and T- and C-levels at various MAP rates. The loudness judgment dynamic range and MAP dynamic range (T- and C-levels) varied notably for different stimulation rates. As a result, the relation of NRT thresholds to these measures also varied with stimulation rate. Overall, the mean vNRT thresholds fell higher in the loudness judgment dynamic range than the tNRT thresholds. Mean NRT thresholds fell between the judgments of medium soft and maximum acceptable loudness for all stimulation rates. Mean vNRT thresholds fell above C-levels, whereas almost half of tNRT thresholds fell just below C-levels. However, the relation between NRT thresholds and C-levels varied substantially for different MAP stimulation levels. In addition, there is substantial individual variability in the relation between NRT thresholds and MAP levels that is not reflected in the group data. The prediction of the contour of T- and C-levels from the contour of NRT thresholds across electrodes would not be appropriate for half of the subjects. Therefore, great care should be taken when applying a fitting rule that incorporates NRT thresholds without considering these individual differences. For adults who can provide appropriate loudness judgments and threshold responses it appears to be most efficient to primarily use behavioral measures to create MAPs.     

Effect of Pulse Rate and Polarity on the Sensitivity of Auditory Brainstem and Cochlear Implant Users to Electrical Stimulation

To further understand the response of the human brainstem to electrical stimulation, a series of experiments compared the effect of pulse rate and polarity on detection thresholds between auditory brainstem implant (ABI) and cochlear implant (CI) patients. Experiment 1 showed that for 400-ms pulse trains, ABI users’ thresholds dropped by about 2 dB as pulse rate was increased from 71 to 500 pps, but only by an average of 0.6 dB as rate was increased further to 3500 pps. This latter decrease was much smaller than the 7.7-dB observed for CI users. A similar result was obtained for pulse trains with a 40-ms duration. Furthermore, experiment 2 showed that the threshold difference between 500- and 3500-pps pulse trains remained much smaller for ABI than for CI users, even for durations as short as 2 ms, indicating the effect of a fast-acting mechanism. Experiment 3 showed that ABI users’ thresholds were lower for alternating-polarity than for fixed-polarity pulse trains, and that this difference was greater at 3500 pps than at 500 pps, consistent with the effect of pulse rate on ABI users’ thresholds being influenced by charge interactions between successive biphasic pulses. Experiment 4 compared thresholds and loudness between trains of asymmetric pulses of opposite polarity, in monopolar mode, and showed that in both cases less current was needed when the anodic, rather than the cathodic, current was concentrated into a short time interval. This finding is similar to that previously observed for CI users and is consistent with ABI users being more sensitive to anodic than cathodic current. We argue that our results constrain potential explanations for the differences in the perception of electrical stimulation by CI and ABI users, and have potential implications for future ABI stimulation strategies.     

Loudness growth in cochlear implants: effect of stimulation rate and electrode configuration

In cochlear implant speech processor design, acoustic amplitudes are mapped to electric currents with the intention of preserving loudness relationships across electrodes. Many parameters may affect the growth of loudness with electrical stimulation. The present study measured the effects of stimulation rate and electrode configuration on loudness growth in six Nucleus-22 cochlear implant users. Loudness balance functions were measured for stimuli that differed in terms of stimulation rate, electrode configuration and electrode location; a 2-alternative, forced-choice adaptive procedure (double-staircase) was used. First, subjects adaptively adjusted the amplitude of a 100-pulse-per-second (pps) pulse train to match the loudness of a 1000-pps standard pulse train. For a range of reference stimulation levels, the loudness of the 100-pps stimulus was matched to that of the 1000-pps standard stimulus; loudness balancing was performed for three electrode pairs [(20,22), (1,3), (1,22)]. The results showed that the loudness balance functions between the 100- and 1000-pps stimulation rates were highly subject-dependent. Some subjects' loudness balance functions were logarithmic, while others' were nearly linear. Loudness balance functions were also measured across electrode locations [(20,22) vs. (1,3)] for two stimulation rates (100, 1000 pps). Results showed that the loudness balance functions between the apical and basal electrode pairs highly depended on the stimulation rate. For all subjects, at the 1000-pps rate, the loudness balance functions between the two electrode locations were nearly linear; however, at the 100-pps rate, the loudness balance function was highly nonlinear in two out of six subjects. These results suggest that, for some cochlear implant patients, low-frequency stimulation may be processed differently at different electrode locations; for these patients, acoustic-to-electric amplitude mapping may need to be sensitive to this place-dependent processing when relatively low stimulation rates are used.     

Modulation threshold functions for chronically impaired Menière patients

Detection thresholds for sinusoidally amplitude-modulated broad-band noise were measured as a function of modulation frequency for both ears of 6 chronic Menière patients who suffered unilateral hearing impairments. Modulation thresholds were measured with an adaptive cued-standard forced-choice psychophysical method. Better-ear modulation thresholds were similar to normative data previously reported [Formby, C.: J. acoust. Soc.Am. 78:70-77, 1985], whereas 5 of the 6 patients exhibited deficits in modulation detection with their poorer ears. Modulation thresholds averaged across the patients' poorer ears were similar to the normative thresholds through 60-100 Hz; at higher modulation frequencies, sensitivity declined at approximately twice the normal attenuation rate (i.e., 6 vs. 3 dB per octave). The poorer-ear data can be described by the mathematical representation for a simple low-pass filter with a cutoff frequency of 60 Hz. This pattern of the Menière modulation thresholds is consistent with broadened peripheral tuning due to hydrops.     

Effects of programming threshold and maplaw settings on acoustic thresholds and speech discrimination with the MED-EL COMBI 40+ cochlear implant

OBJECTIVES: The principal task in the programming of a cochlear implant (CI) speech processor is the setting of the electrical dynamic range (output) for each electrode, to ensure that a comfortable loudness percept is obtained for a range of input levels. This typically involves separate psychophysical measurement of electrical threshold ([theta] e) and upper tolerance levels using short current bursts generated by the fitting software. Anecdotal clinical experience and some experimental studies suggest that the measurement of [theta]e is relatively unimportant and that the setting of upper tolerance limits is more critical for processor programming. The present study aims to test this hypothesis and examines in detail how acoustic thresholds and speech recognition are affected by setting of the lower limit of the output ("Programming threshold" or "PT") to understand better the influence of this parameter and how it interacts with certain other programming parameters. DESIGN: Test programs (maps) were generated with PT set to artificially high and low values and tested on users of the MED-EL COMBI 40+ CI system. Acoustic thresholds and speech recognition scores (sentence tests) were measured for each of the test maps. Acoustic thresholds were also measured using maps with a range of output compression functions ("maplaws"). In addition, subjective reports were recorded regarding the presence of "background threshold stimulation" which is occasionally reported by CI users if PT is set to relatively high values when using the CIS strategy. RESULTS: Manipulation of PT was found to have very little effect. Setting PT to minimum produced a mean 5 dB (S.D. = 6.25) increase in acoustic thresholds, relative to thresholds with PT set normally, and had no statistically significant effect on speech recognition scores on a sentence test. On the other hand, maplaw setting was found to have a significant effect on acoustic thresholds (raised as maplaw is made more linear), which provides some theoretical explanation as to why PT has little effect when using the default maplaw of c = 500. Subjective reports of background threshold stimulation showed that most users could perceive a relatively loud auditory percept, in the absence of microphone input, when PT was set to double the behaviorally measured electrical thresholds ([theta]e), but that this produced little intrusion when microphone input was present. CONCLUSIONS: The results of these investigations have direct clinical relevance, showing that setting of PT is indeed relatively unimportant in terms of speech discrimination, but that it is worth ensuring that PT is not set excessively high, as this can produce distracting background stimulation. Indeed, it may even be set to minimum values without deleterious effect.     

Loudness perception and late auditory evoked potentials in adult cochlear implant users.

In clinical routine the adjustment of speech processors in cochlear implant users is based on the patients' subjective statements about the loudness of specific electrical stimuli. From hearing patients it is known that the latencies and amplitudes of late auditory evoked potentials (LAEP) which are generated within the auditory cortex correlate with the loudness perception of acoustical stimuli. The aim of this study was to investigate the correlation between LAEP and loudness perception in adult cochlear implant users. We investigated 8 adult subjects who had been provided with a 22 electrode Cochlear Implant (nucleus CI24M) at least 6 months prior to the investigation. All subjects showed open speech understanding. Electrical pulse trains of 300 ms duration presented at a single electrode served as stimuli. Electrically LAEP morphologies were similar to normal hearing subjects. In all subjects and each intracochlear electrode position LAEP were well identifiable down to low loudness sensations. Both amplitudes and latencies depended on the loudness perception. The best correlation was observed for the N1 deflection. The results show that LAEP can be used for estimation of both hearing thresholds and most comfortable levels.     

Modulation Frequency Discrimination with Modulated and Unmodulated Interference in Normal Hearing and in Cochlear-Implant Users

Differences in fundamental frequency (F0) provide an important cue for segregating simultaneous sounds. Cochlear implants (CIs) transmit F0 information primarily through the periodicity of the temporal envelope of the electrical pulse trains. Successful segregation of sounds with different F0s requires the ability to process multiple F0s simultaneously, but it is unknown whether CI users have this ability. This study measured modulation frequency discrimination thresholds for half-wave rectified sinusoidal envelopes modulated at 115 Hz in CI users and normal-hearing (NH) listeners. The target modulation was presented in isolation or in the presence of an interferer. Discrimination thresholds were strongly affected by the presence of an interferer, even when it was unmodulated and spectrally remote. Interferer modulation increased interference and often led to very high discrimination thresholds, especially when the interfering modulation frequency was lower than that of the target. Introducing a temporal offset between the interferer and the target led to at best modest improvements in performance in CI users and NH listeners. The results suggest no fundamental difference between acoustic and electric hearing in processing single or multiple envelope-based F0s, but confirm that differences in F0 are unlikely to provide a robust cue for perceptual segregation in CI users.     

The relationship between the intraoperative ECAP threshold and postoperative behavioral levels: the difference between postlingually deafened adults and prelingually deafened pediatric cochlear implant users

The purpose of this study was to determine the relationship between electrically evoked compound-action potential (ECAP) thresholds, electrically evoked auditory brain-stem response (EABR) thresholds, behavioral thresholds (T levels) and maximum comfort levels (C levels) in profoundly deaf cochlear-implant users. The ECAP thresholds were measured intraoperatively in eight postlingually deafened adults and nine (eight prelingually and one postlingually deafened) children implanted with the Nucleus CI24 M cochlear implant. The mean ECAP thresholds did not differ between children and adults. The average behavioral T and C levels after at least 6 months of experience with a cochlear implant were significantly higher in children than those in adults. The ECAP thresholds were more strongly correlated with T and C levels in children than in adults. The stronger correlation between ECAP thresholds and behavioral T and C levels in children than in adults might result from differences in loudness sensation, which should in turn depend on auditory experience.     

Dynamic current focusing for loudness encoding in cochlear implants: a take-home trial

Objective: This study aimed to evaluate a more energy-efficient dynamic current focussing (DCF) speech-processing strategy after long-term listening experience. In DCF, tripolar stimulation is used near the threshold and loudness is controlled by the compensation coefficient σ. A recent acute pilot study showed improved spectral-temporally modulated ripple test (SMRT) scores at low loudness levels, but battery life was reduced to 1.5–4?hours.

Design: Within-subject comparisons were made for the clinical versus. DCF strategy after 5?weeks of at-home usage. Speech intelligibility in noise, spectral ripple discrimination, temporal modulation detection, loudness growth, and subjective ratings were assessed.

Study sample: Twenty HiRes90K (Advanced Bionics, Valencia, USA) cochlear implant (CI) users.

Results: Average battery life was 9?hours with the newly implemented DCF compared to 13.4?hours with the clinical strategy. Compared with measurements made at the beginning of the study, SMRT-scores and speech intelligibility in noise were significantly improved with DCF. However, both measures suffered from unexpected learning effects over time. The improvement disappeared and speech intelligibility in noise declined significantly relative to the final control measurement with the clinical strategy.

Conclusion: Most CI users can adapt to the DCF strategy in a take-home setting. Although DCF has the potential to improve performance on the SMRT test, learning effects complicate the interpretation of the current results.     

Rate pitch discrimination in cochlear implant users with the use of double pulses and different interpulse intervals

The rate pitch discrimination ability of cochlear implant (CI) users is poor compared to normal-hearing (NH) listeners. At low pulse rates, the just noticeable difference (JND) is on average 20% of the base rate, while NH listeners can discriminate small frequency differences of 0.2% at 1?kHz. Recent investigations suggest that double pulses with short interpulse intervals (IPIs) may have a beneficial effect on rate pitch discrimination in CI users. In a first experiment psychophysical tests were carried out to establish whether rate pitch in CI users could be improved by applying double pulses with equal amplitude and short IPIs. Pulse trains with base rates of 200 and 400?pps, composed of either single pulses or double pulses with IPIs of 15, 50, and 150?μs were presented. In a second experiment pairwise comparisons were carried out between pitch of a pulse train composed of alternating double and single pulses with pitch of pulse trains composed of single pulses. The alternating pulse train had a base rate of 400?pps, the pulse trains with solely single pulses had base rates of 200, 300, and 400?pps. The loudness and pitch perception of the different stimulus types were evaluated and compared. A significant loudness difference was found between single and double pulses for both pulse rates. The JND for pitch discrimination between double-pulse IPIs had a high inter-subject variability, and no significant group effect was found. No subject reported a pitch change between double pulse and single pulse stimulation. In contrast, most of the subjects recognized a change in pitch between single-pulse trains and pulse trains with alternating double and single pulses. The latter was lower in pitch than the single-pulse train stimulation. To conclude, using (equal amplitude) double pulses instead of single pulses in a pulse train does not effect pitch perception. Instead, loudness differs between double pulses and single pulses with the same amplitude.     

Estimates of loudness, loudness discomfort, and the auditory dynamic range: normative estimates, comparison of procedures, and test-retest reliability

The purpose of this series of experiments was to establish normative reference values for absolute and relative judgements of loudness discomfort and for the auditory dynamic range (DR), and to evaluate intersubject variability and intra-subject test-retest reliability for the respective measures of loudness discomfort. To establish the normal auditory DR, audiometric thresholds and loudness discomfort levels (LDLs) were measured from a group of 59 normal-hearing adults without sound tolerance problems. The resulting estimates of the LDL and DR were on the order of 100 dB HL and 95 dB, respectively. A subset (n = 18) of this larger group participated in further studies in which loudness growth functions and the upper limit of the auditory DR were measured by categorical scaling judgments. The findings revealed no significant differences between the test methods for absolute (LDL) and relative (categorical scaling) judgements of loudness discomfort, intersubject variability, or intrasubject test-retest reliability, and suggest that the simple LDL estimate of loudness discomfort is an efficient and valid clinical measure for characterizing the "threshold of discomfort."     

Measurement of first- and second-order modulation detection thresholds in listeners with cochlear hearing loss

First- and second-order modulation detection thresholds were measured in normal hearing and hearing-impaired listeners. 'First-order' modulation detection thresholds correspond to the ability of listeners to detect sinusoidal amplitude modulation (SAM); they are measured as a function of the frequency of that modulation, f(m). 'Second-order' modulation detection thresholds correspond to the ability to detect sinusoidal modulation applied to the modulation depth of a SAM signal; they are measured as a function of the frequency of the modulation applied to the modulation depth (referred to as f(m)'). In this case, the SAM signal acts as a 'carrier' stimulus of frequency f(m) and sinusoidal modulation of the SAM-signal's modulation depth (at rate f(m)') generates two additional components in the modulation spectrum at f(m)-f(m)' and f(m)+f(m)'. In both groups of listeners, first-order modulation detection thresholds were measured for modulation frequencies f(m) ranging between 4 Hz and 32 Hz, and second-order modulation detection thresholds were measured for second-order modulation frequencies f(m)' ranging between 1 Hz and 11 Hz, using a fixed first-order modulation frequency f(m) of 16 Hz. The results showed that, in hearing-impaired listeners: first-order modulation detection thresholds were within the normal range up to f(m) = 16 Hz and poorer than normal at f(m) = 32 Hz; second-order modulation detection thresholds were within the normal range at f(m)' = 3, 5 and 11 Hz, and poorer than normal at f(m)' = 1 Hz and 7 Hz. These results suggest that cochlear damage has little effect on the detection of both sinusoidal and complex temporal envelopes.