Effects of amplitude compression on first- and second-order modulation detection thresholds in cochlear implant listeners

The aim of this study was to examine the effects of instantaneous non-linear amplitude mapping on the detection of single-component and multicomponent temporal envelopes. To address this issue, first- and second-order amplitude modulation detection thresholds were measured in four cochlear implant users with the intervention of the compression device of the implant processor. The compression device is set to produce either a strongly or a weakly logarithmic mapping of stimulus amplitude to electrical amplitude. 'First-order' modulation detection thresholds indicate the ability of listeners to detect sinusoidal amplitude modulation (SAM) applied to a white noise carrier; they are measured as a function of the rate of that modulation, fm. 'Second-order' modulation detection thresholds indicate the ability to detect sinusoidal modulation applied to the depth of a sinusoidally amplitude-modulated signal (here, a 16-Hz sinusoidally amplitude-modulated white noise); they are measured as a function of the rate of the modulation applied to the modulation depth (referred to as fm'). In each task, stimuli are transformed by the implant processor and are presented through one electrode at approximately the same level. The results show that, in cochlear implant listeners, both first- and second-order modulation detection thresholds measured at the lower rates (< or =7 Hz) decrease slightly by about 3-6dB when the stronger compression is used. No effect of compression is observed at higher rates. These results suggest that instantaneous logarithmic amplitude mapping has beneficial- but limited-effects on the detection of single-component and multicomponent temporal envelopes. These results are discussed in light of current models of temporal envelope processing.     

Effect of cochlear damage on the detection of complex temporal envelopes

Recent studies have demonstrated that the detection of complex temporal envelopes relies - at least partially - on the perception of a distortion component generated by a peripheral (cochlear) and/or central (post-cochlear) non-linearity. In the present study, first- and second-order amplitude modulation (AM) detection thresholds were obtained in normally hearing (NH) and hearing-impaired (HI) listeners using a 2-kHz pure-tone carrier. In both groups of listeners, first-order AM detection thresholds were measured for AM rates fm ranging between 4 and 87 Hz, and second-order AM detection thresholds were measured for second-order AM rates fm' ranging between 4 and 23 Hz, using a fixed first-order 'carrier' AM rate fm of 64 Hz. When the sound pressure level was adjusted in order to yield equal detectability in both groups for the 64-Hz first-order carrier modulation, (i) first-order AM detection thresholds for the HI listeners were normal at fm=87 Hz, and better-than-normal at fm=4 and 16 Hz, and (ii) second-order AM detection thresholds were identical at all modulation rates in NH and HI listeners. Similar results were obtained when the audibility of the 2-kHz pure-tone carrier was equated for both groups, i.e. when listeners were tested at the same sensation level. These results demonstrate clearly that cochlear damage has no effect on the detection of complex temporal envelopes, and indicate that the distortion component must be generated by a more central non-linearity than cochlear compression, transduction, or short-term adaptation.     

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.     

Psychophysical measures from electrical stimulation of the human cochlear nucleus

Auditory performance on basic psychophysical tasks was measured in ten deaf patients with electrodes positioned near their cochlear nucleus. The device is called the auditory brainstem implant (ABI). Electrodes were placed during surgery to remove an acoustic neuroma, which results in the removal of the VIII nerve and, thus deafness. In patients who received auditory sensation from electrical stimulation we measured auditory performance on standard psychophysical tasks: thresholds, loudness growth, intensity discrimination, temporal integration, temporal modulation detection, gap detection, and forward masking. Plots of threshold as a function of frequency or biphasic pulse duration were markedly different from those of patients with cochlear implants. The difference in threshold functions is probably partly due to the biophysical difference in the neural elements stimulated. Another possibility is that part of the difference is due to the highly abnormal spatial pattern of activation in the cochlear nucleus from electrical stimulation, which prevents normal spatial integration of activity. The usable range of electrical amplitudes above threshold is comparable with that of cochlear implants, typically 10-15 dB. Little temporal integration occurs over a range of stimulus durations from 2-1000 ms. When compared at equivalent loudness levels, gap detection thresholds are similar to, or a bit longer than, gap thresholds in normal-hearing listeners and cochlear implant patients. Forward masking recovery functions are similar to those of normal listeners and cochlear implant patients. Patients' ability to detect amplitude modulation as a function of modulation frequency is similar to that of cochlear implant patients and normal listeners. Thus, direct electrical stimulation of the brainstem produces temporal resolution that does not significantly differ from that of normal listeners when compared in equivalent amplitude units. This implies that the limiting factors for these tasks are more centrally located, and not directly related to threshold mechanisms. Thus, a properly designed speech processor could preserve the important temporal features of speech for these patients.     

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.     

Intervention Planning for Children with Communication Disorders

Modulation thresholds for sinusoidally amplitude-modulated broadband noise were obtained from normal-hearing and sensorineural hearing-impaired listeners as a function of modulation frequency. The resulting temporal modulation transfer functions (TMTFs) indicated that the impaired listeners were generally less sensitive than the normals to amplitude modulation and, unlike previously published data from normal-hearing listeners, TMTFs in the impaired listeners were level dependent: sensitivity to modulation, particularly for modulation frequencies greater than 100 Hz, decreased with decreases in level. TMTFs were also obtained with band-limited noise from the normal-hearing listeners: the noise was low-pass filtered at 1.6 kHz after modulation and was generally presented with a 1.6-kHz high-pass masker. The TMTFs in the low-pass condition were similar to the TMTFs obtained with broadband noise from the impaired listeners, suggesting that the impaired temporal processing in the hearing-impaired listeners is a result of a narrower effective, ‘internal’ bandwidth. Increment thresholds for continuous broadband and low-pass noise were obtained in conditions similar to those in which TMTFs were obtained. In general, a similar power-law relationship between modulation threshold and increment threshold was found to exist for both the normal-hearing and the hearing-impaired listeners.     

Temporal modulation transfer functions in normal-hearing and hearing-impaired listeners

Modulation thresholds for sinusoidally amplitude-modulated broadband noise were obtained from normal-hearing and sensorineural hearing-impaired listeners as a function of modulation frequency. The resulting temporal modulation transfer functions (TMTFs) indicated that the impaired listeners were generally less sensitive than the normals to amplitude modulation and, unlike previously published data from normal-hearing listeners, TMTFs in the impaired listeners were level dependent: sensitivity to modulation, particularly for modulation frequencies greater than 100 Hz, decreased with decreases in level. TMTFs were also obtained with band-limited noise from the normal-hearing listeners: the noise was low-pass filtered at 1.6 kHz after modulation and was generally presented with a 1.6-kHz high-pass masker. The TMTFs in the low-pass condition were similar to the TMTFs obtained with broadband noise from the impaired listeners, suggesting that the impaired temporal processing in the hearing-impaired listeners is a result of a narrower effective, 'internal' bandwidth. Increment thresholds for continuous broadband and low-pass noise were obtained in conditions similar to those in which TMTFs were obtained. In general, a similar power-law relationship between modulation threshold and increment threshold was found to exist for both the normal-hearing and the hearing-impaired listeners.     

Masking of tone bursts by modulated noise in normal, noise-masked normal, and hearing-impaired listeners

Threshold of 4.6-ms tone bursts was measured in quiet and in the presence of a 100% sinusoidally amplitude-modulated speech-shaped noise. For the modulated-noise conditions, the onset of the tone burst coincided either with the maximum or the minimum modulator amplitude. The difference in these two masked thresholds provided an indication of the psychoacoustic modulation depth, or the modulation depth preserved within the auditory system. Modulation frequencies spanning the modulation spectrum of speech (2.5 to 20 Hz) were examined. Tone bursts were 500, 1400, and 4000 Hz. Subjects included normal listeners, normal listeners with a hearing loss simulated by high-pass noise, and hearing-impaired listeners having high-frequency sensorineural hearing loss. Normal listeners revealed a psychoacoustic modulation depth of 30-40 dB for the lowest modulation frequencies which decreased to about 15 dB at 20 Hz. The psychoacoustic modulation depth was decreased in the normal listeners with simulated hearing loss and in the hearing-impaired listeners. There was general agreement in the data, however, for the latter two groups of listeners suggesting that the normal listeners with hearing loss simulated by an additional masking noise provided a good representation of the performance of hearing-impaired listeners on this task.     

Changes in Auditory Nerve Responses Across the Duration of Sinusoidally Amplitude-Modulated Electric Pulse-Train Stimuli

Response rates of auditory nerve fibers (ANFs) to electric pulse trains change over time, reflecting substantial spike-rate adaptation that depends on stimulus parameters. We hypothesize that adaptation affects the representation of amplitude-modulated pulse trains used by cochlear prostheses to transmit speech information to the auditory system. We recorded cat ANF responses to sinusoidally amplitude-modulated (SAM) trains with 5,000 pulse/s carriers. Stimuli delivered by a monopolar intracochlear electrode had fixed modulation frequency (100 Hz) and depth (10%). ANF responses were assessed by spike-rate measures, while representation of modulation was evaluated by vector strength (VS) and the fundamental component of the fast Fourier transform (F₀ amplitude). These measures were assessed across the 400 ms duration of pulse-train stimuli, a duration relevant to speech stimuli. Different stimulus levels were explored and responses were categorized into four spike-rate groups to assess level effects across ANFs. The temporal pattern of rate adaptation to modulated trains was similar to that of unmodulated trains, but with less rate adaptation. VS to the modulator increased over time and tended to saturate at lower spike rates, while F₀ amplitude typically decreased over time for low driven rates and increased for higher driven rates. VS at moderate and high spike rates and degree of F₀ amplitude temporal changes at low and moderate spike rates were positively correlated with the degree of rate adaptation. Thus, high-rate carriers will modify the ANF representation of the modulator over time. As the VS and F₀ measures were sensitive to adaptation-related changes over different spike-rate ranges, there is value in assessing both measures.     

Vocal emotion recognition by normal-hearing listeners and cochlear implant users

The present study investigated the ability of normal-hearing listeners and cochlear implant users to recognize vocal emotions. Sentences were produced by 1 male and 1 female talker according to 5 target emotions: angry, anxious, happy, sad, and neutral. Overall amplitude differences between the stimuli were either preserved or normalized. In experiment 1, vocal emotion recognition was measured in normal-hearing and cochlear implant listeners; cochlear implant subjects were tested using their clinically assigned processors. When overall amplitude cues were preserved, normal-hearing listeners achieved near-perfect performance, whereas listeners with cochlear implant recognized less than half of the target emotions. Removing the overall amplitude cues significantly worsened mean normal-hearing and cochlear implant performance. In experiment 2, vocal emotion recognition was measured in listeners with cochlear implant as a function of the number of channels (from 1 to 8) and envelope filter cutoff frequency (50 vs 400 Hz) in experimental speech processors. In experiment 3, vocal emotion recognition was measured in normal-hearing listeners as a function of the number of channels (from 1 to 16) and envelope filter cutoff frequency (50 vs 500 Hz) in acoustic cochlear implant simulations. Results from experiments 2 and 3 showed that both cochlear implant and normal-hearing performance significantly improved as the number of channels or the envelope filter cutoff frequency was increased. The results suggest that spectral, temporal, and overall amplitude cues each contribute to vocal emotion recognition. The poorer cochlear implant performance is most likely attributable to the lack of salient pitch cues and the limited functional spectral resolution.     

Modulation Enhancement in the Electrical Signal Improves Perception of Interaural Time Differences with Bimodal Stimulation

Interaural timing cues are important for sound source localization and for binaural unmasking of speech that is spatially separated from interfering sounds. Users of a cochlear implant (CI) with residual hearing in the non-implanted ear (bimodal listeners) can only make very limited use of interaural timing cues with their clinical devices. Previous studies showed that bimodal listeners can be sensitive to interaural time differences (ITDs) for simple single- and three-channel stimuli. The modulation enhancement strategy (MEnS) was developed to improve the ITD perception of bimodal listeners. It enhances temporal modulations on all stimulated electrodes, synchronously with modulations in the acoustic signal presented to the non-implanted ear, based on measurement of the amplitude peaks occurring at the rate of the fundamental frequency in voiced phonemes. In the first experiment, ITD detection thresholds were measured using the method of constant stimuli for five bimodal listeners for an artificial vowel, processed with either the advanced combination encoder (ACE) strategy or with MEnS. With MEnS, detection thresholds were significantly lower, and for four subjects well within the physically relevant range. In the second experiment, the extent of lateralization was measured in three subjects with both strategies, and ITD sensitivity was determined using an adaptive procedure. All subjects could lateralize sounds based on ITD and sensitivity was significantly better with MEnS than with ACE. The current results indicate that ITD cues can be provided to bimodal listeners with modified sound processing.     

Amplitude compression in cochlear implants artificially restricts the perception of temporal asymmetry

This paper presents a study in which five cochlear implantees were asked to discriminate the timbre of stimuli with temporally asymmetric envelopes. Stimuli were damped and ramped sinusoids presented acoustically. They were transformed by the speech processor of the implant and were presented through one electrode. All cochlear implantees could discriminate the damped and ramped sinusoids when the half-life was 4 ms, the carrier frequency was 400 Hz, and the period of the envelope was 50 ms. In a second experiment, timbre discrimination performance was measured as a function of half-life for two cochlear implantees. Both showed that timbre discrimination was possible over the range 1-24 ms. In normal-hearing listeners, the range is 1-64 ms and in cochlear implantees, stimulated directly without the speech processor, the range is 1-300 ms. At long half-lives, the decrease in discrimination performance observed with the speech processor appears to be due to the amplitude compression applied by the device. The present results suggest that it may be important to ensure that cochlear implants do not restrict temporal asymmetry unduly when applying compression to control level.     

Modulation detection by patients with eighth-nerve tumors

Detection thresholds for sinusoidally amplitude-modulated broad-band noise were measured as a function of modulation frequency for 4 normally hearing listeners and for 6 patients suffering eighth-nerve tumors. Measurements were obtained using a method-of-adjustment (MOA) procedure. On average, the threshold values revealed that more modulation was needed across all modulation frequencies for the patients' affected ears relative to either the normally hearing listeners or the patients' better ears. The cutoff frequency derived from the average modulation-threshhold function (MTF) for the affected ears was about half the normal value. For a simple lowpass-filter model of the process, the latter result suggested a doubling of the auditory time constant in the affected ears. These patients, on average, exhibited the characteristic high-frequency audiometric hearing loss most often associated with eighth-nerve tumors. Their MTFs closely resembled MTFs described previously for high-frequency audiometric hearing loss. The results of statistical analyses suggested that high frequency audiometric hearing loss, irrespective of other influences, is the most parsimonious explanation for the increased modulation thresholds measured for the eighth-nerve tumor patients.     

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.     

Improved Electrically Evoked Auditory Steady-State Response Thresholds in Humans

Electrically evoked auditory steady-state responses (EASSRs) are EEG potentials in response to periodic electrical stimuli presented through a cochlear implant. For low-rate pulse trains in the 40-Hz range, electrophysiological thresholds derived from response amplitude growth functions correlate well with behavioral T levels at these rates. The aims of this study were: (1) to improve the correlation between electrophysiological thresholds and behavioral T levels at 900 pps by using amplitude-modulated (AM) and pulse-width-modulated (PWM) high-rate pulse trains, (2) to develop and evaluate the performance of a new statistical method for response detection which is robust in the presence of stimulus artifacts, and (3) to assess the ability of this statistical method to determine reliable electrophysiological thresholds without any stimulus artifact removal. For six users of a Nucleus cochlear implant and a total of 12 stimulation electrode pairs, EASSRs to symmetric biphasic bipolar pulse trains were recorded with seven scalp electrodes. Responses to six different stimuli were analyzed: two low-rate pulse trains with pulse rates in the 40-Hz range as well as two AM and two PWM high-rate pulse trains with a carrier rate of 900 pps and modulation frequencies in the 40-Hz range. Responses were measured at eight different stimulus intensities for each stimulus and stimulation electrode pair. Artifacts due to the electrical stimulation were removed from the recordings. To determine the presence of a neural response, a new statistical method based on a two-sample Hotelling T (2) test was used. Measurements from different recording electrodes and adjacent stimulus intensities were combined to increase statistical power. The results show that EASSRs to modulated high-rate pulse trains account for some of the temporal effects at 900 pps and result in improved electrophysiological thresholds that correlate very well with behavioral T levels at 900 pps. The proposed statistical method for response detection based on a two-sample Hotelling T (2) test has comparable performance to previously used one-sample tests and does not require stimulus artifacts to be removed from the EEG signal for the determination of reliable electrophysiological thresholds.     

Detection of amplitude-modulated tones by frogs: Implications for temporal processing mechanisms

he whole nerve action potential (AP) from the auditory nerve and midbrain averaged evoked potential (AEP) were recorded in Hyla chrysoscelis and H. versicolor in response to synthesized amplitude-modulated stimuli with variable modulation frequencies (F_m). The AP from these frogs is similar to the potential described for mammals and showed a bandpass characteristic in its ability to follow sinusoidally amplitude-modulated (AM) sound stimuli. A lesioning study suggests that the midbrain AEP is a localized neural response of neurons near the ventral border of the torus semicircularis. The AEP is a complex waveform consisting of fast and slow components. The fast component encodes the temporal structure of acoustic stimuli and is used to measure temporal sensitivity in these two species. The AEP behaves like a low-pass filter with a cutoff frequency of 250 Hz when tracking AM signals. Threshold for detection requires a modulation depth of 8–12% of the total stimulus amplitude (ΔI = 1.5-2.0 dB). Relative to the eighth nerve AP, the AEP displays an enhanced coding of AM signals when F_m < 100 Hz, and a slightly inferior ability to code F_m above 250 Hz. The AEP reflects only that portion of the neural response that encodes amplitude fluctuations. In comparison to the range of amplitude fluctuations coded by single units in the rat inferior colliculus or by human evoked potentials, the frog AEP codes higher rates of F_m. The proposal that these frogs process AM stimuli solely on the basis of amplitude fluctuations, and do not use spectral cues at higher modulation frequencies is considered. The AM sensitivity of the AEP, which encompasses most biologically relevant rates of amplitude fluctuation for the animal, and the limited frequency resolution of the periphery, lend support to this proposal. However, convergent spectral processing at higher auditory centers cannot be excluded by this study. Psychophysical tests will be required to determine whether both of these mechanisms may be operating during temporal information processing in anurans.     

Amplitude mapping and phoneme recognition in cochlear implant listeners

OBJECTIVE: Speech and other environmental sounds must be compressed to accommodate the small electric dynamic range in cochlear implant listeners. The objective of this paper is to study whether and how amplitude compression and dynamic range reduction affect phoneme recognition in quiet and in noise for cochlear implant listeners. DESIGN: Four implant listeners using the Nucleus-22 SPEAK speech processor participated in this study. The amount of compression was varied by manipulating the Q-value in the SPEAK processor. The size of the dynamic range was systematically reduced by increasing the threshold level and decreasing the comfortable level in the processor. Both female- and male-talker vowel and consonant materials were used to evaluate speech recognition performance in quiet and in noise. Speech-spectrum-shaped noise was mixed with the speech signal and presented continuously to the speech processor through a direct electric connection. Signal to noise ratios were changed over a 30 to 40 dB range, within which phoneme recognition increased from chance to asymptotic performance. Phoneme recognition scores were obtained as the number of active electrodes was reduced from 20 to 10 to 4. For purposes of comparison, phoneme recognition data also were collected in four normal-hearing listeners under comparable laboratory conditions. RESULTS: In both quiet and noise, the amount of amplitude compression did not significantly affect phoneme recognition. The reduction of dynamic range marginally affected phoneme recognition in quiet, but significantly degraded phoneme recognition in noise. Generally, the 20- and 10-electrode processors produced similar performance, whereas the 4-electrode processor produced significantly poorer performance. Compared with normal-hearing listeners, cochlear-implant listeners required higher signal to noise ratios to achieve comparable recognition performance and produced significantly lower recognition scores at the same signal to noise ratios. CONCLUSIONS: The amount of amplitude compression does not significantly affect phoneme recognition, whereas reducing dynamic range significantly lowers phoneme recognition, particularly in noise and for vowels. Because the SPEAK processor extracts mostly spectral peaks, the present conclusions may not be applied to other types of processors extracting temporal envelope cues. The present results also suggest that more than four electrodes are required to optimize speech recognition in multiple-talker and noise conditions. A significant performance gap in speech recognition still remains between cochlear implant and normal-hearing listeners at the same signal to noise ratios. Improved cochlear implant designs and fitting procedures are required to narrow and, hopefully, close this performance gap.     

Detection of Modulated Tones in Modulated Noise by Non-human Primates

In natural environments, many sounds are amplitude-modulated. Amplitude modulation is thought to be a signal that aids auditory object formation. A previous study of the detection of signals in noise found that when tones or noise were amplitude-modulated, the noise was a less effective masker, and detection thresholds for tones in noise were lowered. These results suggest that the detection of modulated signals in modulated noise would be enhanced. This paper describes the results of experiments investigating how detection is modified when both signal and noise were amplitude-modulated. Two monkeys (Macaca mulatta) were trained to detect amplitude-modulated tones in continuous, amplitude-modulated broadband noise. When the phase difference of otherwise similarly amplitude-modulated tones and noise were varied, detection thresholds were highest when the modulations were in phase and lowest when the modulations were anti-phase. When the depth of the modulation of tones or noise was varied, detection thresholds decreased if the modulations were anti-phase. When the modulations were in phase, increasing the depth of tone modulation caused an increase in tone detection thresholds, but increasing depth of noise modulations did not affect tone detection thresholds. Changing the modulation frequency of tone or noise caused changes in threshold that saturated at modulation frequencies higher than 20 Hz; thresholds decreased when the tone and noise modulations were in phase and decreased when they were anti-phase. The relationship between reaction times and tone level were not modified by manipulations to the nature of temporal variations in the signal or noise. The changes in behavioral threshold were consistent with a model where the brain subtracted noise from signal. These results suggest that the parameters of the modulation of signals and maskers heavily influence detection in very predictable ways. These results are consistent with some results in humans and avians and form the baseline for neurophysiological studies of mechanisms of detection in noise.     

Speech Perception in Noise with a Harmonic Complex Excited Vocoder

A cochlear implant (CI) presents band-pass-filtered acoustic envelope information by modulating current pulse train levels. Similarly, a vocoder presents envelope information by modulating an acoustic carrier. By studying how normal hearing (NH) listeners are able to understand degraded speech signals with a vocoder, the parameters that best simulate electric hearing and factors that might contribute to the NH-CI performance difference may be better understood. A vocoder with harmonic complex carriers (fundamental frequency, f₀ = 100 Hz) was used to study the effect of carrier phase dispersion on speech envelopes and intelligibility. The starting phases of the harmonic components were randomly dispersed to varying degrees prior to carrier filtering and modulation. NH listeners were tested on recognition of a closed set of vocoded words in background noise. Two sets of synthesis filters simulated different amounts of current spread in CIs. Results showed that the speech vocoded with carriers whose starting phases were maximally dispersed was the most intelligible. Superior speech understanding may have been a result of the flattening of the dispersed-phase carrier’s intrinsic temporal envelopes produced by the large number of interacting components in the high-frequency channels. Cross-correlogram analyses of auditory nerve model simulations confirmed that randomly dispersing the carrier’s component starting phases resulted in better neural envelope representation. However, neural metrics extracted from these analyses were not found to accurately predict speech recognition scores for all vocoded speech conditions. It is possible that central speech understanding mechanisms are insensitive to the envelope-fine structure dichotomy exploited by vocoders.     

小儿耳蜗植入后电诱发复合动作电位的阈值及其临床应用

目的：研究应用神经反应遥测（neural response telemetry,NRT)技术，测试电诱发复合动作电位（electrically-evoked compound action potential,ECAP)阈值以指导小儿人工耳蜗映射调图的策略与时机。方法：应用NRT3．0软件对辐值增长函数进行线性拟合，确定ECAP阈值。比较6例儿童植入者在术后1、2、3个月ECAP阈值的变化，同时比较了7例儿童术中、术后ECAP阈值的差异。结果：ECAP幅值增长函数在接近阈值或进入饱和时不再呈线性。术后ECAP阈值保持稳定。各导电极的术中ECAP阈值比术后阈值平均高约15CL，二者有显著性相关（R2＝0．9154）。结论：应选取幅值增长函数的直线段部分进行拟合以确定ECAP阈值。术后应用ECAP阈值指导小儿映射调图时，测试一次ECAP阈值即可。术中ECAP阈值可用作开机时映射图的C值。