Detection of Tones and Their Modification by Noise in Nonhuman Primates

A fundamental function of the auditory system is to detect important sounds in the presence of other competing environmental sounds. This paper describes behavioral performance in a tone detection task by nonhuman primates (Macaca mulatta) and the modification of the performance by continuous background noise and by sinusoidally amplitude modulating signals or noise. Two monkeys were trained to report detection of tones in a reaction time Go/No-Go task using the method of constant stimuli. The tones spanned a wide range of frequencies and sound levels, and were presented alone or in continuous broadband noise (40 kHz bandwidth). Signal detection theoretic analysis revealed that thresholds to tones were lowest between 8 and 16 kHz, and were higher outside this range. At each frequency, reaction times decreased with increases in tone sound pressure level. The slope of this relationship was higher at frequencies below 1 kHz and was lower for higher frequencies. In continuous broadband noise, tone thresholds increased at the rate of 1 dB/dB of noise for frequencies above 1 kHz. Noise did not change either the reaction times for a given tone sound pressure level or the slopes of the reaction time vs. tone level relationship. Amplitude modulation of tones resulted in reduced threshold for nearly all the frequencies tested. Amplitude modulation of the tone caused thresholds for detection in continuous broadband noise to be changed by smaller amounts relative to the detection of steady-state tones in noise. Amplitude modulation of background noise resulted in reduction of detection thresholds of steady-state tones by an average of 11 dB relative to thresholds in steady-state noise of equivalent mean amplitude. In all cases, the slopes of the reaction time vs. sound level relationship were not modified. These results show that macaques have hearing functions similar to those measured in humans. These studies form the basis for ongoing studies of neural mechanisms of hearing in noisy backgrounds.     

Detection of Tones in Reproducible Noise Maskers by Rabbits and Comparison to Detection by Humans

Processing mechanisms used for detection of tones in noise can be revealed by using reproducible noise maskers and analyzing the pattern of results across masker waveforms. This study reports detection of a 500-Hz tone in broadband reproducible noise by rabbits using a set of masker waveforms for which human results are available. An appetitive-reinforcement, operant-conditioning procedure with bias control was used. Both fixed-level and roving-level noises were used to explore the utility of energy-related cues for detection. An energy-based detection model was able to partially explain the fixed-level results across reproducible noise waveforms for both rabbit and human. A multiple-channel energy model was able to explain fixed-level results, as well as the robust performance observed with roving-level noises. Further analysis using the energy model indicated a difference between species: human detection was influenced most by the noise spectrum surrounding the tone frequency, whereas rabbit detection was influenced most by the noise spectrum at frequencies above that of the tone. In addition, a temporal envelope-based model predicted detection by humans as well as the single-channel energy model did, but the envelope-based model failed to predict detection by rabbits. This result indicates that the contributions of energy and temporal cues to auditory processing differ across species. Overall, these findings suggest that caution must be used when evaluating neural encoding mechanisms in one species on the basis of behavioral results in another.     

Temporary Suppression of Tinnitus by Modulated Sounds

Despite high prevalence of tinnitus and its impact on quality life, there is no cure for tinnitus at present. Here, we report an effective means to temporarily suppress tinnitus by amplitude- and frequency-modulated tones. We systematically explored the interaction between subjective tinnitus and 17 external sounds in 20 chronic tinnitus sufferers. The external sounds included traditionally used unmodulated stimuli such as pure tones and white noise and dynamically modulated stimuli known to produce sustained neural synchrony in the central auditory pathway. All external sounds were presented in a random order to all subjects and at a loudness level that was just below tinnitus loudness. We found some tinnitus suppression in terms of reduced loudness by at least one of the 17 stimuli in 90% of the subjects, with the greatest suppression by amplitude-modulated tones with carrier frequencies near the tinnitus pitch for tinnitus sufferers with relatively normal loudness growth. Our results suggest that, in addition to a traditional masking approach using unmodulated pure tones and white noise, modulated sounds should be used for tinnitus suppression because they may be more effective in reducing hyperactive neural activities associated with tinnitus. The long-term effects of the modulated sounds on tinnitus and the underlying mechanisms remain to be investigated.     

听力正常青年人独立调幅调频反应与言语识别率的关系

目的观察听力正常青年人独立调幅调频(independentamplitudeandfrequencymodulation,IAFM)反应与言语识别率(wordrecognitionscore,WRS)的关系,探讨采用IAFM反应预估WRS的可能性。方法21名听力正常青年受试者(21耳)以0.5、1.0、2.0和4.0kHz为载波,振幅调制频率分别为79、87、95和103Hz,调幅深度分别为55%、50%、45%及35%;频率调制频率分别为85、93、101和109Hz,调制深度分别为35%、30%、20%及35%,四个I-AFM声同时给出,单耳给声。分别测试20、30、40、50、60、70、80和90dBSPL的IAFM有意义反应数和WRS,观察两者之间的相关性。结果IAFM反应数与强度的线性相关系数为0.844(P<0.01),WRS与强度的线性相关系数为0.785(P<0.01),IAFM反应数与WRS的线性相关系数为0.785(P<0.01);IAFM反应数与WRS的偏相关系数(强度为控制因素)为0.371(P<0.01)。结论调制频率在70~110Hz的IAFM反应能够反映耳蜗和脑干对频率和振幅变化的分辨能力,与WRS有显著的相关性,有可能成为评价言语感知所必需的声学信息分辨能力的客观工具,从而成为评价和预估言语识别功能的措施之一。     

Modeling the Anti-masking Effects of the Olivocochlear Reflex in Auditory Nerve Responses to Tones in Sustained Noise

The medial olivocochlear reflex (MOCR) has been hypothesized to provide benefit for listening in noise. Strong physiological support for an anti-masking role for the MOCR has come from the observation that auditory nerve (AN) fibers exhibit reduced firing to sustained noise and increased sensitivity to tones when the MOCR is elicited. The present study extended a well-established computational model for normal-hearing and hearing-impaired AN responses to demonstrate that these anti-masking effects can be accounted for by reducing outer hair cell (OHC) gain, which is a primary effect of the MOCR. Tone responses in noise were examined systematically as a function of tone level, noise level, and OHC gain. Signal detection theory was used to predict detection and discrimination for different spontaneous rate fiber groups. Decreasing OHC gain decreased the sustained noise response and increased maximum discharge rate to the tone, thus modeling the ability of the MOCR to decompress AN fiber rate-level functions. Comparing the present modeling results with previous data from AN fibers in decerebrate cats suggests that the ipsilateral masking noise used in the physiological study may have elicited up to 20 dB of OHC gain reduction in addition to that inferred from the contralateral noise effects. Reducing OHC gain in the model also extended the dynamic range for discrimination over a wide range of background noise levels. For each masker level, an optimal OHC gain reduction was predicted (i.e., where maximum discrimination was achieved without increased detection threshold). These optimal gain reductions increased with masker level and were physiologically realistic. Thus, reducing OHC gain can improve tone-in-noise discrimination even though it may produce a “hearing loss” in quiet. Combining MOCR effects with the sensorineural hearing loss effects already captured by this computational AN model will be beneficial for exploring the implications of their interaction for the difficulties hearing-impaired listeners have in noisy situations.     

Effects of Noise Reduction on AM Perception for Hearing-Impaired Listeners

Noise reduction (NR) systems are commonplace in modern digital hearing aids. Though not improving speech intelligibility, NR helps the hearing-aid user in terms of lowering noise annoyance, reducing cognitive load and improving ease of listening. Previous psychophysical work has shown that NR does in fact improve the ability of normal-hearing (NH) listeners to discriminate the slow amplitude-modulation (AM) cues representative of those found in speech. The goal of this study was to assess whether this improvement of AM discrimination with NR can also be observed for hearing-impaired (HI) listeners. AM discrimination was measured at two audio frequencies of 500 Hz and 2 kHz in a background noise with a signal-to-noise ratio of 12 dB. Discrimination was measured for ten HI and ten NH listeners with and without NR processing. The HI listeners had a moderate sensorineural hearing loss of about 50 dB HL at 2 kHz and normal hearing (≤20 dB HL) at 500 Hz. The results showed that most of the HI listeners tended to benefit from NR at 500 Hz but not at 2 kHz. However, statistical analyses showed that HI listeners did not benefit significantly from NR at any frequency region. In comparison, the NH listeners showed a significant benefit from NR at both frequencies. For each condition, the fidelity of AM transmission was quantified by a computational model of early auditory processing. The parameters of the model were adjusted separately for the two groups (NH and HI) of listeners. The AM discrimination performance of the HI group (with and without NR) was best captured by a model simulating the loss of the fast-acting amplitude compression applied by the normal cochlea. This suggests that the lack of benefit from NR for HI listeners results from loudness recruitment.     

Statistical Analyses of Temporal Information in Auditory Brainstem Responses to Tones in Noise: Correlation Index and Spike-distance Metric

Gai and Carney (J Neurophysiol 96:2451-2464, 2006) previously explored the detection of tones in noise based on responses in the anteroventral cochlear nucleus; that study focused on temporal information in discharge reliability and analyses of neural responses related to the fine structure or envelope of the stimulus. Two additional temporal approaches, the correlation index (Joris et al., Hearing Res 216-217:19-30, 2006) and the spike-distance metric (Victor and Purpura, J Neurophysiol 76:1310-1326, 1996; Netw Comput Neural Syst 8:127-164, 1997), are tested in the present study. Trends in the correlation index as a function of stimulus level are similar to those of the synchronization coefficient (also called the vector strength) when the tone is presented alone. However, the present study found that trends in the correlation index did not agree with those of the synchronization coefficient for tones presented with relatively high-level background noise. Instead, trends in the correlation index generally agreed with those of the temporal reliability metric discussed in Gai and Carney (J Neurophysiol 96:2451-2464, 2006); that is, the correlation index decreased with increased tone level in the presence of relatively high-level background noise. The spike-distance metric, which was based on absolute spike times or on interspike intervals, was compared to the temporal measures described above, which were generally based on relative spike times. The results confirm that the spike-distance metric is not an optimal temporal metric. In addition, absolute spike times of primary-like responses generally contained much less temporal information than absolute spike times of chopper response types. The present study highlights the importance of relative spike-timing information as characterized by traditional and novel temporal measures.     

Auditory Cortical Images of Tones and Noise Bands

We examined the representation of stimulus center frequencies by the distribution of cortical activity. Recordings were made from the primary auditory cortex (area A1) of ketamine-anesthetized guinea pigs. Cortical images of tones and noise bands were visualized as the simultaneously recorded spike activity of neurons at 16 sites along the tonotopic gradient of cortical frequency representation. The cortical image of a pure tone showed a restricted focus of activity along the tonotopic gradient. As the stimulus frequency was increased, the location of the activation focus shifted from rostral to caudal. When cochlear activation was broadened by increasing the stimulus level or bandwidth, the cortical image broadened. An artificial neural network algorithm was used to quantify the accuracy of center-frequency representation by small populations of cortical neurons. The artificial neural network identified stimulus center frequency based on single-trial spike counts at as few as ten sites. The performance of the artificial neural network under various conditions of stimulus level and bandwidth suggests that the accuracy of representation of center frequency is largely insensitive to changes in the width of cortical images.     

Exploring the Role of Medial Olivocochlear Efferents on the Detection of Amplitude Modulation for Tones Presented in Noise

Journal of the Association for Research in Otolaryngology - The medial olivocochlear reflex has been hypothesized to improve the detection and discrimination of dynamic signals in noisy...     

Responses of neurons in the inferior colliculus of the rat to AM and FM tones

The responses of units in the inferior colliculus of the urethane-anaesthetized rat were recorded extracellularly. They responded to sinusoidal AM and FM tones with a modulation of their spike discharge usually at the same, or occasionally at twice, the modulation rate of the stimulus. The modulation depth of the response initially increased with the modulation depth of the stimulus, hut usually saturated or decreased at higher stimulus depths. The units showed a bandpass tuning to stimulus modulation rate which was independent of modulation depth and, in all cases, the most effective modulation rate was below 120 Hz. The modulated response to temporally varying stimuli could not be predicted from the pure tone discharge patterns or, in some cases, the unit's mean firing rate to modulated tones; temporally varying stimuli gave temporally varying responses. When compared with the data available from units at other levels in the auditory system, the results indicate a trend in which units at successively higher levels in the pathway respond most effectively to progressively lower rates of modulation.     

Rate Representation of Tones in Noise in the Inferior Colliculus of Decerebrate Cats

Neurons in the central nucleus of the inferior colliculus (ICC) of decerebrate cats show three major response patterns when tones of different frequencies and sound-pressure levels (SPLs) are presented to the contralateral ear. The frequency response maps of type I units are uniquely defined by a narrow excitatory area at best frequency (BF: a unit's most sensitive frequency) and surrounding inhibition at higher and lower frequencies. As a result of this receptive field organization, type I units exhibit strong excitatory responses to BF tones but respond only weakly to broadband noise (BBN). These response characteristics predict that type I units are well suited to encode narrowband signals in the presence of background noise. To test this hypothesis, the dynamic range properties of ICC unit types were measured under quiet conditions and in multiple levels of continuous noise. As observed in previous studies of the auditory nerve and cochlear nucleus, type I units showed upward threshold shifts and discharge rate compression in background noise that partially degraded the dynamic range properties of neural representations at high noise levels. Although the other two unit types in the ICC showed similar trends in threshold shift and noise compression, their ability to encode auditory signals was compromised more severely in increasing noise levels. When binaural masking effects were simulated, only type I units showed an enhanced representation of spatially separated signals and maskers that was consistent with human perceptual performance in independent psychoacoustic observations. These results support the interpretation that type I units play an important role in the auditory processing of narrowband signals in background noise and suggest a physiological basis for spatial factors that govern signal detection under free-field listening conditions.     

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.     

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.     

Response modulation of auditory-nerve fibers by am stimuli: effects of average intensity

Auditory-nerve responses were obtained for characteristic frequency tones which were amplitude modulated by sinusoids. Response modulation (RM) was determined from folded histograms which were synchronized to the modulating wave form. As the average intensity increased from threshold, RM increased to a maximum and then decreased, and the shape of the RM function resembled that described previously for incremental responses. However, unlike the latter, the RM function could not he predicted directly from the steady-state rate-intensity function. In general, the maximum RM occurred at a higher intensity than predicted, and RM occurred over a wider range of average intensities than predicted. The results are interpreted as reflecting a dynamic response characteristic with an operating range that exceeds that determined from the steady-state rate-intensity function.     

Cues for Diotic and Dichotic Detection of a 500-Hz Tone in Noise Vary with Hearing Loss

Hearing in noise is a challenge for all listeners, especially for those with hearing loss. This study compares cues used for detection of a low-frequency tone in noise by older listeners with and without hearing loss to those of younger listeners with normal hearing. Performance varies significantly across different reproducible, or “frozen,” masker waveforms. Analysis of these waveforms allows identification of the cues that are used for detection. This study included diotic (N₀S₀) and dichotic (N₀S_π) detection of a 500-Hz tone, with either narrowband or wideband masker waveforms. Both diotic and dichotic detection patterns (hit and false alarm rates) across the ensembles of noise maskers were predicted by envelope-slope cues, and diotic results were also predicted by energy cues. The relative importance of energy and envelope cues for diotic detection was explored with a roving-level paradigm that made energy cues unreliable. Most older listeners with normal hearing or mild hearing loss depended on envelope-related temporal cues, even for this low-frequency target. As hearing threshold at 500 Hz increased, the cues for diotic detection transitioned from envelope to energy cues. Diotic detection patterns for young listeners with normal hearing are best predicted by a model that combines temporal- and energy-related cues; in contrast, combining cues did not improve predictions for older listeners with or without hearing loss. Dichotic detection results for all groups of listeners were best predicted by interaural envelope cues, which significantly outperformed the classic cues based on interaural time and level differences or their optimal combination.     

Sound Localization in Noise by Gerbils and Humans

Detection and localization of a target sound in the presence of concurrent, spatially distributed masking sounds is one of the most challenging tasks for the mammalian auditory system. Previous studies demonstrated that the ability to localize signals is decreased by interfering noise. In order to directly compare the behavioral performance in a signal processing task in noise between gerbils and humans in the free sound field, we quantified their localization ability for a low-frequency signal in the presence of six masking noise sources surrounding the subject. Thresholds were measured both for masking noises that were correlated or uncorrelated across the masking sources. Overall, the gerbils required a higher signal/noise ratio to detect the low-frequency signal than the humans; that is, the behavioral performance of the gerbils was considerably worse than that of the humans. Moreover, switching from maskers that were uncorrelated across the masking sources to correlated maskers resulted in more masking in gerbils but in a release from masking in humans. These results would suggest that the gerbil may not be a good animal model for binaural processing. However, simulations of the localization thresholds in a numerical model of binaural processing in gerbils and humans reveal that both the inferior overall performance in gerbils and the opposite effect of masker correlation on the detection thresholds can be attributed to the smaller head size and the wider peripheral auditory filters in gerbils. Thus, the current data indicate that the binaural processor itself (i.e., the evaluation of signals coming from the two ears) is equally sensitive in gerbils and humans. However, the physical limitations imposed by the small head prevent the gerbil from performing equally well in the current paradigm.     

Discrimination of Direction in Fast Frequency-Modulated Tones by Rats

Fast frequency modulations (FM) are an essential part of species-specific auditory signals in animals as well as in human speech. Major parameters characterizing non-periodic frequency modulations are the direction of frequency change in the FM sweep (upward/downward) and the sweep speed, i.e., the speed of frequency change. While it is well established that both parameters are represented in the mammalian central auditory pathway, their importance at the perceptual level in animals is unclear. We determined the ability of rats to discriminate between upward and downward modulated FM-tones as a function of sweep speed in a two-alternative-forced-choice-paradigm. Directional discrimination in logarithmic FM-sweeps was reduced with increasing sweep speed between 20 and 1,000 octaves/s following a psychometric function. Average threshold sweep speed for FM directional discrimination was 96 octaves/s. This upper limit of perceptual FM discrimination fits well the upper limit of preferred sweep speeds in auditory neurons and the upper limit of neuronal direction selectivity in the rat auditory cortex and midbrain, as it is found in the literature. Influences of additional stimulus parameters on FM discrimination were determined using an adaptive testing-procedure for efficient threshold estimation based on a maximum likelihood approach. Directional discrimination improved with extended FM sweep range between two and five octaves. Discrimination performance declined with increasing lower frequency boundary of FM sweeps, showing an especially strong deterioration when the boundary was raised from 2 to 4 kHz. This deterioration corresponds to a frequency-dependent decline in direction selectivity of FM-encoding neurons in the rat auditory cortex, as described in the literature. Taken together, by investigating directional discrimination of FM sweeps in the rat we found characteristics at the perceptual level that can be related to several aspects of FM encoding in the central auditory pathway.     

Use of Amplitude Modulation Cues Recovered from Frequency Modulation for Cochlear Implant Users When Original Speech Cues Are Severely Degraded

Won et al. (J Acoust Soc Am 132:1113–1119, 2012) reported that cochlear implant (CI) speech processors generate amplitude-modulation (AM) cues recovered from broadband speech frequency modulation (FM) and that CI users can use these cues for speech identification in quiet. The present study was designed to extend this finding for a wide range of listening conditions, where the original speech cues were severely degraded by manipulating either the acoustic signals or the speech processor. The manipulation of the acoustic signals included the presentation of background noise, simulation of reverberation, and amplitude compression. The manipulation of the speech processor included changing the input dynamic range and the number of channels. For each of these conditions, multiple levels of speech degradation were tested. Speech identification was measured for CI users and compared for stimuli having both AM and FM information (intact condition) or FM information only (FM condition). Each manipulation degraded speech identification performance for both intact and FM conditions. Performance for the intact and FM conditions became similar for stimuli having the most severe degradations. Identification performance generally overlapped for the intact and FM conditions. Moreover, identification performance for the FM condition was better than chance performance even at the maximum level of distortion. Finally, significant correlations were found between speech identification scores for the intact and FM conditions. Altogether, these results suggest that despite poor frequency selectivity, CI users can make efficient use of AM cues recovered from speech FM in difficult listening situations.     

Coding of amplitude-modulated tones in the central auditory system of catfish

In the catfish central acoustic system information is coded by two subsets of units. Type I units show little or no adaptation, type II units adapt rapidly, and some units are transitional, showing moderate adaptation. The two groups of units also respond differently when exposed to sinusoidal amplitude modulation of the signal's carrier frequency. The tonic, less readily adapting type I units code over an intensity range of about 30 dB, are fairly insensitive to intensity changes, and follow stimulus envelopes of 60 Hz and less. They apparently discharge in response to the actual intensity of the signal rather than in response to something in its temporal pattern. The onset-sensitive, fast-adapting type II units on the other hand are restricted to an intensity range of only 10 dB, show greater sensitivity to intensity changes, and are capable of following the temporal pattern of amplitude-modulated stimuli exceeding 100 Hz. These units appear to code the temporal changes in the stimulus intensity irrespective of the absolute intensity of the signal.     

Audibility of short-duration tone-glides as a function of rate of frequency change

Thresholds for rising and falling tone-glides were determined against a background of 50–6000 Hz noise at a level of 60 dB re 20 μPa. Glides were centered around 2000 Hz and changed frequencies linearly at rates of 24, 48, 96 and 192 Hz/ms; tone-glide durations were 5, 10, 20 and 40 ms. Results demonstrate a rate-dependent asymmetry in the detectability of rising and falling tone-glides, with rising tone-glides detected at lower signal intensities for the higher rates of frequency change (i.e., 96 and 192 Hz/ms).